AN ABSTRACT OF THE THESIS OF Ryan L. Gerber for the degree of Master of Science in Radiation Health Physics presented on December 6, 2010. Title: A COMPARISON OF COMPACT IONIZATION CHAMBER PERFORMANCE AND RELATIVE READINGS Abstract approved: Kathryn A. Higley The purpose of this study is to investigate the relative performance of compact ionization chambers as it changes based on the speed of detector motion and collection volume. To quantify changes, multiple scans were made with each of a selection of compact chambers and repeated varying detector speed. Each scan was then used to compute the necessary statistics for each field sampled. These results were then compared to analyze differences in relative ionization readings across entire scan ranges. The results and conclusions of this study further reinforce existing studies, in particular those released since 2007 relating to the study of newly available compact ionization chambers. When choosing a chamber, one should use the smallest chamber available that has been proven to respond appropriately for the field sizes to be measured. As for detector speed, generally smaller field sizes are shown to be more sensitive to detector speed changes. There is not one recommendation for detector speed, as the optimum speed is determined by the type of scan being performed, the energy being scanned, the field size being scanned, and the end use of the data being captured. Finally, optimizing pdd scan depths and profile penumbra margins is an important step to maximizing efficient use of time when capturing LINAC beam characteristics. © Copyright by Ryan L. Gerber December 6, 2010 All Rights Reserved A COMPARISON OF COMPACT IONIZATION CHAMBER PERFORMANCE AND RELATIVE READINGS by Ryan L. Gerber A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented December 6, 2010 Commencement June 2011 Master of Science thesis of Ryan L. Gerber presented on December 6, 2010. APPROVED: ________________________________________ Major Professor, representing Radiation Health Physics ________________________________________ Head of the Department of Nuclear Engineering & Radiation Health Physics ________________________________________ Dean of the Graduate School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. ____________________________________ Ryan L. Gerber, Author TABLE OF CONTENTS Page Introduction ................................................................................................................ 1 Background ................................................................................................................ 4 Radiation Therapy with Linear Accelerators .........................................................4 Medical Linear Accelerators ..................................................................................... 4 Treatment Planning ..................................................................................................... 8 Beam Measurement ..............................................................................................10 Ionization Chambers ................................................................................................. 10 Electrometers ............................................................................................................. 12 3D Beam Scanning Systems .................................................................................... 13 Equipment Used and Data Collection Conditions ................................................... 14 Equipment/Parameters ..........................................................................................14 Data Collection Conditions ..................................................................................14 Data Analysis Legend ..........................................................................................15 Additional Notes on Data Collection ...................................................................15 Data Presentation and Analysis ............................................................................... 16 Detector Speed Change ........................................................................................16 Small fields – Data .................................................................................................... 16 Small fields - Analysis ............................................................................................. 17 Small fields - Recommendation .............................................................................. 21 Medium fields – Data ............................................................................................... 21 Medium fields – Analysis ........................................................................................ 22 Medium fields – Reommendation........................................................................... 27 Large Fields – Data ................................................................................................... 27 Large Fields – Analysis ............................................................................................ 28 Large fields – Recommendation ............................................................................. 32 Physical Detector Change.....................................................................................33 Chamber response to field size change – Data...................................................... 33 TABLE OF CONTENTS (Continued) Page Chamber response to field size change – Analysis............................................... 37 Small fields – Data .................................................................................................... 37 Small fields - Analysis ............................................................................................. 37 Small field – Recommendation ............................................................................... 39 Medium field – Data ................................................................................................. 39 Medium field – Analysis .......................................................................................... 39 Medium fields – Recommendation ......................................................................... 43 Large fields – Data .................................................................................................... 44 Large fields – Analysis ............................................................................................. 44 Large fields – Recommendation ............................................................................. 47 Conclusion ............................................................................................................... 48 Bibliography ............................................................................................................ 49 Appendix .................................................................................................................. 51 LIST OF FIGURES Figure Page 1 – Varian Trilogy LINAC......................................................................................... 1 2 – IBA Welhoefer Blue Phantom Scanning System ................................................ 2 3 – LINAC Console with Scanning System............................................................... 3 4 – PTW 30013 Farmer Chamber ............................................................................ 11 5 – IBA Welhoefer cc01 Compact Chamber ........................................................... 12 6 – 6 MV Photons, .5x.5cm2, cc01, profile, all speeds ............................................ 20 7 – 6 MV Photons, .5x.5cm2, cc01, pdd, speeds ...................................................... 21 8 – Field Size Dependence – 6 MV photons ............................................................ 35 9 – Field Size Dependence – 18 MV photons .......................................................... 36 10 – 6 MV Photons, .5x.5cm2, cc01(blue)/cc04(green)/cc13(red), pdd .................. 38 11 – 6 MV Photons, .5x.5cm2, cc01(blue)/cc04(green)/cc13(red), profile .............. 39 12 – 6 MeV electrons, 3cm circle, cc01/cc04/cc13, pdd .........................................42 13 – 6 MeV electrons, 10x10cm2, cc01/cc04/cc13, pdd .......................................... 42 14 – 6 MV photons, 10x10cm2, cc01/cc04/cc13, pdd .............................................43 15 – 6 MV photons, 40x40 cm2, cc01/cc04/cc13, pdd ............................................46 16 – 6 MeV, 25x25cm2, cc01, profile ...................................................................... 47 17 – 6 MV photons, .5x.5cm2, cc01, fast pdd ..........................................................52 18 – 6 MV photons, .5x.5cm2, cc01, medium pdd ...................................................53 19 – 6 MV photons, .5x.5cm2, cc01, slow pdd ........................................................54 20 – 6 MV photons, .5x.5cm2, cc01, all speeds, pdd ...............................................55 21 – 6 MV photons, .5x.5cm2, cc01, fast profile .....................................................56 22 – 6 MV photons, .5x.5cm2, cc01, medium profile ..............................................57 23 – 6 MV photons, .5x.5cm2, cc01, slow profile....................................................58 24 – 6 MV photons, .5x.5cm2, cc01, all speeds profile ...........................................59 25 – 6 MV photons, .5x.5cm2, cc04, fast pdd ..........................................................60 26 – 6 MV photons, .5x.5cm2, cc04, medium pdd ...................................................61 27 – 6 MV photons, .5x.5cm2, cc04, slow pdd ........................................................62 LIST OF FIGURES (Continued) Figure Page 28 – 6 MV photons, .5x.5cm2, cc04, all speeds pdd ................................................63 29 – 6 MV photons, .5x.5cm2, cc04, fast profile .....................................................64 30 – 6 MV photons, .5x.5cm2, cc04, medium profile ..............................................65 31 – 6 MV photons, .5x.5cm2, cc04, slow profile....................................................66 32 – 6 MV photons, .5x.5cm2, cc04, all speeds profile ...........................................67 33 – 6 MeV electrons, 3cm circle, cc01, fast pdd ....................................................68 34 – 6 MeV electrons, 3cm circle, cc01, medium pdd.............................................69 35 – 6 MeV electrons, 3cm circle, cc01, slow pdd .................................................. 70 36 – 6 MeV electrons, 3cm circle, cc01, all speeds pdd .......................................... 71 37 – 6 MeV electrons, 3cm circle, cc01, fast profile ............................................... 72 38 – 6 MeV electrons, 3cm circle, cc01, medium profile ........................................73 39 – 6 MeV electrons, 3cm circle, cc01, slow profile ............................................. 74 40 – 6 MeV electrons, 3cm circle, cc01, all speeds profile .....................................75 41 – 6 MeV electrons, 3cm circle, cc04, fast pdd ....................................................76 42 – 6 MeV electrons, 3cm circle, cc04, medium pdd.............................................77 43 – 6 MeV electrons, 3cm circle, cc04, slow pdd .................................................. 78 44 – 6 MeV electrons, 3cm circle, cc04, all speeds pdd .......................................... 79 45 – 6 MeV electrons, 3cm circle, cc04, fast profile ............................................... 80 46 – 6 MeV electrons, 3cm circle, cc04, medium profile ........................................81 47 – 6 MeV electrons, 3cm circle, cc04, slow profile ............................................. 82 48 – 6 MeV electrons, 3cm circle, cc04, all speeds profile .....................................83 49 – 6 MeV electrons, 3cm circle, cc13, fast pdd ....................................................84 50 – 6 MeV electrons, 3cm circle, cc13, medium pdd.............................................85 51 – 6 MeV electrons, 3cm circle, cc13, slow pdd .................................................. 86 52 – 6 MeV electrons, 3cm circle, cc13, all speeds pdd .......................................... 87 53 – 6 MeV electrons, 3cm circle, cc13, fast profile ............................................... 88 54 – 6 MeV electrons, 3cm circle, cc13, medium profile ........................................89 LIST OF FIGURES (Continued) Figure Page 55 – 6 MeV electrons, 3cm circle, cc13, slow profile ............................................. 90 56 – 6 MeV electrons, 3cm circle, cc13, all speeds profile .....................................91 57 – 6 MeV electrons, 10x10 cm2, all chambers, medium pdd ............................... 92 58 – 6 MeV electrons, 25x25 cm2, cc01, fast pdd ................................................... 93 59 – 6 MeV electrons, 25x25 cm2, cc01, med pdd .................................................. 94 60 – 6 MeV electrons, 25x25 cm2, cc01, slow pdd.................................................. 95 61 – 6 MeV electrons, 25x25 cm2, cc01, all speeds pdd.......................................... 96 62 – 6 MeV electrons, 25x25 cm2, cc01, fast profile ............................................... 97 63 – 6 MeV electrons, 25x25 cm2, cc01, med profile.............................................. 98 64 – 6 MeV electrons, 25x25 cm2, cc01, slow profile ............................................. 99 65 – 6 MeV electrons, 25x25 cm2, cc01, all speeds profile ................................... 100 66 – 6 MeV electrons, 25x25 cm2, cc04, fast pdd ................................................. 101 67 – 6 MeV electrons, 25x25 cm2, cc04, med pdd ................................................ 102 68 – 6 MeV electrons, 25x25 cm2, cc04, slow pdd................................................ 103 69 – 6 MeV electrons, 25x25 cm2, cc04, all speeds pdd........................................ 104 70 – 6 MeV electrons, 25x25 cm2, cc04, fast profile ............................................. 105 71 – 6 MeV electrons, 25x25 cm2, cc04, med profile............................................ 106 72 – 6 MeV electrons, 25x25 cm2, cc04, slow profile ........................................... 107 73 – 6 MeV electrons, 25x25 cm2, cc04, all speeds profile ................................... 108 74 – 6 MeV electrons, 25x25 cm2, cc13, fast pdd ................................................. 109 75 – 6 MeV electrons, 25x25 cm2, cc13, med pdd ................................................ 110 76 – 6 MeV electrons, 25x25 cm2, cc13, slow pdd................................................ 111 77 – 6 MeV electrons, 25x25 cm2, cc13, all speeds pdd........................................ 112 78 – 6 MeV electrons, 25x25 cm2, cc13, fast profile ............................................. 113 79 – 6 MeV electrons, 25x25 cm2, cc13, medium profile ..................................... 114 80 – 6 MeV electrons, 25x25 cm2, cc13, slow profile ........................................... 115 81 – 6 MeV electrons, 25x25 cm2, cc13, all speeds profile ................................... 116 LIST OF FIGURES (Continued) Figure Page 82 – 6MV photons, .5x.5cm2, cc01, pdd ................................................................ 117 83 – 6MV photons, .5x.5cm2, cc04, pdd ................................................................ 118 84 – 6MV photons, .5x.5cm2, cc13, pdd ................................................................ 119 85 – 6MV photons, .5x.5cm2, all chambers pdd ....................................................120 86 – 6MV photons, .5x.5cm2, cc01, profile ...........................................................121 87 – 6MV photons, .5x.5cm2, cc04, profile ...........................................................122 88 – 6MV photons, .5x.5cm2, cc13, profile ...........................................................123 89 – 6MV photons, .5x.5cm2, all chambers, profile .............................................. 124 90 – 6 MeV electrons, 3cm circle, cc01, pdd ......................................................... 125 91 – 6 MeV electrons, 3cm circle, cc04, pdd ......................................................... 126 92 – 6 MeV electrons, 3cm circle, cc13, pdd ......................................................... 127 93 – 6 MeV electrons, 3cm circle, all chambers pdd ............................................. 128 94 – 6 MeV electrons, 10x10 cm2, all chambers, pdd ........................................... 129 95 – 6 MV photons, 10x10 cm2, all chambers, pdd ...............................................130 96 – 6 MV photons, 10x10 cm2, all chambers, pdd (near surface zoom) ..............131 97 – 6 MeV electrons, 3cm circle, cc01, profile .................................................... 132 98 – 6 MeV electrons, 3cm circle, cc04, profile .................................................... 133 99 – 6 MeV electrons, 3cm circle, cc13, profile .................................................... 134 100 – 6 MeV electrons, 3cm circle, all chambers, profile .....................................135 101 – 6 MeV electrons, 25x25cm2, cc01, pdd ....................................................... 136 102 – 6 MeV electrons, 25x25cm2, cc04, pdd ....................................................... 137 103 – 6 MeV electrons, 25x25cm2, cc13, pdd ....................................................... 138 104 – 6 MeV electrons, 25x25cm2, all chambers pdd ........................................... 139 105 – 6 MV photons, 40x40cm2, all chambers, pdd .............................................. 140 106 – 6 MeV electrons, 25x25cm2, cc01, profile................................................... 141 107 – 6 MeV electrons, 25x25cm2, cc04, profile................................................... 142 108 – 6 MeV electrons, 25x25cm2, cc13, profile................................................... 143 LIST OF FIGURES (Continued) Figure Page 109 – 6 MeV electrons, 25x25cm2, all chambers, profile ...................................... 144 LIST OF TABLES Table Page 1 – 6 MV Photons, .5x.5cm2, cc01, profile, fast ...................................................... 17 2 – 6 MV Photons, .5x.5cm2, cc01, profile, medium ............................................... 17 3 – 6 MV Photons, .5x.5cm2, cc01, profile, slow ....................................................17 4 – 6 MV Photons, .5x.5cm2, cc01, profile, fast ...................................................... 18 5 – 6 MV Photons, .5x.5cm2, cc01, profile, medium ............................................... 18 6 – 6 MV Photons, .5x.5cm2, cc01, profile, slow ....................................................18 7 – 6 MV Photons, .5x.5cm2, cc01, pdd, fast ........................................................... 18 8 – 6 MV Photons, .5x.5cm2, cc01, pdd, medium ................................................... 19 9 – 6 MV Photons, .5x.5cm2, cc01, pdd, slow ......................................................... 19 10 – 6 MV Photons, .5x.5cm2, cc01, pdd, fast ......................................................... 19 11 – 6 MV Photons, .5x.5cm2, cc01, pdd, medium ................................................. 19 12 – 6 MV Photons, .5x.5cm2, cc01, pdd, slow ....................................................... 19 13 – 6 MeV electrons, 3cm circle, cc01, profile, fast ..............................................22 14 – 6 MeV electrons, 3cm circle, cc01, profile, medium .......................................22 15 – 6 MeV electrons, 3cm circle, cc01, profile, slow ............................................ 23 16 – 6 MeV electrons, 3cm circle, cc04, profile, fast ..............................................23 17 – 6 MeV electrons, 3cm circle, cc04, profile, medium .......................................23 18 – 6 MeV electrons, 3cm circle, cc04, profile, slow ............................................ 23 19 – 6 MeV electrons, 3cm circle, cc13, profile, fast ..............................................24 20 – 6 MeV electrons, 3cm circle, cc13, profile, medium .......................................24 21 – 6 MeV electrons, 3cm circle, cc13, profile, slow ............................................ 24 22 – 6 MeV electrons, 3cm circle, cc01, pdd, fast ...................................................24 23 – 6 MeV electrons, 3cm circle, cc01, pdd, medium............................................ 25 24 – 6 MeV electrons, 3cm circle, cc01, pdd, slow ................................................. 25 25 – 6 MeV electrons, 3cm circle, cc04, pdd, fast ...................................................25 26 – 6 MeV electrons, 3cm circle, cc04, pdd, medium............................................ 25 27 – 6 MeV electrons, 3cm circle, cc04, pdd, slow ................................................. 25 LIST OF TABLES (Continued) Table Page 28 – 6 MeV electrons, 3cm circle, cc13, pdd, fast ...................................................26 29 – 6 MeV electrons, 3cm circle, cc13, pdd, medium............................................ 26 30 – 6 MeV electrons, 3cm circle, cc13, pdd, slow ................................................. 26 31 – 6 MeV electrons, 25x25 cm2, cc01, profile, fast .............................................. 28 32 – 6 MeV electrons, 25x25 cm2, cc01, profile, medium ...................................... 28 33 – 6 MeV electrons, 25x25 cm2, cc01, profile, slow ............................................ 29 34 – 6 MeV electrons, 25x25 cm2, cc04, profile, fast .............................................. 29 35 – 6 MeV electrons, 25x25 cm2, cc04, profile, medium ...................................... 29 36 – 6 MeV electrons, 25x25 cm2, cc04, profile, slow ............................................ 29 37 – 6 MeV electrons, 25x25 cm2, cc13, profile, fast .............................................. 30 38 – 6 MeV electrons, 25x25 cm2, cc13, profile, medium ...................................... 30 39 – 6 MeV electrons, 25x25 cm2, cc13, profile, slow ............................................ 30 40 – 6 MeV electrons, 25x25 cm2, cc01, pdd, fast .................................................. 30 41 – 6 MeV electrons, 25x25 cm2, cc01, pdd, medium ........................................... 31 42 – 6 MeV electrons, 25x25 cm2, cc01, pdd, slow................................................. 31 43 – 6 MeV electrons, 25x25 cm2, cc04, pdd, fast .................................................. 31 44 – 6 MeV electrons, 25x25 cm2, cc04, pdd, medium ........................................... 31 45 – 6 MeV electrons, 25x25 cm2, cc04, pdd, slow................................................. 31 46 – 6 MeV electrons, 25x25 cm2, cc13, pdd, fast .................................................. 32 47 – 6 MeV electrons, 25x25 cm2, cc13, pdd, medium ........................................... 32 48 – 6 MeV electrons, 25x25 cm2, cc13, pdd, slow................................................. 32 49 – 6 MeV electrons, 3cm circle, cc01, profile ...................................................... 40 50 – 6 MeV electrons, 3cm circle, cc04, profile ...................................................... 40 51 – 6 MeV electrons, 3cm circle, cc13, profile ...................................................... 40 52 – 6 MeV electrons, 3cm circle, cc01, pdd ........................................................... 40 53 – 6 MeV electrons, 3cm circle, cc04, pdd ........................................................... 41 54 – 6 MeV electrons, 3cm circle, cc13, pdd ........................................................... 41 LIST OF TABLES (Continued) Table Page 55 – 6 MeV electrons, 25x25 cm2, cc01, profile...................................................... 44 56 – 6 MeV electrons, 25x25 cm2, cc04, profile...................................................... 44 57 – 6 MeV electrons, 25x25 cm2, cc13, profile...................................................... 45 58 – 6 MeV electrons, 25x25 cm2, cc01, pdd ..........................................................45 59 – 6 MeV electrons, 25x25 cm2, cc04, pdd ..........................................................45 60 – 6 MeV electrons, 25x25 cm2, cc13, pdd ..........................................................45 61 – Ion Chamber Technical Details........................................................................ 51 62 – Ion Chamber Wall Material Information ......................................................... 51 COMMONLY USED TERMS AND ABBREVIATIONS LINAC: Linear Accelerator RT: Radiation Therapy/Treatment RTPS: Radiation Treatment Planning System PDD: Percent Depth Dose CAX: Central Axis MLC: Multi-Leaf Collimator Farmer: PTW Farmer Chamber model 30013 cc01: IBA Welhoefer Compact Chamber model cc01 (volume .01 cm3) cc04: IBA Welhoefer Compact Chamber model cc04 (volume .04 cm3) cc13: IBA Welhoefer Compact Chamber model cc13 (volume .13 cm3) Introduction Accurate modeling of the multiple poly-energetic beams available for use in clinical external beam radiotherapy is of utmost importance to patient care. The first and foremost need for understanding the beam characteristics is from a safety and efficacy of treatment standpoint. Without this accurate understanding the potential penalties include the potential for death in the patient population from either a lack of curative treatment if undertreated or an unintended overexposure to a beam with a higher output than modeled. Modern radiation therapy (RT) involves a complex network of interconnected systems to allow for all steps of the treatment planning process from diagnostic imaging to actual treatment on a clinical Linear Accelerator (LINAC, see Figure 1). An important key in the middle of this process is the radiation therapy treatment planning system (RTPS). Today’s RT treatments are increasingly complex and are rarely anymore done by hand. However, whether performed by hand or a RTPS, the calculations rely on data gathered either during the initial machine commissioning or during an additional RTPS commissioning session. Figure 1 – Varian Trilogy LINAC 2 Capturing commissioning data has traditionally been a time intensive process that utilized large square water phantoms (see Figure 2) connected to computer control equipment that moves and tracks the location in 3 dimensions of ionization chambers (see Figure 3). The ion chambers used for scanning have continued to shrink in size, and improve accuracy over time. However, as noted by many authors, with the number of treatments where field sizes are being reduced to well below the traditional small 4x4 cm2 field and into the sub centimeter dimensions, one of the main dosimetric challenges is the availability of even smaller, minimally perturbing detectors appropriate for measuring these newer very small fields. (Sauer & Wilbert, 2007) (Das, Ding, & Ahnesjo, 2007) (Klein, Tailor, Archambault, Wang, Therriault-Proulx, & Beddar, 2010) These smaller chambers for use in measuring very small fields hold the potential to provide more accurate readings of the penumbra and other high dose gradient regions. Figure 2 – IBA Welhoefer Blue Phantom Scanning System 3 Figure 3 – LINAC Console with Scanning System In addition to the potential for higher accuracy from smaller detectors, the newer 3-d water scanning systems also allow for customizable detector motion speeds for different scan types. Increasing detector speed will save time during the data acquisition, but there has been no study on how the change in detector speed affects the data collected. The biggest benefits of shortened scanning time are the ability to get the LINAC treating patients sooner, and saving the organization the costs associated with additional physics commissioning time. This thesis will attempt to address these two questions; 1 - How will changing the speed of the chambers affect their relative readings? 2 - How will changing the chamber size affect the readings of a range of fields sizes from .5x.5 cm2 to 40x40 cm2? 4 Background Many of the separate items here could each be written on at length, and could easily deserve a complete, independent study. I have attempted to summarize a relevant history and working functional understanding of each topic as necessary for the express purpose of having a framework for the reader of this study to place the results into. None of these backgrounds should be considered all encompassing. Radiation Therapy with Linear Accelerators Medical Linear Accelerators A brief history The modern medical LINAC has had a long history that started with the first rf powered linear accelerator build by Wideroe in 1928. However, it wasn’t until the advent of the Klystron in the summer of 1937 and introduced in 1939 (Ginzton, 2010) that would allow for the construction of the first generation of modern medical LINACs. The first LINAC used to treat a patient worldwide was London in 1953. The first use in the USA was at Stanford in 1956. Since these first relatively simple single low energy photon (4-8MV) machines were developed, accelerators have gone through a rapid development in the past ~60 years. Today’s LINACs can produce multiple low and high energy electron beams along with a selection of typically five or six available electron beam energies. These machines also possess the ability to highly modulate the radiation beam with on-board computer controlled shaping equipment known as multi-leaf collimators. How they work The LINAC used for final data set capture was a Varian Trilogy unit installed in the fall of 2008. The following discussion of how LINACs work will be constrained to this and other similar units available from a few different vendors worldwide. Each vendor has a slightly different method of creating their desired beams, but this background will not attempt to tease out all of individual details. One should consult vendor documentation for machine operations specifics. 5 Podgorsak et al classifies the main beam forming components of the modern medical LINAC into six classes: (i) Injection system; (ii) Radiofrequency (RF) power generation system; (iii) Accelerating waveguide; (iv) Source beam transport system; (v) Clinical beam production, collimation, and monitoring system; (vi) Auxiliary system. (Podgorsak, Metcalf, & Van Dyk, 1999) In addition to the beam forming components there are several machine structure and control components. The machine structure is composed primarily of the gantry stand that houses the bulk of the equipment necessary, a gantry which will rotate around a patient 360, a patient support table that may move in up to six different directions; angle, in/out, left/right, up/down, tilt, and roll. (see Figure 1) Finally, the majority of control components are housed at the treatment console. (see Figure 2) The injection system is the source of electrons for the accelerator and consists of an electrostatic accelerator known as an electron gun and its associated control systems. The electron gun may be of the diode or triode type, which consists of a heated cathode, a perforated grounded anode, and in the case of the triode electron gun an additional grid. In the triode electron gun, used in the Varian Trilogy, “the cathode is held at a static negative potential (typically -20 kV). The grid of the triode gun is normally held sufficiently negative with respect to the cathode to cut off the current to the anode. The injection of electrons into the accelerating waveguide is then controlled by voltage pulses, which are applies to the grid and must be synchronized with the pulses applied to the microwave generator.” (Podgorsak, Metcalf, & Van Dyk, 1999) The RF power generation system is comprised of three main components. First is the RF power source, either a magnetron or a klystron and RF driver. The Varian Trilogy unit uses a klystron, which is a RF power amplifier that amplifies the RF signal created by the RF driver. Second is a pulsed modulator. The modulator creates the high voltage (~100 kV), high current (~100 A), short duration (~1s) synchronized pulses needed by both the RF power source and electron gun. The third and final component of the RF power system is the RF 6 power transmission waveguides. The RF transmission waveguides transport the RF from the RF generator to the accelerating waveguide. These waveguides are typically either evacuated to a near vacuum or pressurized with a dielectric gas. The accelerating waveguide, as used in a LINAC, is an evolution of a basic cylindrical waveguide. In its most generic form the LINAC accelerating waveguide is a cylindrical waveguide with a series of discs spaced equidistant along the cylinder dividing the cylinder into cavities. Each disc has a hole in its center aligned on the central axis of the cylinder and in-line with the pencil beam created by the electron gun, which is coupled to one end of the accelerating waveguide. Once the waveguide has either been evacuated to near vacuum or filled with a dielectric gas the electrons can be accelerated using the high power RF waves. It should be noted that there are two main types of LINAC accelerating waveguides, the traveling wave structure and the standing wave structure, the Varian Trilogy used for data capture utilizes a standing wave guide. Once the electron pencil beam has been accelerated it must strike a target and/or other shaping devices to become a clinically useful beam. On many low energy machines the target is attached directly to the accelerating waveguide. However, in the case of the Varian Trilogy and all other high energy machines, the electron pencil beam must go through the electron beam transport system. This transport system is a complicated, interconnected group of components that include, high voltage power supplies, electro-magnets, coils, and drift tubes. This system must bend, steer, and focus the source beam into the LINAC head where the actual clinical beam “production” takes place. The final piece of the beam transport system is the window through which the electron beam must pass. The window is typically made of Beryllium, which with its low atomic number Z, minimizes the pencil beam scattering and bremstrahlung photon production. As noted earlier, modern medical LINACs operate with multiple photon and electron beam energies available for use in treatment. These multiple modalities and energies create a special problem of each needing its own target and filter, or 7 foil to pass through to created the desired clinical beam. LINAC designers have addressed this by creating moveable, typically pneumatically driven, targets, filters, and foils. To create a clinical photon beam, the source electron beam passes through a target, a tungsten alloy in the Varian Trilogy, then a flattening filter, which results in a sufficiently large, flat, and symmetrical beam for clinical use. For clinical electron beams, the target and flattening filter are retracted and a scattering foil is driven into place to again create a sufficiently large, flat, and symmetrical beam of the appropriate energy and modality to be used. At this point the clinical beam is shaped to its maximum size by a fixed primary collimator and is monitored by multiple internal ionization chambers. Next, it continues on to multiple levels of beam shape modifiers. The modifiers that a beam may pass through are specific to its modality. For a clinical photon beam the general levels of beam shape modifiers, in order of passage by the beam, are: (i) upper and lower collimator jaws; (ii) multi-leaf collimator; (iii) physical wedge; (iv) physical field block; (v) physical compensating filter. For a clinical electron beam the general levels of beam shape modifiers, in order of passage by the beam, are: (i) upper and lower collimator jaws; (ii) physical electron cone; (iii) physical field block; (iv) physical compensating filter. The auxiliary systems attached to the LINAC provide critically important resources and actions necessary to making a LINAC work. Some of these items have been referenced above. There are many electric motors and tracking systems for providing motion to the table, gantry, collimator, jaws, and mlc. Also needed is a constant source of air pressure for driving targets, filters, and flattening foils. The transport and accelerating waveguides also require either a source of dielectric gas or a vacuum pump to provide the necessary conditions within the guides. Finally, a water cooling system is needed to remove the massive amounts of heat being created in parts of the machine such as accelerating waveguide and target. A final detail to the understanding of how a LINAC works is how the machine is calibrated in a clinical setting. Generally, the internal ion chambers 8 used the machine for measuring the beams are sealed and therefore not temperature and pressure dependant. This allows the machine to provide a consistent daily output. The internally measured output of each beam is displayed in monitor units. A LINAC is then calibrated such that a known number of monitor units will deliver a known dose to given point in a homogeneous medium under certain setup conditions. For example, a 6MV photon beam may be calibrated so that 100 monitor units delivered through a 10x10 cm2 field size to a point 1.6 cm deep in a water phantom that is set to a measured distance of 100cm will result in the measured dose of 100 cGy. The output for each beam must be calibrated in a known set of similar conditions. Treatment Planning A course of treatment It’s helpful to understand the general sequence of events in today’s planning process, so here is a brief summary that begins after diagnosis and oncologist consultations. A patient must first be imaged by CT and any additional modalities needed to identify tumor(s). These study sets are transferred to a radiotherapy treatment planning system (RTPS) where a dosimetrist will outline the patient’s critical structures, and a physician will outline the tumor(s) to be targeted. Next the physician prescribes a total dose to be delivered to the target volume(s) in a given number of treatment fractions, along with any additional special details specific to this course of treatment. The dosimetrist then, utilizing a RTPS, develops the best way to deliver the prescribed dose to the target(s), while sparing critical structures. Upon completion and review by the physician and physicist, the treatment plan is considered ready for patient treatment. The patient will then come in the prescribed number of days, where each day they will be precisely positioned and treated with that day’s planned fraction. A brief history Treatment planning for external beam radiation therapy has evolved even more than the LINACS used to deliver the therapy of the past 60 years. There is 9 one fundamental reason for this evolution, and that is to provide better patient care and outcome. However, there are many supporting new/improved technologies and expansions in knowledge that make this evolution in treatment planning and improvement in treatment efficacy possible. In the early days of treatment planning, all planning was performed in 2D. This relied upon a pair of orthogonal x-ray films and physical clinical measurements to provide the necessary information needed to perform the calculations that determined the necessary number of monitor units needed for each beam to deliver the desired dose to a prescribed point. These calculations were performed by hand with the assistance of calculators. The next major leap in treatment planning would not take place until minicomputers allowed for CT scanners to become available for medical use and greatly expanding processing capabilities. With these new computing and imaging capabilities came the development of 3D RTP systems and 3D conformal therapy. 3D conformal therapy is just like 2D in that the goal is to hit a target while minimizing harm to critical structures. However, now these RTP systems are capable of prescribing, computing, and visualizing dose to a volume instead of a single point. This method and these capabilities allow for much more complex field shapes and overall plans After 3D conformal planning came Intensity Modulated Radiation Therapy (IMRT). IMRT utilized the ever increasing processing power of minicomputers and a piece of new machine hardware known as the multi-leaf collimator (MLC). The MLC is two opposing banks of “leaves”. The individual leaves are anywhere from .25cm to 1cm wide and each bank is anywhere from 40 to 80 leaves. IMRT plans still prescribe dose to a volume, just as 3D conformal, but now the planning systems will create a series of segments which together make up a single beam. Each segment of the beam has a different set of MLC leaves open to a different pattern, creating final field where the intensity of the dose has been modulated over the field to maximize target dose while minimizing dose to critical structures. To 10 create very highly modulated fields, the RTP system must use a large number of small to very small segments. As noted in the introduction, this increasing reliance upon small fields has largely driven the development of small measurement chambers. A few notes on other advances in planning and treatment. There are of course on-going advances in planning and therapy, such as image guided radiation therapy (IGRT) for more precise patient positioning and 4D planning/treatment that will track and adjust beam on time to patient breathing cycle. Both of these build on IMRT as an underlying principle of many small segments. Stereotactic radiosurgery (SRS) is another form of advanced external beam radiation therapy that also relies upon very small fields for treatment. SRS mainly differs from traditional external beam therapy based on its exclusive use of small fields and a much higher dose per fraction with a smaller number of fractions. SRS may be delivered with some the same machines that can deliver traditional external beam or it may be delivered with a specialty machine. I have not attempted to cover the history or workings of dedicated SRS machines, but the following research is relevant due their use of similar energies and field sizes. Beam Measurement Ionization Chambers As stated by Knoll, Ion chambers in principle are the simplest of all gasfilled detectors. Their normal operation is based on collection of all the charges created by direct ionization within the gas through the application of an electric field. (Knoll, 2000) As simple as ion chambers may be, there have been entire texts written them. In their most basic construction there are two conductive materials with a gas between them and a constant dc voltage is applied. Once a charged particle enters the chamber and interacts with a neutral molecule an ion pair is formed. Ideally this freed electron is attracted to the positive electrode and counted in the form of current. However, this electron may recombine, interact again, or scatter entirely out of the detector. 11 Ion chambers used for beam measurements in radiation therapy have traditionally been either of thimble or parallel plate constructions and open to the atmosphere. This unsealed or vented construction means that the fill gas is not a specific mixture, but rather just air, and the chamber readings must be corrected for temperature and pressure variation. The chambers have needed to be robust and respond consistently to a variety of poly-energetic beams from 4 to 25 megavolts. By far the most common chamber traditionally used in radiation therapy is the Farmer chamber. (see Figure 4) In 1955 a chamber was designed to provide a stable and reliable secondary standard for x-rays and γ rays for all energies in the therapeutic range. This chamber connected to a specific electrometer (to measure ionization charge) and is known as the Baldwin-Farmer substandard dosimeter. (Khan, 2010) The chamber came to be traditionally known as the Farmer chamber. Later in 1972 the chamber was modified. By substituting pure graphite and pure aluminum for the existing chamber materials the response curve over the therapeutic range was flattened and made more consistent from one instrument to another. (Aird & Farmer, 1972) Today’s Farmer chambers are minor evolutions of the 1972 improvements. Figure 4 – PTW 30013 Farmer Chamber 12 Compact chambers have arisen out of a need to measure more accurately small fields and high dose gradient regions of beams. These chambers are also of the thimble design, vented, and may or may not have the same wall and electrode materials. The biggest difference is the physical dimensions of the chamber. The Farmer chamber cavity is 23.1mm long and 6.1mm in diameter. Currently available compact chamber cavities vary from 3 to 6mm long and 1 to 3mm in diameter. (see Figure 5) Figure 5 – IBA Welhoefer cc01 Compact Chamber Using air filled chambers with walls made of air-equivalent material also allows for dose to be computed and measured based on the Bragg-Gray principle. Bragg-Gray states that the absorbed dose in a given material can be deduced from the ionization produced in a small gas-filled cavity within that material. (Knoll, 2000) This same principle allows for the computation of dose in water when using the same chambers in a water phantom. Electrometers Defined by Knoll, an electrometer indirectly measures the current by sensing the voltage drop across a series resistance placed in the measuring circuit. (Knoll, 2000) In a basic sense, and without getting into electrical circuits, the 13 electrometer performs two important functions. One, the electrometer provides a consistent and stable 300 volts DC to the ion chamber. Two the electrometer is able to measure the accumulated charge while the chamber is being exposed to the beam. 3D Beam Scanning Systems The ability to accurately measure and visualize what was happening in 3D with a clinical beam is another problem that has been greatly alleviated with advancements in computing technology. It’s also a subject whose history has not been well documented in the literature. 3D beam scanning systems utilize large water tanks, and a collection of motors, sensors, ion chambers, electrometers, and computer control equipment. These systems allow for an ion chamber to be moved and tracked in 3D through a field while keeping track of the relative dose readings along the prescribed course of motion. The most common scans performed are percent depth dose (PDD) scans and profile scans. PDD scans drive the ion chamber to a given depth along the central axis (CAX), then measures the relative dose along the CAX from that depth to the surface. A profile scan measures the beam at a given depth along the in-plane or cross-plane direction from a few cm outside one field edge to a few cm beyond the opposite field edge. 14 Equipment Used and Data Collection Conditions Equipment/Parameters LINAC: Varian Trilogy 3D scanning system: IBA Wellhofer Blue Phantom with CU500E controller Ion Chambers: PTW Type 30013 Farmer; IBA Wellhofer cc13, cc04, and cc01 (see Appendix 1 for technical specs) Electrometer: Fluke Biomedical Systems Model 35040 LINAC rep rate: 400MU/min SSD to water for scans: 100 cm SSD to water for field size dependence: 90 cm Depth for profile scans: Dmax for energy chosen (dmax is the point of maximum dose deposition) Depth for field size dependence: 10 cm Data Collection Conditions Due to the large amount of data collected, it was impossible to collect all data in one session. Each set of scans to be inter-compared were captured back to back in the same data collection session. Before each session the water tank was leveled and centered. A centering routine was run within the scanning software to verify centering for each energy before capturing the corresponding data set. All scans were normalized to the given energy’s established dmax. No data is intercompared that was collected during different collection sessions. For all scans the detector was horizontally placed with the cable pointing towards the gantry. All percent depth dose (pdd) scans were captured from some depth moving towards the surface. All profile scans compared are cross-plane scans. The cross-plane direction sweeps the chamber in the left/right axis. For each profile scan the direction of chamber motion was reversed. There were no changes made to the chamber mounting or orientation during scanning. Three scans or measurements were made with each detector at each speed for all of the following samples. As necessary for inter-chamber or inter-speed comparison, 15 the three measurements were averaged, and each corresponding result is then used for these comparisons. The detector speed chosen for inter-chamber comparison was slow speed for small fields, and medium speed for all others. Data Analysis Legend For the purposes of data analysis we must define small, medium, and large field sizes: Small fields < 3x3 cm2 <Medium fields < 20x20 cm2 < Large fields. When comparing speed changes on graphs for a given chamber, fast scans are represented red, medium scans green and slow scans blue. When comparing chamber changes on graphs, the cc13 chamber is represented in red, the cc04 chamber in green, and the cc01 chamber in blue. Additional Notes on Data Collection The scans chosen to be presented here are the result of analysis of many data sets collected by many physicists on many different machines. The number of combinations of energies and field sizes available for treatment is very large, and it’s simply not feasible to compare all energies and field sizes in one study. An attempt has been made to create reasonable sample for discussion of established questions. 16 Data Presentation and Analysis Detector Speed Change As stated in the introduction, collecting data with a 3D scanning system can be a very time consuming process. LINACs and RTP systems are large capital expenditures made by the organizations that do no good sitting idle. Whether it is a new installation or verifying a machine after major repair, it is important to both patients and administrators that the LINAC be brought to a clinical status in the most efficient and accurate way possible. The ability to change the detector motion is not entirely new for scanning systems, but it has been made much easier to manipulate in the more recent versions. The IBA Wellhofer Blue Phantom scanning system used for these scans offered three pre-defined speeds (slow, medium, and fast) for detector motion during scans. The scanning software is programmed to attempt a minimum number of counts relative to its speed of motion. The result is that the speed of detector motion is relative to the rep rate of the LINAC, i.e. the slow speed with the LINAC running at 100MU/min is slower than the slow speed with the LINAC running at 400MU/min and the size of the chamber being used if there is a large enough difference in collection volume. The most common rep rates used in clinical treatment are 300MU/min and 600MU/min, which led the choice of 400 MU/min for this data set. With the LINAC running at 400 MU/min the variable speed results were: slow = .17cm/sec; medium = .51cm/sec; fast = 1.36cm/sec. Small fields – Data The small field chosen to sample for comparing speed change with different chambers was a 6MV photon field of size .5x.5cm2. The IBA Welhoefer cc13 was excluded from the comparison due to its size relative to the field size. IBA Welhoefer - cc01 Figures 17 thru 24 in the appendix show the graphs and statistics for all cc01 scans for this field size. Figures 25 thru 32 show the graphs and statistics for all cc04 scans for this field size 17 Small fields - Analysis Regardless of which compact ion chamber was used the analytical results are the same. Tables 1 thru 13 show the cc01 and cc04 data statistics for profile and pdd scans. There was little difference in the spread of results, and unexpectedly for profiles, the slow speed scans did not necessarily give the tightest statistical data spread. However, the medium and slow speed scans do have the appearance of being smoother overall. With that being said there are some differences between pdds and profiles. Table 1 – 6 MV Photons, .5x.5cm2, cc01, profile, fast Chamber: 2 .5x.5 cm Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc01 Speed: fast Scan 1 Scan 2 Scan 3 Spread 2.0 3.1 2.4 1.1 21.8 21.7 23.0 1.3 .21:.22 .22:.22 .21:.21 0.01 0.46 0.47 0.46 0.01 Table 2 – 6 MV Photons, .5x.5cm2, cc01, profile, medium Chamber: .5x.5 cm2 Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc01 Speed: med Scan 1 Scan 2 Scan 3 Spread 5.1 2.3 2.1 2.9 22.2 22.2 22.1 0.1 .21:.20 .21:.21 .22:.21 0.01 0.45 0.45 0.45 0 Table 3 – 6 MV Photons, .5x.5cm2, cc01, profile, slow Chamber: 2 .5x.5 cm Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc01 Speed: slow Scan 1 Scan 2 Scan 3 Spread 2.0 3.8 2.3 1.8 22.1 22.6 22.0 0.6 .19:.20 .20:.20 .21:.20 0.02 0.46 0.45 0.45 0.01 18 Table 4 – 6 MV Photons, .5x.5cm2, cc01, profile, fast Chamber: 2 .5x.5 cm Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc04 Speed: fast Scan 1 Scan 2 Scan 3 Spread 2.3 1.7 2.2 0.6 22.0 22.8 22.4 0.8 .27:.26 .27:.27 .28:.27 0.01 0.56 0.56 0.55 0.01 Table 5 – 6 MV Photons, .5x.5cm2, cc01, profile, medium Chamber: 2 .5x.5 cm Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc04 Speed: med Scan 1 Scan 2 Scan 3 Spread 2.7 2.0 2.4 1.7 22.9 21.0 22.5 1.9 .27:.26 .26:.26 .27:.26 0.01 0.55 0.55 0.56 0.01 Table 6 – 6 MV Photons, .5x.5cm2, cc01, profile, slow Chamber: 2 .5x.5 cm Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc04 Speed: slow Scan 1 Scan 2 Scan 3 Spread 2.9 2.2 3.2 1.0 21.8 22.9 21.9 1.1 .26:.26 .26:.25 .26:.26 0.01 0.54 0.56 0.54 0.02 Table 7 – 6 MV Photons, .5x.5cm2, cc01, pdd, fast Chamber: 2 .5x.5 cm R100 (dmax,cm): D100 (dose at 10cm,%): cc01 Scan 1 1.06 55.9 Scan 2 1.23 55 Speed: fast Scan 3 1.06 54.1 Spread 0.17 1.8 19 Table 8 – 6 MV Photons, .5x.5cm2, cc01, pdd, medium Chamber: 2 .5x.5 cm R100 (dmax,cm): D100 (dose at 10cm,%): cc01 test Speed: med Scan 1 1.07 53.8 Scan 2 1.15 54 Scan 3 1.07 53.2 Spread 0.08 0.8 Table 9 – 6 MV Photons, .5x.5cm2, cc01, pdd, slow Chamber: 2 .5x.5 cm R100 (dmax,cm): D100 (dose at 10cm,%): cc01 Scan 1 1.05 53.9 Scan 2 0.95 54.1 Speed: slow Scan 3 1.04 53.9 Spread 0.1 0.2 Table 10 – 6 MV Photons, .5x.5cm2, cc01, pdd, fast Chamber: cc04 .5x.5 cm2 R100 (dmax,cm): D100 (dose at 10cm,%): Scan 1 1.2 56.3 Scan 2 1.13 57.2 Speed: fast Scan 3 1.25 56.7 Spread 0.12 0.9 Table 11 – 6 MV Photons, .5x.5cm2, cc01, pdd, medium Chamber: cc04 .5x.5 cm2 R100 (dmax,cm): D100 (dose at 10cm,%): Scan 1 1.03 57.6 Scan 2 1.14 56.3 Speed: med Scan 3 1.09 56.3 Spread 0.11 1.3 Table 12 – 6 MV Photons, .5x.5cm2, cc01, pdd, slow Chamber: 2 .5x.5 cm R100 (dmax,cm): D100 (dose at 10cm,%): cc04 Scan 1 1.18 56.7 Scan 2 1.11 56.1 Speed: slow Scan 3 1.05 56.4 Spread 0.13 0.6 20 For profiles, there is not a question that the slow speed scans for both chambers appear to give the best results. Figure 6 shows the cc01 profile scans for all speeds. In this graph the smoother shape of the slower speed scans can plainly be seen. There is also very small time penalty to be paid for scanning small field profiles at slow speed, as the width of scans is typically only the field width + a 1-3 cm margin to capture the penumbra. To scan one cross-plane profile of a .5x.5cm2 field, total width with margin of 3x3cm2, at slow speed takes 18 seconds, while at medium speed takes 6 seconds. If the margin is shrunk to a total field width of 2x2cm2, slow speed takes 12 seconds, and medium speed takes 4 seconds. By optimizing the total profile scan width the time penalty can also be made as minimal as possible. Figure 6 – 6 MV Photons, .5x.5cm2, cc01, profile, all speeds However, for pdds, the appearance of the slow speed scans is very noisy in its raw, unsmoothed form, but still gave the overall best data results. The medium speed scans also provided very consistent statistical results, and the time savings is fairly significant. Figure 7 shows the cc01 profile scans for all speeds. Photon pdd 21 scans are typically made from a depth of 30cm moving the detector towards the surface regardless of field size. A slow speed photon pdd scan for any field size, therefore takes 2 minutes and 56 seconds. At medium speed the same pdd scan takes 59 seconds, a savings of nearly 2 minutes. Figure 7 – 6 MV Photons, .5x.5cm2, cc01, pdd, speeds Small fields - Recommendation Run profiles at slow speed and pdds at medium speed. Medium fields – Data The medium field chosen to sample for comparing speed change with different chambers was a circular 6MeV electron field of diameter 3cm. An electron field was chosen due to the fact that ion chambers are generally more sensitive to perturbation effects in electron fields than in photon fields. As a general rule a 3 cm circle or square is the smallest electron field scanned and modeled for treatment. Smaller electron fields may be used, but special dosimetric measurements in water must be performed on the fields before they can be computed for dose to a patient. 22 Figures 33 thru 40 in the appendix show the graphs and statistics for all cc01 scans for the 6 MeV electrons, 3cm field. Figures 41 thru 48 show the graphs and statistics for all cc04 scans for the 6 MeV electrons, 3cm field. Figures 49 thru 56 show the graphs and statistics for all cc13 scans for the 6 MeV electrons, 3cm field. Figure 57 in the appendix shows the 6 MeV electrons, 10x10 cm2, all chambers, pdd. Medium fields – Analysis As with the small fields, regardless of which compact ion chamber was used the analytical results are the same. Tables 13 thru 30 show the cc01, cc04, and cc13 data statistics for both profile and pdd scans. The medium field analysis does break down to some small differences based on beam modality. Table 13 – 6 MeV electrons, 3cm circle, cc01, profile, fast Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc01 Speed: Scan 1 Scan 2 Scan 3 1.8 2.1 1.8 19.4 19.1 19.5 1.17:1.10 1.16:1.11 1.14:1.11 3.23 3.26 3.26 fast Spread 0.3 0.4 0.03 0.03 Table 14 – 6 MeV electrons, 3cm circle, cc01, profile, medium Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc01 Speed: Scan 1 Scan 2 Scan 3 1.7 2.3 2.1 20.0 19.7 19.3 1.25:1.18 1.25:1.17 1.24:1.18 3.26 3.25 3.26 med Spread 0.6 0.7 0.01 0.01 23 Table 15 – 6 MeV electrons, 3cm circle, cc01, profile, slow Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc01 Speed: Scan 1 Scan 2 Scan 3 2.0 2.0 2.1 19.0 19.5 18.9 1.20:1.11 1.17:1.10 1.16:1.09 3.22 3.23 3.24 slow Spread 0.1 0.6 0.04 0.02 Table 16 – 6 MeV electrons, 3cm circle, cc04, profile, fast Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc04 Speed: Scan 1 Scan 2 Scan 3 1.8 1.3 1.1 20.2 19.8 19.3 1.26:1.24 1.27:1.22 1.26:1.21 3.25 3.27 3.26 fast Spread 0.7 0.9 0.03 0.02 Table 17 – 6 MeV electrons, 3cm circle, cc04, profile, medium Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc04 Speed: Scan 1 Scan 2 Scan 3 1.3 1.8 1.2 19.9 20.5 19.9 1.27:1.22 1.28:1.23 1.29:1.24 3.26 3.26 3.25 med Spread 0.6 0.6 0.02 0.01 Table 18 – 6 MeV electrons, 3cm circle, cc04, profile, slow Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc04 Speed: Scan 1 Scan 2 Scan 3 1.1 0.9 1.4 19.9 20.2 19.6 1.26:1.21 1.26:1.22 1.27:1.20 3.27 3.27 3.26 slow Spread 0.5 0.6 0.01 0.01 24 Table 19 – 6 MeV electrons, 3cm circle, cc13, profile, fast Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc13 Speed: Scan 1 Scan 2 Scan 3 1.7 1.1 1.9 19.5 19.4 19.6 1.19:1.13 1.19:1.14 1.20:1.13 3.22 3.23 3.22 fast Spread 0.8 0.2 0.01 0.01 Table 20 – 6 MeV electrons, 3cm circle, cc13, profile, medium Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc13 Speed: Scan 1 Scan 2 Scan 3 2.1 1.6 1.9 19.2 19.4 19.6 1.20:1.13 1.20:1.15 1.20:1.14 3.23 3.24 3.23 med Spread 0.5 0.4 0.02 0.01 Table 21 – 6 MeV electrons, 3cm circle, cc13, profile, slow Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc13 Speed: Scan 1 Scan 2 Scan 3 1.4 1.4 1.7 19.4 19.3 19.7 1.21:1.15 1.21:1.14 1.22:1.15 3.23 3.23 3.22 slow Spread 0.3 0.4 0.01 0.01 Table 22 – 6 MeV electrons, 3cm circle, cc01, pdd, fast Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc01 Scan 1 1.20 2.29 Scan 2 1.21 2.29 Speed: Scan 3 1.22 2.30 fast Spread 0.02 0.01 25 Table 23 – 6 MeV electrons, 3cm circle, cc01, pdd, medium Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc01 Scan 1 1.09 2.30 Scan 2 1.10 2.29 Speed: Scan 3 1.11 2.30 med Spread 0.02 0.01 Table 24 – 6 MeV electrons, 3cm circle, cc01, pdd, slow Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc01 Scan 1 1.12 2.29 Scan 2 1.07 2.29 Speed: Scan 3 1.08 2.29 slow Spread 0.04 0 Table 25 – 6 MeV electrons, 3cm circle, cc04, pdd, fast Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc04 Scan 1 1.08 2.29 Scan 2 1.05 2.28 Speed: Scan 3 1.1 2.27 fast Spread 0.05 0.02 Table 26 – 6 MeV electrons, 3cm circle, cc04, pdd, medium Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc04 Scan 1 0.94 2.28 Scan 2 1.12 2.28 Speed: Scan 3 1.02 2.26 med Spread 0.16 0.02 Table 27 – 6 MeV electrons, 3cm circle, cc04, pdd, slow Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc04 Scan 1 1.11 2.27 Scan 2 1.04 2.29 Speed: Scan 3 1.09 2.28 slow Spread 0.07 0.02 26 Table 28 – 6 MeV electrons, 3cm circle, cc13, pdd, fast Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc13 Scan 1 1.05 2.27 Scan 2 0.98 2.26 Speed: Scan 3 1.2 2.27 fast Spread 0.22 0.01 Table 29 – 6 MeV electrons, 3cm circle, cc13, pdd, medium Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc13 Scan 1 0.98 2.28 Scan 2 1.12 2.29 Speed: Scan 3 1.06 2.28 med Spread 0.14 0.01 Table 30 – 6 MeV electrons, 3cm circle, cc13, pdd, slow Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc13 Scan 1 1.08 2.29 Scan 2 0.99 2.28 Speed: Scan 3 1.12 2.28 slow Spread 0.13 0.01 Unexpectedly, the cc01 and cc04 medium speed electron pdd scans show two odd humps in the graph, as seen on figures 34 and 42 in the appendix, at 2.5 cm deep and 2.7 cm deep. The cc13 scan, figure 50, does not show the same humps, probably due to the volume averaging effects of the larger chamber. The humps do not appear as pronounced on the high speed scans, but this can be attributed to the stretching/smoothing effect on the pdd of running the chamber at high speed. Only the slow speed pdd scans appear to capture enough data to fully represent these beam scans accurately. The time penalty for slow speed electron pdd scans is energy dependant. Unlike photons, the depth of the pdd scan must increase in proportion to the electron energy. As a general rule, the electron pdd is 10% when the depth is equal to half of the energy. Therefore to capture a full electron pdd the total depth must 27 equal half the energy, in cm, plus 1-3 cm. So, for 6 MeV electrons the total pdd depth is typically 4-5 cm, but for 21e, the highest available electron energy, the total pdd depth is typically 12-13 cm. At this highest available electron energy, the slow speed scan time is 1 minute and 16 seconds, assuming the total depth of 13 cm. A medium speed scan takes 26 seconds, and a fast speed scan takes 10 seconds. This time penalty is not insignificant, but necessary to obtain the necessary quality of scans. Electron profiles did not differ much as a function of detector speed change. The slow speed scans generally produced the tightest groups of data statistics, with the medium and fast speed each close behind. Electron profiles must also include a margin to capture the penumbra just as with photon profiles, therefore also have a field size dependent time penalty. The medium speed appears to be a good compromise of time and repeatability of data. For photon fields, it was already established with the small field analysis that photon pdds can be run at medium speed. There were not a selection of medium size photon field profiles run, but instead we can infer the best speed based on the electron analysis. Since the perturbation effect is generally accepted to be smaller for photon fields, it’s reasonable to conclude that the photon pdds can also be run at medium speed to again, reach a good compromise of time and repeatability of data. Medium fields – Recommendation Electron pdds should be run at slow speed. Photon pdds should be run at medium speed. Electron and photon profiles should be run at medium speed. Large Fields – Data For examining the speed change on large fields, as 6 MeV electrons 25x25cm2 field was chosen. Figures 58 thru 65 in the appendix show the graphs and statistics for all cc01 scans for this field size. Figures 66 thru 73 show the graphs and statistics for all cc04 scans for this field size. Figures 74 thru 81 show the graphs and statistics for all cc13 scans for this field size. 28 Large Fields – Analysis As with the small and medium fields, regardless of which compact ion chamber was used the analytical results are the same. The profiles and pdds did not differ much as a function of detector speed change. Again, the slow speed scans generally produced the tightest groups of data statistics, with the medium and fast speed scans each giving a slightly wider grouping with the each increase in speed. . Tables 31 thru 48 show the cc01, cc04, and cc13 data statistics for both profile and pdd scans. As the field sizes continue to increase so do the time penalties for using the slow or medium speeds relative to the fast speed. Table 31 – 6 MeV electrons, 25x25 cm2, cc01, profile, fast Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc01 Scan 1 2.1 2.1 1.01:1.00 25.63 Scan 2 2.7 1.8 .99:1.00 25.63 Speed: Scan 3 1.8 1.9 .98:.99 25.63 fast Spread 0.9 0.3 0.03 0 Table 32 – 6 MeV electrons, 25x25 cm2, cc01, profile, medium Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc01 Scan 1 2.2 1.8 1.02:1.01 25.62 Scan 2 3.1 2.3 .99:1.02 25.65 Speed: Scan 3 2.7 2.1 1.00:.99 25.63 med Spread 0.9 0.5 0.03 0.03 29 Table 33 – 6 MeV electrons, 25x25 cm2, cc01, profile, slow Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc01 Scan 1 1.7 1.6 1.00:1.01 25.63 Scan 2 2.0 2.0 .98:1.02 25.63 Speed: Scan 3 2.0 1.7 .99:.99 25.63 slow Spread 0.3 0.4 0.03 0 Table 34 – 6 MeV electrons, 25x25 cm2, cc04, profile, fast Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc04 Speed: Scan 1 Scan 2 Scan 3 1.7 1.9 1.9 1.8 1.9 1.7 1.06:1.08 1.08:1.09 1.07:1.09 25.65 25.63 25.66 fast Spread 0.2 0.2 0.02 0.03 Table 35 – 6 MeV electrons, 25x25 cm2, cc04, profile, medium Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc04 Speed: Scan 1 Scan 2 Scan 3 1.5 1.7 1.8 1.5 1.8 1.6 1.07:1.09 1.08:1.09 1.08:1.07 25.63 25.64 25.64 med Spread 0.3 0.3 0.02 0.01 Table 36 – 6 MeV electrons, 25x25 cm2, cc04, profile, slow Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc04 Speed: Scan 1 Scan 2 Scan 3 2.1 2.6 2.5 1.7 1.9 1.9 1.07:1.08 1.07:1.07 1.08:1.09 25.65 25.65 25.63 slow Spread 0.5 0.2 0.02 0.02 30 Table 37 – 6 MeV electrons, 25x25 cm2, cc13, profile, fast Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc13 Scan 1 1.8 1.6 .95:.95 25.58 Scan 2 2.1 1.4 .94:.94 25.59 Speed: Scan 3 1.8 1.3 .95:.94 25.61 fast Spread 0.3 0.3 0.01 0.03 Table 38 – 6 MeV electrons, 25x25 cm2, cc13, profile, medium Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc13 Scan 1 1.6 1.7 .95:.94 25.59 Speed: Scan 3 1.5 1.5 .93:.95 25.58 Scan 2 1.7 1.5 .93:.94 25.58 med Spread 0.2 0.2 0.02 0.01 Table 39 – 6 MeV electrons, 25x25 cm2, cc13, profile, slow Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc13 Scan 1 1.6 1.4 .93:.95 25.61 Scan 2 1.6 1.5 .93:.94 25.59 Speed: Scan 3 1.7 1.6 .94:.94 25.59 slow Spread 0.1 0.2 0.01 0.02 Table 40 – 6 MeV electrons, 25x25 cm2, cc01, pdd, fast Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc01 Scan 1 1.30 2.27 Scan 2 1.18 2.28 Speed: Scan 3 1.32 2.28 fast Spread 0.14 0.01 31 Table 41 – 6 MeV electrons, 25x25 cm2, cc01, pdd, medium Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc01 Scan 1 1.29 2.27 Scan 2 1.15 2.27 Speed: Scan 3 1.28 2.27 med Spread 0.14 0 Table 42 – 6 MeV electrons, 25x25 cm2, cc01, pdd, slow Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc01 Scan 1 1.29 2.28 Scan 2 1.23 2.27 Speed: Scan 3 1.18 2.28 slow Spread 0.11 0.01 Table 43 – 6 MeV electrons, 25x25 cm2, cc04, pdd, fast Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc04 Scan 1 1.24 2.28 Scan 2 1.31 2.27 Speed: Scan 3 1.27 2.26 fast Spread 0.07 0.02 Table 44 – 6 MeV electrons, 25x25 cm2, cc04, pdd, medium Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc04 Scan 1 1.25 2.26 Scan 2 1.24 2.26 Speed: Scan 3 1.3 2.26 med Spread 0.06 0 Table 45 – 6 MeV electrons, 25x25 cm2, cc04, pdd, slow Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc04 Scan 1 1.22 2.27 Scan 2 1.26 2.27 Speed: Scan 3 1.26 2.27 slow Spread 0.04 0 32 Table 46 – 6 MeV electrons, 25x25 cm2, cc13, pdd, fast Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc13 Scan 1 1.24 2.29 Scan 2 1.34 2.31 Speed: Scan 3 1.31 2.31 fast Spread 0.1 0.02 Table 47 – 6 MeV electrons, 25x25 cm2, cc13, pdd, medium Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc13 Scan 1 1.22 2.32 Scan 2 1.33 2.32 Speed: Scan 3 1.34 2.33 med Spread 0.12 0.01 Table 48 – 6 MeV electrons, 25x25 cm2, cc13, pdd, slow Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc13 Scan 1 1.23 2.33 Scan 2 1.3 2.32 Speed: Scan 3 1.28 2.32 slow Spread 0.07 0.01 Margins for profiles must also increase with field size, which further increases the time penalty for slower speed scans. An example 25x25cm2 field needs a total width of 32x32cm2 to completely capture the needed profile information. A slow speed scan takes 3 minutes and 14 seconds. The same scan at medium speed takes 1 minute and 3 seconds, and a fast scan 24 seconds. Large fields – Recommendation If a LINAC is being spot checked after repair to verify that there have not been major changes, then running scans at high speed is sufficient. However, if collecting data for beam model commissioning, the medium speed is a better choice for the best compromise of time versus quality of data. 33 Physical Detector Change Studying small chamber profile scanning and output measurements is not a new endeavor. There have been numerous studies done and journal articles written on the subjects. (Das, Ding, & Ahnesjo, 2007) (Fox C. , et al., 2010) (Klein, Tailor, Archambault, Wang, Therriault-Proulx, & Beddar, 2010) (Sauer & Wilbert, 2007) (Pappas E. , et al., 2008) (Agostinelli, Garelli, Piergentili, & Foppiano, 2008) (Sahoo, Kazi, & Hoffman, 2008) (Fox C. , Simon, Li, Palta, Liu, & Simon, 2007) (Pappas T. G., et al., 2006) (Dawson, Schroeder, & Hoya, 1985) (Sibata, Mota, Bedder, Higgins, & Shin, 1991) The lessons and recommendations of these studies can be brought together into several relevant points. One, use a detector that is as minimally perturbing as reasonably achievable that has the appropriate response characteristics for the field size being measured. Two, the cc01 and other small diode detectors are known to over-respond to large fields. Three, small fields need as small a detector as possible and the cc01 is good down to a .5x.5cm2 field. Four, the measured penumbra width varies linearly with detector radius. Five, the stem effect is fairly significant for farmer chambers, but not nearly as significant for compact chambers. Six, the near surface response of detectors improves with decreasing measurement volume dimensions. For the following comparisons, three measurements were made with each detector. As necessary when putting all chambers on one graph, the three measurements were averaged, and each corresponding result used for that comparison. Chamber response to field size change – Data As was just noted, the cc01 is known to over-respond to large fields. Agostinelli et al., studied this phenomenon and also established that the overresponse could be eliminated by applying a polarity correction. (Agostinelli, Garelli, Piergentili, & Foppiano, 2008) To examine the response characteristics for each compact chamber a series of field size dependence measurements were made with each chamber and all measurements normalized to the 10x10 cm2 reading of 34 that particular chamber. These were then compared against the traditional Farmer chamber. As the over-response only becomes significant at the largest field sizes, it is only necessary to run these measurements for photon fields. Figure 8 shows the graphs of field size dependence for 6MV photons, and figure 9 shows the same for 18MV photons. Normalized Output 0.345 0.299 0.204 0.067 cc04 cc13 Farmer 0.5 cc01 0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 0.253 0.577 0.638 0.670 1 0.616 0.776 0.782 0.778 2 0.809 0.825 0.827 0.818 3 0.858 0.860 0.862 0.853 4 0.919 0.919 0.920 0.911 6 0.960 0.964 0.964 0.959 8 1.000 1.000 1.000 1.000 10 1.029 1.029 1.030 1.028 12 1.062 1.064 1.065 1.067 15 20 1.106 1.106 1.106 1.115 Figure 8 – Field Size Dependence – 6 MV photons 1.131 1.138 1.135 1.152 25 1.162 1.164 1.160 1.183 30 1.180 1.184 1.177 1.205 35 1.197 1.200 1.189 1.221 40 35 Normalized Output 0.477 0.248 0.178 cc13 Farmer 0.059 0.525 0.246 cc04 0.550 1 0.297 0.5 cc01 0.000 0.200 0.400 0.600 0.800 1.000 1.200 0.599 0.732 0.744 0.757 2 0.813 0.833 0.837 0.835 3 0.877 0.886 0.887 0.884 4 0.942 0.943 0.943 0.940 6 0.970 0.977 0.976 0.975 8 1.000 1.000 1.000 1.000 10 1.018 1.019 1.019 1.019 12 1.037 1.038 1.038 1.041 15 1.061 1.061 1.061 1.067 20 Figure 9 – Field Size Dependence – 18 MV photons 1.075 1.079 1.078 1.087 25 1.094 1.095 1.093 1.106 30 1.101 1.108 1.106 1.120 35 1.117 1.119 1.116 1.132 40 36 37 Chamber response to field size change – Analysis The results of these output readings support previous work done on the subject. The cc01 begins to over-respond at 20x20 cm2 for 6MV photons, and 25x25 cm2 for 18MV photons. As expected based on its size, the Farmer chamber begins to under-respond at 4x4 cm2 for both photon energies. Also of consideration are the responses of the compact chambers. For 6 MV photons the cc01, cc04, and cc13 all agree down to a 2x2 cm2 field size. For 18 MV photons, they only agree to a 3x3 cm2 field size. Below the noted small field sizes only the cc01 gives accurate output readings to field sizes down to .5x.5 cm2. (Sauer & Wilbert, 2007) Small fields – Data Again, a 6 MV photon, .5x.5 cm2 field was chosen for the small field comparison. Figures 82 thru 85 in the appendix show the pdds for each chamber, then all chambers combined on one graph. Figures 86 thru 89 show the same for profiles. Small fields - Analysis The pdd data presented here is strictly for the interest of discussion. As already referenced from other papers and shown with the field size dependence graphs, the only valid chamber for this field size is the cc01. All of the chambers were normalized to the same depth, so that does make the combined pdd graph , figure 10 below and 85 in appendix, a little interesting. Up to the established energy dmax of 1.6 cm the cc01 records a higher relative dose. However, beyond dmax the graph lines cross and both the cc04 and cc13 begin to record more dose. The amount of dose discrepancy increases with depth and is larger for the cc13 than the cc04. The reason for this discrepancy was not study, but for a quick hypothesis, I would suggest that it is caused by the increased scatter component at depth combined with the larger collection volume creating this effect. Further study would be interesting, but not clinically significant as it has already been determined that the cc04 and cc13 are too large for this field size. 38 Figure 10 – 6 MV Photons, .5x.5cm2, cc01(blue)/cc04(green)/cc13(red), pdd With all of the profiles graphed together, as shown in figure 11 below and 89 in the appendix, and normalized to 100% and field center, one can see the field widening effect of the larger volume chambers. Also apparent from the statistics (cc13 = 3 mm, cc04 = 2.6 mm, cc01 = 2 mm) is the penumbra widening effect of the increasing radius of the ion chambers. Again, all of this data merely reinforces studies already cited. 39 Figure 11 – 6 MV Photons, .5x.5cm2, cc01(blue)/cc04(green)/cc13(red), profile Small field – Recommendation For small fields down to the size sampled the cc01 is clearly the most appropriate chamber of those tested for use. Medium field – Data The same 6 MeV, 6x6 cm2 cone with 3 cm diameter circle cutout electron field was used for chamber size change comparison. In addition pdd scans for a 6 MeV electron and 6 MV photon 10x10 cm2 fields are included. Figures 90 thru 93 in the appendix show the pdds for each chamber, then all chambers combined on one graph. Figures 94 thru 96 show the additional 10x10 cm2 pdd scans. Figures 97 thru 100 show the data and statistics for profiles. Medium field – Analysis When analyzed individually there is virtually no statistical difference in the grouping of results. Tables 49 thru 54 show the cc01, cc04, and cc13 data statistics for both profile and pdd scans. However, the raw appearance of the larger chamber graphs at this speed is smoother. This smoother appearance can be attributed to the volumetric smoothing effect of the larger chamber. As noted in the speed change 40 analysis the bumps that appear at depths 2.5 cm and 2.75 cm with the smaller chambers can be avoided by scanning at a slower speed. Table 49 – 6 MeV electrons, 3cm circle, cc01, profile Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc01 Speed: Scan 1 Scan 2 Scan 3 1.7 2.3 2.1 20.0 19.7 19.3 1.25:1.18 1.25:1.17 1.24:1.18 3.26 3.25 3.26 med Spread 0.6 0.7 0.01 0.01 Table 50 – 6 MeV electrons, 3cm circle, cc04, profile Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc04 Speed: Scan 1 Scan 2 Scan 3 1.3 1.8 1.2 19.9 20.5 19.9 1.27:1.22 1.28:1.23 1.29:1.24 3.26 3.26 3.25 med Spread 0.6 0.6 0.02 0.01 Table 51 – 6 MeV electrons, 3cm circle, cc13, profile Chamber: 3 cm circle Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc13 Speed: Scan 1 Scan 2 Scan 3 2.1 1.6 1.9 19.2 19.4 19.6 1.20:1.13 1.20:1.15 1.20:1.14 3.23 3.24 3.23 med Spread 0.5 0.4 0.02 0.01 Table 52 – 6 MeV electrons, 3cm circle, cc01, pdd Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc01 Scan 1 1.09 2.30 Scan 2 1.10 2.29 Speed: Scan 3 1.11 2.30 med Spread 0.02 0.01 41 Table 53 – 6 MeV electrons, 3cm circle, cc04, pdd Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc04 Scan 1 0.94 2.28 Scan 2 1.12 2.28 Speed: Scan 3 1.02 2.26 med Spread 0.16 0.02 Table 54 – 6 MeV electrons, 3cm circle, cc13, pdd Chamber: 3 cm circle R100 (dmax,cm): R50 (depth of 50% dose,cm): cc13 Scan 1 0.98 2.28 Scan 2 1.12 2.29 Speed: Scan 3 1.06 2.28 med Spread 0.14 0.01 The biggest and most interesting difference in the pdds is the shallow depth response difference of the compact chambers. The 3 cm circle cutout pdds responded in unexpected way, as shown in figure 12. Previous studies and research would suggest that the cc01 chamber would provide the highest relative near surface dose. This characteristic is supported by the 6 MeV electrons and 6 MV photons 10x10cm2 pdd graphs and shown in figures 13 and 14. For both of these fields the cc01 provides a more accurate near surface response. However, for the 3cm circle cutout, the cc13 provided the highest relative near surface dose response. Further study is needed to completely explain this response, however it should be noted that the cerrobend used to make the 3 cm cutout is known to provide a hard to quantify energy and field size dependant photon scatter contamination component. I would suggest this additional photon scatter component combined with the larger collection volumes should largely compensate for this unexpected result. 42 Figure 12 – 6 MeV electrons, 3cm circle, cc01/cc04/cc13, pdd Figure 13 – 6 MeV electrons, 10x10cm2, cc01/cc04/cc13, pdd 43 Figure 14 – 6 MV photons, 10x10cm2, cc01/cc04/cc13, pdd For the profile scans there is no statistically significant difference in any parameter as a result of the analysis. Interestingly, the cc13, which has the largest radius, actually reported the narrowest penumbra. However, for practical purposes, all chambers reported essentially the same penumbral width, as the variation in measurements was only .2 mm. Medium fields – Recommendation The recommendations for this set of fields are not quite as clear. Generally, the cc01 shows to be a solid performing chamber. However, as field sizes move up from the smallest in this range, there is no statistical difference among any of the compact chambers tested except near the surface on pdd scans, and any could be considered a satisfactory chamber for use. A special area of consideration for these field sizes are the small electron cutouts. Further study needs to be done to further explain and characterize the near surface response of the compact chambers, before one can be suggested as the best for use. 44 Large fields – Data As with the speed change comparison, a 6 MeV, 25x25 cm2, electron field was used for chamber size change comparison. Also shown is a 6 MV, 40x40 cm2 photon field. Figures 101 thru 104 in the appendix show the pdds for each chamber, and then all chambers combined on one graph. Figure 105 shows the photon pdd graph. Figures 106 thru 109 show the data and statistics for the profile scans. Large fields – Analysis Changing the chamber had virtually no effect on the largest electron field pdd. All compact chambers provided comparable results. Tables 55 thru 60 show the statistics for profile and pdd scans for the 25x25 cm2 electron scans. The only exception was as expected in the near surface regions where the smaller chambers provide a more accurate response. The 40x40 cm2 photon field provides further evidence of the cc01 over-response to large fields. For the largest photon field, both the cc04 and cc13 provided equivalent results. Table 55 – 6 MeV electrons, 25x25 cm2, cc01, profile Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc01 Scan 1 2.2 1.8 1.02:1.01 25.62 Scan 2 3.1 2.3 .99:1.02 25.65 Speed: Scan 3 2.7 2.1 1.00:.99 25.63 med Spread 0.9 0.5 0.03 0.03 Table 56 – 6 MeV electrons, 25x25 cm2, cc04, profile Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc04 Speed: Scan 1 Scan 2 Scan 3 1.5 1.7 1.8 1.5 1.8 1.6 1.07:1.09 1.08:1.09 1.08:1.07 25.63 25.64 25.64 med Spread 0.3 0.3 0.02 0.01 45 Table 57 – 6 MeV electrons, 25x25 cm2, cc13, profile Chamber: Symmetry (%) Flatness (%) Penumbra (cm) Field Width (cm) cc13 Scan 1 1.6 1.7 .95:.94 25.59 Scan 2 1.7 1.5 .93:.94 25.58 Speed: Scan 3 1.5 1.5 .93:.95 25.58 med Spread 0.2 0.2 0.02 0.01 Table 58 – 6 MeV electrons, 25x25 cm2, cc01, pdd Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc01 Scan 1 1.29 2.27 Scan 2 1.15 2.27 Speed: Scan 3 1.28 2.27 med Spread 0.14 0 Table 59 – 6 MeV electrons, 25x25 cm2, cc04, pdd Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc04 Scan 1 1.25 2.26 Scan 2 1.24 2.26 Speed: Scan 3 1.3 2.26 med Spread 0.06 0 Table 60 – 6 MeV electrons, 25x25 cm2, cc13, pdd Chamber: R100 (dmax,cm): R50 (depth of 50% dose,cm): cc13 Scan 1 1.22 2.32 Scan 2 1.33 2.32 Speed: Scan 3 1.34 2.33 med Spread 0.12 0.01 46 Figure 15 – 6 MV photons, 40x40 cm2, cc01/cc04/cc13, pdd Observation of the profile scans show that the cc01, shown in figure 16 below and 106 in the appendix, is very noisy and upon analysis, results in the widest data spread of all the chambers. It is reasonable to conclude that this poor quality of data is because this field size is just beyond the upper limit of field sizes where it has been established that the cc01 can accurately be used. The larger cc04 and cc13 chambers proved to be better for this field size. Both provided smoother and more consistent data. There was no statistical difference between the cc04 and cc13. 47 Figure 16 – 6 MeV, 25x25cm2, cc01, profile Large fields – Recommendation For the large field spectrum, the use of either the cc04 or cc13 would provide adequate results across the entire range. 48 Conclusion The results of this study further reinforce the importance of choosing the correct chamber for the chosen field size. Whether the mistake is choosing too large of a chamber for a small field size, or choosing a chamber that over-responds, an incorrect modeling of beam parameters will result in incorrect calculations of dose to be delivered to patients. Scanning at the most appropriate speed has also been shown to affect both the quality and reproducibility of data acquired. Generally, the smallest, minimally perturbing chamber available and appropriate for the field sizes to be measured should be used for scanning. The smallest chamber in these tests consistently, until it reached it maximum usable field size, provided to the most consistent data. However, if field sizes are restricted to the 4x4 cm2 to 20x20 cm2, then any of the compact chambers tested can be used for data acquisition with a high degree of confidence in the data. The cc13 chamber does begin to show some apparent volume averaging, but it is not significant enough to disqualify its results. The penumbra widening effect was also not an issue when comparing all but the smallest of field sizes. The use of the slowest scanning speed also showed to be very important when scanning small field sizes. The effect of detector speed change diminished with field size, but still provided measurable changes. This is fortunate, since as the penalty of the slower speed increases, the detector speed can be increased with decreasing penalty. To maximize efficiency of time, it is also important to optimize the field scan parameters. Scanning needlessly beyond field edges and maximum depths of dose delivery can easily add more time than is saved by increasing the speed. 49 Bibliography Agostinelli, S., Garelli, S., Piergentili, M., & Foppiano, F. (2008). Response to high-energy photons of PTW31014 PinPoint ion chamber with a central aluminum electrode. Medical Physics , 3293-3302. Aird, E. G., & Farmer, F. T. (1972). The design of a thimble chamber for the Farmer dosemeter. Physics in Medicine and Biology , 169. Das, I. J., Ding, G. X., & Ahnesjo, A. (2007). Small Fields: Nonequilibrium radiation dosimetry. Medical Physics , 206-215. Dawson, D. J., Schroeder, N. J., & Hoya, J. D. (1985). 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Medical Accelerators - The Modern Technology in Radiation Oncology: A Copendium for Medical Physicists and Radiation Oncologists. Madison: Medical Physics Publishiing. Sahoo, N., Kazi, A., & Hoffman, M. (2008). Study of the Effect of Size of the Beam Scanning Diodes On Measurement of Beam Profiles of the Clinical High Energy Photon Beams. Medical Physics , 2772-2773. Sauer, O., & Wilbert, J. (2007). Measurement of output factors for small photom beams. Medical Physics , 1983-1988. Scandale, W. (2006). Introduction to Particle Accelerators. Rome: CERN-AT Department. Sibata, C. H., Mota, H. C., Bedder, A. S., Higgins, P. D., & Shin, K. H. (1991). Influence of detector size in photon beam measurements. Physics in Medicine and Biology , 621-628. 3 0.01 3.6 1 Shonka C552 0.4 Steel 1 cc01 0.04 3.6 2 Shonka C552 0.4 C552 1 cc04 0.13 5.8 3 Shonka C552 0.4 C552 1 Description Commonly used air equivalent plastic Polymethyl methacylate (acrylic) Graphite Material Shonka C552 PMMA Graphite Farmer 0.6 23 3.05 PMMA/graphite .335 PMMA / .09 graphite Aluminum (99.98%) 1.1 1.76 1.19 1.85 Density (g/cm3) Table 62 – Ion Chamber Wall Material Information Cavity Volume (cm ) Cavity Length (mm) Cavity Radius (mm) Wall Material Wall Thickness (mm) Center Electrode Material Diameter Center Electrode (mm) Specification Chamber cc13 Table 61 – Ion Chamber Technical Details Appendix 51 Figure 17 – 6 MV photons, .5x.5cm2, cc01, fast pdd 52 Figure 18 – 6 MV photons, .5x.5cm2, cc01, medium pdd 53 Figure 19 – 6 MV photons, .5x.5cm2, cc01, slow pdd 54 Figure 20 – 6 MV photons, .5x.5cm2, cc01, all speeds, pdd 55 Figure 21 – 6 MV photons, .5x.5cm2, cc01, fast profile 56 Figure 22 – 6 MV photons, .5x.5cm2, cc01, medium profile 57 Figure 23 – 6 MV photons, .5x.5cm2, cc01, slow profile 58 Figure 24 – 6 MV photons, .5x.5cm2, cc01, all speeds profile 59 Figure 25 – 6 MV photons, .5x.5cm2, cc04, fast pdd 60 Figure 26 – 6 MV photons, .5x.5cm2, cc04, medium pdd 61 Figure 27 – 6 MV photons, .5x.5cm2, cc04, slow pdd 62 Figure 28 – 6 MV photons, .5x.5cm2, cc04, all speeds pdd 63 Figure 29 – 6 MV photons, .5x.5cm2, cc04, fast profile 64 Figure 30 – 6 MV photons, .5x.5cm2, cc04, medium profile 65 Figure 31 – 6 MV photons, .5x.5cm2, cc04, slow profile 66 Figure 32 – 6 MV photons, .5x.5cm2, cc04, all speeds profile 67 Figure 33 – 6 MeV electrons, 3cm circle, cc01, fast pdd 68 Figure 34 – 6 MeV electrons, 3cm circle, cc01, medium pdd 69 Figure 35 – 6 MeV electrons, 3cm circle, cc01, slow pdd 70 Figure 36 – 6 MeV electrons, 3cm circle, cc01, all speeds pdd 71 Figure 37 – 6 MeV electrons, 3cm circle, cc01, fast profile 72 Figure 38 – 6 MeV electrons, 3cm circle, cc01, medium profile 73 Figure 39 – 6 MeV electrons, 3cm circle, cc01, slow profile 74 Figure 40 – 6 MeV electrons, 3cm circle, cc01, all speeds profile 75 Figure 41 – 6 MeV electrons, 3cm circle, cc04, fast pdd 76 Figure 42 – 6 MeV electrons, 3cm circle, cc04, medium pdd 77 Figure 43 – 6 MeV electrons, 3cm circle, cc04, slow pdd 78 Figure 44 – 6 MeV electrons, 3cm circle, cc04, all speeds pdd 79 Figure 45 – 6 MeV electrons, 3cm circle, cc04, fast profile 80 Figure 46 – 6 MeV electrons, 3cm circle, cc04, medium profile 81 Figure 47 – 6 MeV electrons, 3cm circle, cc04, slow profile 82 Figure 48 – 6 MeV electrons, 3cm circle, cc04, all speeds profile 83 Figure 49 – 6 MeV electrons, 3cm circle, cc13, fast pdd 84 Figure 50 – 6 MeV electrons, 3cm circle, cc13, medium pdd 85 Figure 51 – 6 MeV electrons, 3cm circle, cc13, slow pdd 86 Figure 52 – 6 MeV electrons, 3cm circle, cc13, all speeds pdd 87 Figure 53 – 6 MeV electrons, 3cm circle, cc13, fast profile 88 Figure 54 – 6 MeV electrons, 3cm circle, cc13, medium profile 89 Figure 55 – 6 MeV electrons, 3cm circle, cc13, slow profile 90 Figure 56 – 6 MeV electrons, 3cm circle, cc13, all speeds profile 91 Figure 57 – 6 MeV electrons, 10x10 cm2, all chambers, medium pdd 92 Figure 58 – 6 MeV electrons, 25x25 cm2, cc01, fast pdd 93 Figure 59 – 6 MeV electrons, 25x25 cm2, cc01, med pdd 94 Figure 60 – 6 MeV electrons, 25x25 cm2, cc01, slow pdd 95 Figure 61 – 6 MeV electrons, 25x25 cm2, cc01, all speeds pdd 96 Figure 62 – 6 MeV electrons, 25x25 cm2, cc01, fast profile 97 Figure 63 – 6 MeV electrons, 25x25 cm2, cc01, med profile 98 Figure 64 – 6 MeV electrons, 25x25 cm2, cc01, slow profile 99 Figure 65 – 6 MeV electrons, 25x25 cm2, cc01, all speeds profile 100 Figure 66 – 6 MeV electrons, 25x25 cm2, cc04, fast pdd 101 Figure 67 – 6 MeV electrons, 25x25 cm2, cc04, med pdd 102 Figure 68 – 6 MeV electrons, 25x25 cm2, cc04, slow pdd 103 Figure 69 – 6 MeV electrons, 25x25 cm2, cc04, all speeds pdd 104 Figure 70 – 6 MeV electrons, 25x25 cm2, cc04, fast profile 105 Figure 71 – 6 MeV electrons, 25x25 cm2, cc04, med profile 106 Figure 72 – 6 MeV electrons, 25x25 cm2, cc04, slow profile 107 Figure 73 – 6 MeV electrons, 25x25 cm2, cc04, all speeds profile 108 Figure 74 – 6 MeV electrons, 25x25 cm2, cc13, fast pdd 109 Figure 75 – 6 MeV electrons, 25x25 cm2, cc13, med pdd 110 Figure 76 – 6 MeV electrons, 25x25 cm2, cc13, slow pdd 111 Figure 77 – 6 MeV electrons, 25x25 cm2, cc13, all speeds pdd 112 Figure 78 – 6 MeV electrons, 25x25 cm2, cc13, fast profile 113 Figure 79 – 6 MeV electrons, 25x25 cm2, cc13, medium profile 114 Figure 80 – 6 MeV electrons, 25x25 cm2, cc13, slow profile 115 Figure 81 – 6 MeV electrons, 25x25 cm2, cc13, all speeds profile 116 Figure 82 – 6MV photons, .5x.5cm2, cc01, pdd 117 Figure 83 – 6MV photons, .5x.5cm2, cc04, pdd 118 Figure 84 – 6MV photons, .5x.5cm2, cc13, pdd 119 Figure 85 – 6MV photons, .5x.5cm2, all chambers pdd 120 Figure 86 – 6MV photons, .5x.5cm2, cc01, profile 121 Figure 87 – 6MV photons, .5x.5cm2, cc04, profile 122 Figure 88 – 6MV photons, .5x.5cm2, cc13, profile 123 Figure 89 – 6MV photons, .5x.5cm2, all chambers, profile 124 Figure 90 – 6 MeV electrons, 3cm circle, cc01, pdd 125 Figure 91 – 6 MeV electrons, 3cm circle, cc04, pdd 126 Figure 92 – 6 MeV electrons, 3cm circle, cc13, pdd 127 Figure 93 – 6 MeV electrons, 3cm circle, all chambers pdd 128 Figure 94 – 6 MeV electrons, 10x10 cm2, all chambers, pdd 129 Figure 95 – 6 MV photons, 10x10 cm2, all chambers, pdd 130 Figure 96 – 6 MV photons, 10x10 cm2, all chambers, pdd (near surface zoom) 131 Figure 97 – 6 MeV electrons, 3cm circle, cc01, profile 132 Figure 98 – 6 MeV electrons, 3cm circle, cc04, profile 133 Figure 99 – 6 MeV electrons, 3cm circle, cc13, profile 134 Figure 100 – 6 MeV electrons, 3cm circle, all chambers, profile 135 Figure 101 – 6 MeV electrons, 25x25cm2, cc01, pdd 136 Figure 102 – 6 MeV electrons, 25x25cm2, cc04, pdd 137 Figure 103 – 6 MeV electrons, 25x25cm2, cc13, pdd 138 Figure 104 – 6 MeV electrons, 25x25cm2, all chambers pdd 139 Figure 105 – 6 MV photons, 40x40cm2, all chambers, pdd 140 Figure 106 – 6 MeV electrons, 25x25cm2, cc01, profile 141 Figure 107 – 6 MeV electrons, 25x25cm2, cc04, profile 142 Figure 108 – 6 MeV electrons, 25x25cm2, cc13, profile 143 Figure 109 – 6 MeV electrons, 25x25cm2, all chambers, profile 144