A Monte Carlo study of detector dose response in small megavoltage photon beams Hugo Bouchard1, Jan Seuntjens2, Simon Duane1, Yuji Kamio3, Hugo Palmans1,4 1. Acoustics and Ionising Radiation Team, National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom 2. Medical Physics Unit, McGill University, Montreal, Quebec H3G 1A4, Canada 3. Centre hospitalier de l'Universite de Montreal (CHUM), 1560 Sherbrooke est, Montreal, Quebec H2L 4M1, Canada 4. Medical Physics, EBG MedAustron GmbH, A-2700 Wiener Neustadt, Austria Correction factors in small fields • In small photon fields, radiation detectors cannot be used in the same way as in reference beams • A new CoP for small fields is underway to allow small field dosimetry New dosimetry technique Machine MSR SSD/SAD Machine SSD/SAD dref dref H2O Alfonso et al. 2008 f clin , f msr Qclin ,Qmsr Ω CLIN = Dwf clin ,Qclin f msr w ,Qmsr D = MH2QOf clin clin M f msr Qmsr clin , f msr kQfclin ,Qmsr Motivations • Several papers addressed nonstandard beam dosimetry – Sanchez-Dobaldo et al. (2003, 2005), Capote et al. (2004), Bouchard et al. (2004, 2009, 2012), Das et al. (2008), Francescon et al. (2008, 2012, 2014), Scott et al. (2012), Underwood et al. (2013), Fenwick et al. (2013), Charles et al. (2013), Czarnecki and Zink (2013) • A comprehensive overview of the problem might be lacking – This can be helpful to prepare our community for the upcoming IAEAAAPM CoP • The goal of the present work is to: – Address the fundamental differences between absorbed dose measurements in standard and nonstandard beams – Highlight potential misconceptions in the applicability of cavity theory in nonstandard beams 3 Why do small fields require correction factors? • One isolates 4 main effects related to the characteristics of the detector 1. 2. 3. 4. The presence of extracameral components The atomic properties of the sensitive volume The density of the sensitive volume of the detector Volume averaging • To characterize these effects quasi-individually, we define a series of geometries in which absorbed dose is evaluated 4 Monte Carlo factorization of perturbation factors • Standard cavity theory approach (e.g., Bouchard et al. 2009) • “Fano” cavity theory (this work) Pencil Beam Decomposition Method • Pencil beam decomposition method to model dose in the cavity (xi,yi) Dose response function Fluence weight distribution in [0,1] dcav(xi,yi) 6 Pencil Beam Decomposition Method • Cavity response to pencil beams: DRF 7 Pencil Beam Decomposition Method • Perturbation function – We define Pencil Beam Decomposition Method • Perturbation function – From the definition – Having the property – We calculate kQ factors Denominator is field factor (also known as output factor) 9 Monte Carlo simulations EGSnrc user code cavity C++ library (egs++) ECUT = AE = 512 keV PCUT = AP = 1 keV Range rejection up to 10 keV (ESAVE = 521 keV) All physics “on” (spin effects, atomic relaxation, etc..) • 1.25 MeV parallel photon pencil beams • Geometry/simulation passes Fano test to 0.1% or better • • • • • • Air detector π x 5 x 2 mm3 volume 1 mm graphite wall 10 cm 30 x 30 x 30 cm3 H2O phantom Silicon detector π x 2 x 2 mm3 volume 1 mm aluminium wall 1. Extracameral components • Components such as wall, electrodes, stems, etc., can affect the detector dose response Air detector Air cavity Silicon detector Silicon cavity 2. Atomic properties • Assuming the detector to be a cavity with the electron density of water, relevant atomic properties are – The atomic number (photo-electric effect, pair production) – The I-value (stopping power) – The density effect parameter δ (stopping power) • All have an effect on the interaction cross sections (i.e., electronic cross sections) 2. Atomic properties • Monte Carlo simulations of cavity dose response to 1.25 MeV photon pencil beams in cavity of identical electron density Air cavity Air-density water cavity Silicon Silicon-density water 3. Density perturbation effects • Monte Carlo simulation of cavity dose response to 1.25 MeV photon pencil beams in water cavities made of different density Air-density water Water cavity Silicon-density water Water Perturbation functions Remind that • They govern quality correction factors cavity wall π x 52 x 2 mm3 air cavity cavity wall π x 12 x 2 mm3 silicon cavity 4. Volume averaging • In nonstandard beams, volume averaging effect can be as important as density perturbation effects Density perturbation effect Scott et al. Phys. Med. Biol. 2012 Volume averaging effects 16 Density perturbation effects • Results show that by far, detector density is the most important effect, taking volume averaging effects aside • This can be explain by the interaction between lack of CPE and the detector density using Fano’s theorem 17 Fano’s theorem • In a medium of uniform properties irradiated by uniform photon fluence, the fluence of charged particles is uniform and independent of the mass density distribution in space. Water cavity Vapor cavity Dense-water cavity 18 • We decompose the beam Beam directed at cavity Broad beam = + Beam not directed at cavity • By Fano’s theorem • And we find 19 Fano calculations of DRF 20 How small is small? • Y. Kamio and H. Bouchard, Phys. Med. Biol. 59 (2014) 4973-5002 – Pencil beam decomposition method (as in this talk) – Tolerance on kQ

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# A Monte Carlo study of detector dose response Hugo Bouchard