AAPM Task Group 106
Accelerator Beam Data Commissioning
Equipment and Procedures
New Measurement Dosimetry –
New Developments
John P. Gibbons, Jr., Ph.D.
Med Phys 35 (9): 4186-4210 (2008)
Relative Dose Measurements
Relative Dose Measurements
Beam Scanning
Beam Scanning
Beam Scanning System Setup:
• Water Phantom
Scanning Arm Tilt
– Water conditions
• Water temperature is equilibrated
• Periodically check for evaporation
– Leveling
• Level (horizontal) scanning arm, not tank
• Can verify visually, or scan in gradient region (e.g., R50) of
low-energy electron beam
• Verify vertical (depth) scans are parallel to central axis
Das et al,
TG-106
1
Relative Dose Measurements
Relative Dose Measurements
Beam Scanning
Beam Scanning
Gantry Tilt - Photons
Gantry Tilt - Electrons
Das et al,
TG-106
Das et al,
TG-106
Relative Dose Measurements
Relative Dose Measurements
Beam Scanning
Electrometer amplification
Comparison of chamber and bias
Beam Scanning System Setup:
• Scanning System
110
100
6 MV bad chamber, incorrect gain
– Range of motion sufficient
(Offset tank scans for 40 x 40 cm2 scans)
– Sufficient scatter material for field sizes and depths
measured
– Detectors positioned appropriately
– Detection electronics adjusted for measurement
conditions
6 MV bad chamber, correct gain
15 MV good chamber
15 MV bad chamber, incorrect gain
80
15 MV bad chamber, correct gain
Percent Depth Dose
(TG40: +1mm over 300 mm movement)
6 MV good chamber
90
– Detector motion calibrated
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
Depth(cm)
Das et al,
TG-106
2
Relative Dose Measurements
Relative Dose Measurements
Beam Scanning - Photons
Beam Scanning - Photons
Percent Depth Dose scanning:
• Corroborate with integrated measurements
• Adjust for effective point of measurement
• Should ensure sufficient scatter for measured
depths
• Detector alignment should be verified for
wedged fields
Percent Depth Dose scanning:
• Corroborate with integrated measurements
• Adjust for effective point of measurement
• Should ensure sufficient scatter for measured
depths
• Detector alignment should be verified for
wedged fields
Relative Dose Measurements
Relative Dose Measurements
Cylindrical chamber shift
Beam Scanning - Photons
Percent Depth Dose scanning:
• Corroborate with integrated measurements
• Adjust for effective point of measurement
• Should ensure sufficient scatter for measured
depths
• Detector alignment should be verified for
wedged fields
Almond et al., AAPM TG-51, Med Phys 26:1854 (1999)
3
Relative Dose Measurements
Relative Dose Measurements
Detector alignment for wedged beams
Beam Scanning - Photons
Beam profile scanning:
• Should ensure sufficient scatter for measured
field sizes
• Penumbra dependent on detector size
• Scanning speed should not disturb surface
• Careful with electronic amplification for gradient
fields
CAX
Scanning
direction
Relative Dose Measurements
Relative Dose Measurements
Beam Scanning
Beam Scanning - Photons
Scanning Speed
Beam profile scanning:
• MLC-defined fields:
– Detector volume centered under leaf center
– Avoid scanning abutted or close leaves
• Diagonal profiles:
– Confirm calibration of detector movement
– Scatter material more important
– Consider detector orientation
• Special fields (e.g., blocks, compensators)
– Avoid scanning through mounting tray holes, slots, etc.
Das et al,
TG-106
4
Relative Dose Measurements
Relative Dose Measurements
Beam Scanning - Electrons
Beam Scanning - Electrons
Percent Depth Dose scanning :
• Ionization chamber data require corrections
(stopping power ratio, prepl)
• Depth accuracy important
Determination of Surface Position
– R50 determines energy and calibration
– Effective point of measurement more critical
– Water ripple has more effect
Das et al., Phys Med Phys 43(11) : 3419 (1998)
Relative Dose Measurements
Beam Scanning - Electrons
Relative Dose Measurements
Beam Scanning - Electrons
Effect of Scanning Speed - Electrons
Das et al, TG-106
5
Relative Dose Measurements
Relative Dose Measurements
Integrated Measurements - Photons
Integrated Measurements - Photons
General Considerations:
• Choice of Normalization Depth
– All quantities should be measured at this depth
• Periodically repeat normalization readings
Output factor measurements (Scp):
• Phantom large enough for larger fields
• Detector small enough for smaller fields
• Should verify minimal stem effect, especially for
smaller volume, non-farmer type chambers
Relative Dose Measurements
Relative Dose Measurements
Integrated Measurements - Photons
Integrated Measurements - Photons
Sc Collimator Exchange Effect
In-Air output ratio measurements (Sc):
• Mini-Phantoms recommended
• Collimator exchange effect:
– Defined as Sc(a,b)
Sc(b,a)
– Verify small (<2%) for range of clinical
field sizes
Lam et al., MU Calculations for
Photon and Electrons Fields, 2000
6
Relative Dose Measurements
Relative Dose Measurements
Integrated Measurements - Photons
Integrated Measurements - Electrons
Transmission factor measurements (WF, TF, etc.):
• Should be measured at normalization depth
Output factor measurements (Se):
• Function of applicator size and jaw setting, so record and
periodically verify both.
• Should verify both wedge orientations, especially
for steeper wedges
• Careful with wedge factor normalization.
• Account for the effective point of measurement of the
detector
• Should be measured at R100, which may vary with
applicator size.
• If depth changes between measured and reference
applicator, should convert ionization to dose for each
measurement. (S/p is a function of depth)
– TPS may require WF = Dw(r)/Do(10x10)
• Wedges may have depth and/or field size
dependence
Conclusions
• TG-106 gives specific recommendations for
determining relative doses via scanning or
integrated measurements
•A variety of simple tests should be performed
on the beam scanning systems prior to each
measurement to ensure correct setup.
• Integrated measurements should be
performed using an appropriate detector and
at the correction depth.
7