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Klinik und Poliklinik für StrahlentherapieDirektor: Prof. Dr. Dr. M. Flentje
Small Field DosimetryRelative dosimetry: detector choice and corrections
Otto A SauerOtto A Sauer
Relative dosimetry
Measurement tasks:
• Rel. profiles (OARs)
• Rel. depth dose• Tissue phantom ratios (TPR)
2Otto Sauer: small field dosimetry 20.07.2010
• Total scatter factor (output measured in phantom, Scp)
• Head scatter factor (output measured in air Sc)
• Phantom scatter factor Sp
Necessary and desired detector properties
• High spatial resolution• High signal, low noise• Low energy dependence• Low directional dependence• high dose, resp. dose rate
linearity
• Some detectors
Ionization chamber PTW
31010, 0.125cc
Ionization chamber PTW
31006, PinPoint, 0.015cc
Shielded Si-diode Sc.-Wellhöfer PFD Photon, 0.0003cc
3Otto Sauer: small field dosimetry 20.07.2010
linearity• High stability• Handy, easy to use in a
clinic, robust• inexpensive
0.0003cc
Mini Si-diode Sc.-Wellhöfer SFD stereotactic, 0.000017cc
Si-MOSFET Thomson &
Nielsen TN-502 RD
Diamond detector PTW
60003, ca. 0.002cc
microLion Chamber 0.0017cc
Detector response
Reference dosimetry:• Product of correction factors should be known
(use CoP)
( ) ( ) ( )rrr NMkD ⋅= ∏
4Otto Sauer: small field dosimetry 20.07.2010
(use CoP)
Relative dosimetry:• Product of correction factors k should be constant or
known
Detectors
• Polymer gel – 3D, difficult readout, calibration, preparation
• Radiochromic film – 2D, difficult calibration, stability of scanning
• Micro ionization chamber– Volume effect, signal/noise, background
• TLD, alanine, MOSFET, all non direct reading 1D detectors
5Otto Sauer: small field dosimetry 20.07.2010
• TLD, alanine, MOSFET, all non direct reading 1D detectors– Position uncertainty
• Diamond– Dose rate dependence, volume effect, robustness
• Si-diodes– High spatial resolution, energy dependent response
Polymer gel dosimetry / volume effect
*: MC(Verhagen et al PMB 43 1998 2755–2768)
Gel
Pappas et al MP 32 2005 1513–1520
6Otto Sauer: small field dosimetry 20.07.2010
Volume effect (1)
• Mean relative dose on the sensitive area of the detector perpendicular to the beam axis
• (assumption:
7Otto Sauer: small field dosimetry 20.07.2010
• (assumption: constant sensitivity
• → volume effect is overestimated)
Kawachi et al. Medical Physics, Vol. 35, No. 10, 2008, 4591f
Volume effect (2)
• Deconvolution with the detector response function K
• Dm: measured dose
• D: “true” dose
• F:
8Otto Sauer: small field dosimetry 20.07.2010
• F: Fourier transform
[ ] [ ] [ ])()()(
)()()(
xuKFuDFxDF
duxuKuDxD
m
m
−=
−= ∫+∞
∞−
Garcia-Vicente et al Med. Phys. 25, 1998, 202f
Dose rate dependence
A diamond detector• may have a huge dose rate
dependence• is not really small (> 3mm)• has a huge production
spread• has low energy
liquid ionization chamber
Single & double diode
9Otto Sauer: small field dosimetry 20.07.2010
• has low energy dependence
Natural diamondDetector response for different dose rates in a 6 MV bremsstrahlung beam expressedas ratios of detector response and dose rate, determined by a Farmer ionization chamber, andnormalized at the lowest dose rate. The error bars represent the statistical uncertainty (1SD).[Westermark et al. Phys. Med. Biol. 45 (2000) 685–702]
Energy dependence, cavity theory
Silicon detector
( ) detdetdetdet
1 DkkDL
LkD Ep
wwpw ′′′=′
−+′= α
µµα
10Otto Sauer: small field dosimetry 20.07.2010
• strong increase of response below hν = 200 keV
• Small: width < 1mm
Relative dose from photons below 200 keV for 60Co• 5cm x 5cm @ 1cm and @ 20cm depth
11Otto Sauer: small field dosimetry 20.07.2010
N Chofor et al 2007 Phys. Med. Biol. 52 N137 doi: 10.1088/0031-9155/52/7/N03and personal communication
Possible change of response outside the useful beam
Response versus field size
• Decreasing response of Si-detectors with decreasing field size
• Reason: depletion of low energy photons
• For small fields volume averaging of the 0.125 cm³ large IC is dominant
12Otto Sauer: small field dosimetry 20.07.2010
Sauer, Wilbert MP 34, 2007, 1983-1988
perturbation
Ionization chamber:• wall, central electrode,
displacement, cavity,• (saturation, humidity, T&p, polarization)
Diode:• housing, shielding,
13Otto Sauer: small field dosimetry 20.07.2010
• housing, shielding, contacting
• Field size dependence of perturbation factors?
C. McKerracher, D.I. Thwaites / Radiotherapy and Oncology 79 (2006) 348–351
Assumption: perturbation for solid state detectors is approx. constant as long as field is larger than detector diameter.
5mm7mm
Profile measurements
Recommendations• To detect the penumbra correctly use a small diode• Check the detector response outside the geometrical field• Correct for over/under-response or use an appropriate
detector. [e.g. (shielded) diode or radiochromic film]
Off axis profiles at the
14Otto Sauer: small field dosimetry 20.07.2010
Off axis profiles at the isocentre (FAD D 100 cm), 6 cm deep, measured withdiamond, diode, and film and calculated using MC (EGS4), for (a) 7 mm and
(b) 23 mm SRScollimators.
Heydarian et al PMB 41 (1996) 93–110
Profile measurements: directional dependence
15Otto Sauer: small field dosimetry 20.07.2010
Beddar el al (1994), PMB 37(10)
Dasu el al (1998), PMB 43(1)
Depth dose and TPR measurements
• Use a small diode• (energy dependence, i.e. depth
dependence of response is hardly effective for small fields)
• Estimate the changing volume effect with depth
• Carefully align CAX with scan
16Otto Sauer: small field dosimetry 20.07.2010
• Carefully align CAX with scan direction
• Measure TPR directly• Examine data
– Fit to a function– Corroborate by different means
Output measurement, total scatter factor Scp
Field size dependence of
( ) ( ) ( ) ( )
( )( )zAD
zADS
DAkAkDAksD
refw
wcp
Epw
,
,
detdet
=
==
0 2 4 6 8 10 12 14 16 180.0
0.2
0.4
0.6
0.8
1.06 MV meas.
IC PiP DiGre DiYe MOS1 MOS2 DIAre
lativ
e D
ose
SES / cm
0.0 0.5 1.0 1.5 2.0 2.50.0
0.2
0.4
0.6
0.8
17Otto Sauer: small field dosimetry 20.07.2010
Field size dependence of• perturbation correction factor
kp?• energy correction factor kE
for different detectors ?• volume effect
0 2 4 6 8 10 12 14 16 180.0
0.2
0.4
0.6
0.8
1.06 MV corr.
IC PiP DiGre DiYe MOS1 MOS2 DIA
Dos
e / G
y/10
0MU
SES / cm
0.0 0.5 1.0 1.5 2.0 2.50.0
0.2
0.4
0.6
0.8
( )brnn
n
eSrk
rPD −∞∞ −++
= 1
Output measurement Scp• measurement with an ionization chamber
high perturbation?
( ) ( )( )( )
( ) [ ]A
Aairwater,refref
ref
refrefw
refwcp
reff
z,AM
zA,M
z,AD
zA,DAS ==
18Otto Sauer: small field dosimetry 20.07.2010
•R. Capote et al., Med. Phys. 31(9), (2004) 2454–2465 : ∼ 3% inside the irregular (∼2×2cm2) IMRT-field and outside field ∼ 9%
•H. Bouchard and J. Seuntjens, Med. Phys. 31(9), (2004): 10% in a realistic dynamic IMRT field and large for fields that provoke the lateral electron equilibrium (LEE)
•A. L. McNiven et. al., Med. Phys. 33(11), (2006) 3997-4004: The conventional approach ofcalculating this dose conversion factor, using a ratio of detector readings only, is not suitable for small-
field dosimetry unless the conversion factor can be shown to be unity!
Output measurement Scp• measurement with a silicon detector
high energy dependence
( ) ( )( )( )
( ) [ ] [ ]A
A
diodeA
A
diode
refref
ref
refref
refdiodecp
refErefp
kkz,AM
zA,M
z,AD
zA,DAS ==
19Otto Sauer: small field dosimetry 20.07.2010
kp = 1
for detector < A
kE = as+b
Output measurement Scp• Cross-calibrate a Si-diode at medium
field sizes with a small IC• Evaluate energy/field size dependence• Avoid volume effects (use a small
detector)
1.04 6 MV IC
Li et al MP 22, 1995, 1167-1170
20Otto Sauer: small field dosimetry 20.07.2010
0 2 4 6 8 10 12 14 16 18
0.96
0.98
1.00
1.02
DiYe MOS1 MOS2 DIA
IC PiP DiGre
Sig
nal r
atio
s
SES / cm
Eklund, Ahnesjö PMB 2010
LEE
Sauer, Wilbert MP 34, 2007, 1983-1988
Standard uncertainty: Type B (non -statistical)
0.02
0.03MOSDiGre
DiYe
DIA
STDs
raw06 corr06 raw10 corr10
stan
dard
dev
iatio
n / G
y/10
0MU
0 2 4 6 8 10
0.28
0.30
0.32
0.34
0.36
10 MV
6 MV
corrected dose for SES = 4 mm
dos
e /G
y
approx. detector area / mm²
21Otto Sauer: small field dosimetry 20.07.2010
0.0 0.8 1.6 2.4 4 8 12 160.00
0.01
stan
dard
dev
iatio
n / G
y/10
0MU
SES / cm
approx. detector area / mm²
Decrease for s> 8mm, increase for s=4mm, p?
Head/Collimator scatter SC and phantom scatter SP• Some planning systems want SP and SC data
• SC• Use a cap on a diode• Brass or other high Z is acceptable
• SP• calculate from S and S
22Otto Sauer: small field dosimetry 20.07.2010
• calculate from SCP and SP• Use tabulated SP data
• Alternatively:• Calculate SC from tabulated SP data and measured SCP
Conclusions
• For small photon fields, detector response is not always known with high certainty
• A small (shielded) diode works, • determine dependence of response on energy spectrum and other
dependences for each individual detector
23Otto Sauer: small field dosimetry 20.07.2010
• Corroborate the measurements by– Using different detectors– Fitting data to analytical functions– Survey the literature
Thank you!
24Otto Sauer: small field dosimetry 20.07.2010