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An Independent Audit and Analysis of Small Field Dosimetry Quality Assurance
David FollowillIROC Houston QA Center
Outline of Presentation
• Definition of the problem• Results of audits• Possible solutions• Resources to help
Learning Objectives
• Recognize that small field dosimetry is not easy and that there are tools to help you be as accurate as possible.
Sometimes we all like to believe that all across the kingdom it’s bright and merry!
Ladies and LordsThere is a scourge across the Kingdom
Small field dosimetry
What is the truth?
From Das et al 2000
Recommendation:
The unshielded stereotactic diode and micro-chambers are the detectors of choice formeasurements in radiosurgery beams. However, werecommend comparing the measurements from atleast two different detectors listed in Table 2a, andafter applying corrections taking their average.
TG-155Small Field Dosimetry
TomoTherapy results (since Aug 2010)
0
20
40
60
80
100
120
0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.1
# re
sults
TLD/Institution
Average: 0.970 +/- 0.024552 irradiations
But the manufacturer told me it would work, the manufacturer set the value or I sent my own measured data to the TPS company and they modeled my beams for me.
1. Accuray calibrates all of the tomotherapy units using the Exradin A1SL chamber and the user monitors constancy.
• Correction factor of 0.980 for A1SL based on Gago-Arias et al, Med Phys, 2012
Manufacturer modeling of small field dataYou make the measurementsYou send the measurementsThey create the beam model
BUTThey assume you used the correct dosimeter
They assume you made measurements correctlyThey do not know to apply additional corr. factors
RESULTIncorrect Small Beam Model
Francescon et al2011 data
Small Field Dosimetry Corrections
Situation is even worse if you consider using field sizes less then 0.5 x 0.5 cm2
Field Size (cm x cm)
Detectors 0.5x0.5 0.75x0.75 1.0x1.0 1.25x1.25 1.5x1.5 3.0x3.0
PTW 60012-60017 0.968±0.003 0.984±0.002 0.995±0.001 1.001±0.001 1.006±0.001 1.013±0.001
Sun Nuclear Dedge 0.932±0.003 0.951±0.002 0.967±0.001 0.978±0.003 0.986±0.002 1.001±0.002
IBA SFD 0.972±0.003 0.996±0.002 1.007±0.001 1.011±0.001 1.015±0.001 1.017±0.001
Exradin D1V 0.980±0.003 0.994±0.002 0.999±0.001 1.000±0.001 1.005±0.001 1.012±0.001
Exradin A16 1.112±0.018 1.044±0.007 1.020±0.001 1.007±0.001 1.002±0.001 0.999±0.001
PTW PinPoint 31014 1.128±0.018 1.053±0.007 1.024±0.002 1.010±0.001 1.005±0.001 1.000± 0.001
PTW MicroLion 1.023±0.009 0.997±0.002 0.993±0.001 0.992±0.001 0.994±0.001 0.998±0.001
Exradin W1 PSD 0.998±0.003 0.995±0.002 0.996±0.002 0.996±0.002 0.994±0.002 0.994±0.002
calculated as the mean over all the values obtained by changing the linacmodel, (PrimusTM Siemens 6 MV and Synergy® Elekta 6 MV), the radial FWHM andenergy of the electron source as presented by Francescon et al.97 These values canalso be used with caution for Varian 6 MV Linacs.
clin 10x10clin 10x10
f , fQ ,Qk
Unpublished data from P. Francescon
Gamma Knife results (since Sep 2007)
0
10
20
30
40
50
60
0.90 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10
# re
sults
TLD/Inst.
Average: 0.997 +/- 0.021266 results
Depending on the dosimeter used, the magnitude of the additional correction, down to a 0.5 x 0.5
cm2 field size can be as much as:
6%91%1%2%1% 1. 5%
2. 2%3. No correction needed4. 13%5. 8%
The answer is:
13%
The magnitude of the additional correction, down to a 0.5 x 0.5 cm2 field size can be as much as:
Francescon et al. Medical Physics, Vol. 38, No. 12, December 2011
Recommendation:
It is strongly recommended to independently verifydosimetric measurements in small fields, eitherthrough measurements carried out by a differentperson and/or though an independent external audit,such as that carried out by the RPC .
TG-155Small Field Dosimetry
RPC Measurements during onsite visitsMeasurement configuration:
depth = 10 cm, 100 cm SSD, Exradin A16Secondary jaws kept at a 10 x 10
MLC @ 10 x 10MLC @ 6 x 6MLC @ 4 x 4MLC @ 3 x 3MLC @ 2 x 2
The Problem is that our Dragon is very small!
Tables of standard small field factors
Followill et al 2012 and also will be in TG-155
We even make mistakes
Corrected Varian DataVarian 6 MV Varian 10 MV Varian 15 MV Varian 18 MV
Field Size
(cm x cm) RPC Institution RPC Institution RPC Institution RPC Institution
10 x 10 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000
6 x 6 0.938 0.945 0.959 0.964 0.963 0.964 0.969 0.971(0.005) (0.008) (0.002) (0.003) (0.005) (0.007) (0.003) (0.006)
[0.8%] [0.5%] [0.4%] [0.5%](n=127) (n=22) (n=32) (n=41)
4 x 4 0.886 0.900 0.916 0.927 0.927 0.928 0.928 0.933(0.006) (0.013) (0.006) (0.010) (0.006) (0.009) (0.002) (0.009)
[1.8%] [1.2%] [0.7%] [0.9%](n=122) (n=25) (n=32) (n=37)
3 x 3 0.851 0.870 0.880 0.894 0.892 0.894 0.884 0.891(0.007) (0.012) (0.006) (0.008) (0.006) (0.012) (0.005) (0.019)
[2.4%] [1.6%] [0.9%] [1.4%](n=123) (n=25) (n=31) (n=41)
2 x 2 0.804 0.825 0.823 0.838 0.824 0.823 0.806 0.804(0.008) (0.020) (0.005) (0.015) (0.011) (0.040) (0.007) (0.020)
[2.9%] [2.0%] [2.2%] [1.6%](n=136) (n=23) (n=33) (n=40)
Varian 6 MV Varian 10 MV Varian 15 MV Varian 18 MV Field Size (cm x cm) RPC Institution RPC Institution RPC Institution RPC Institution
10 x 10 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000
6 x 6 0.938
(0.937)* 0.945 0.959 0.964 0.963
(0.965) 0.964 0.969 0.971 (0.005) (0.008) (0.002) (0.003) (0.005) (0.007) (0.003) (0.006) [0.8%] [0.5%] [0.4%] [0.5%]
(n=127) (n=22) (n=32) (n=41)
4 x 4 0.886
(0.885) 0.900 0.916 0.927 0.927
(0.924) 0.928 0.928 0.933 (0.006) (0.013) (0.006) (0.010) (0.006) (0.009) (0.002) (0.009) [1.8%] [1.2%] [0.7%] [0.9%]
(n=122) (n=25) (n=32) (n=37)
3 x 3 0.851
(0.849) 0.870 0.880 0.894 0.892
(0.884) 0.894 0.884 0.891 (0.007) (0.012) (0.006) (0.008) (0.006) (0.012) (0.005) (0.019) [2.4%] [1.6%] [0.9%] [1.4%]
(n=123) (n=25) (n=31) (n=41)
2 x 2 0.804
(0.802) 0.825 0.823 0.838 0.824
(0.814) 0.823 0.806 0.804 (0.008) (0.020) (0.005) (0.015) (0.011) (0.040) (0.007) (0.020) [2.9%] [2.0%] [2.2%] [1.6%]
(n=136) (n=23) (n=33) (n=40)
Discrepancies Regarding:Number of InstitutionsReceiving rec. (n = 206)
Review QA Program 152 (74%)Photon Field Size Dependence 138 (67%)
Wedge Factor (WF) 66 (32%)Off-axis Factors (OAF)/Beam symmetry 60 (29%)
Electron Calibration 35 (17%)Photon Depth Dose 33 (16%)Electron Depth Dose 25 (12%)Photon Calibration 16 (8%)
Discrepancies Discovered (Jan. ’05 – April ’13)
On-Site Dosimetry Review Audit
This is a beam measurement issue and TPS beam modeling challenge.
There is guidance or possibly a fair maiden after the dragon is slain.
The bigger challenge is with Varian machines due to the tertiary collimation (MLC)• Varian/Pinnacle combination:
Followill et al, JACMP, vol. 13, No. 5, 2012
• Varian/Eclipse combination:Kron et al, Med Phys, vol 39 (2), 2012
Huge Resource of Published data:
239 references
5 Tables
15 Figures
13 Conclusions and recommendations
TG-155Small Field Dosimetry
New IROC Houston audit
OSLD
Commissioning and Testing
Three dosimeters were used to estimate the dose for each field size: Exradin A16Exradin D1V diodeExradin D1V diodePTW CC04Exradin W1
First Test Irradiation away from RPC
All of the following are current methods to provide a verification of your small field
dosimetry data except:
2%4%86%8%1% 1. Use of published data
2. Your 2nd set of measurements3. Rely on manufacturer’s data4. Remote audit5. Indep. meas. from another physicist
The answer is:
Reliance on the manufacturers data
All of the following are current methods to provide a verification of your small field dosimetry data except:
Ref: Das et al, AAPM TG 155 report, submitted to Med Phys
• The challenge continues for the <2 x 2 cm2 field sizes
• The correct dosimeter is essential.
• There are independent audits and standard data to help.
• We will defeat thisdragon!
Summary|Conclusion
Thank YouQuestions?
ReferencesFrancescon et al. Medical Physics, Vol. 38, No. 12, December 2011
Das et al, AAPM TG 155 report, submitted to Med Phys
Followill et al. J Appl Clin Med Phys 13(5):282-9, 2012.
Followill DS: Erratum, J Appl Clin Med Phys 15 (2): 4757 3/2014.
AcknowledgementsWork supported by PHS grant CA10953 awarded by NCI, DHHS