An Independent Audit and Analysis of Small Field Dosimetry Quality Assurance. David Followill IROC Houston QA Center

Transcription

1 An Independent Audit and Analysis of Small Field Dosimetry Quality Assurance David Followill IROC Houston QA Center

2 Outline of Presentation Definition of the problem Results of audits Possible solutions Resources to help

3 Learning Objectives Recognize that small field dosimetry is not easy and that there are tools to help you be as accurate as possible.

4 Sometimes we all like to believe that all across the kingdom it s bright and merry!

5 Ladies and Lords There is a scourge across the Kingdom Small field dosimetry

6 What is the truth? From Das et al 2000

7 Recommendation: TG-155 Small Field Dosimetry The unshielded stereotactic diode and microchambers are the detectors of choice for measurements in radiosurgery beams. However, we recommend comparing the measurements from at least two different detectors listed in Table 2a, and after applying corrections taking their average.

8 TomoTherapy results (since Aug 2010) Average: / irradiations 80 # results TLD/Institution

9 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 for A1SL based on Gago-Arias et al, Med Phys, 2012

10 Manufacturer modeling of small field data You make the measurements You send the measurements They create the beam model BUT They assume you used the correct dosimeter They assume you made measurements correctly They do not know to apply additional corr. factors RESULT Incorrect Small Beam Model

11 Small Field Dosimetry Corrections Situation is even worse if you consider using field sizes less then 0.5 x 0.5 cm 2 Francescon et al 2011 data

12 f, f k clin 10x10 Q clin,q10x10 calculated as the mean over all the values obtained by changing the linac model, (Primus TM Siemens 6 MV and Synergy Elekta 6 MV), the radial FWHM and energy of the electron source as presented by Francescon et al. 97 These values can also be used with caution for Varian 6 MV Linacs. Field Size (cm x cm) Detectors 0.5x x x x x x3.0 PTW ± ± ± ± ± ±0.001 Sun Nuclear Dedge 0.932± ± ± ± ± ±0.002 IBA SFD 0.972± ± ± ± ± ±0.001 Exradin D1V 0.980± ± ± ± ± ±0.001 Exradin A ± ± ± ± ± ±0.001 PTW PinPoint ± ± ± ± ± ± PTW MicroLion 1.023± ± ± ± ± ±0.001 Exradin W1 PSD 0.998± ± ± ± ± ±0.002 Unpublished data from P. Francescon

13 Gamma Knife results (since Sep 2007) 60 Average: / results # results TLD/Inst.

14 Depending on the dosimeter used, the magnitude of the additional correction, down to a 0.5 x 0.5 cm 2 field size can be as much as: 1% 1. 5% 2% 2. 2% 1% 3. No correction needed 91% 4. 13% 6% 5. 8%

15 The magnitude of the additional correction, down to a 0.5 x 0.5 cm 2 field size can be as much as: The answer is: 13% Francescon et al. Medical Physics, Vol. 38, No. 12, December 2011

16 TG-155 Small Field Dosimetry Recommendation: It is strongly recommended to independently verify dosimetric measurements in small fields, either through measurements carried out by a different person and/or though an independent external audit, such as that carried out by the RPC.

17 RPC Measurements during onsite visits Measurement configuration: depth = 10 cm, 100 cm SSD, Exradin A16 Secondary jaws kept at a 10 x x x

18 The Problem is that our Dragon is very small!

19 Tables of standard small field factors We even make mistakes Followill et al 2012 and also will be in TG-155

20 Corrected Varian Data 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 (cm x cm) 10 x 10 RPC Institution RPC Institution RPC Institution RPC Institution x x 6 (0.937)* (0.965) x (0.005) (0.008) (0.002) (0.003) (0.005) (0.007) (0.003) (0.006) (0.005)[0.8%] (0.008) (0.002) [0.5%] (0.003) (0.005) [0.4%] (0.007) (0.003) [0.5%] (0.006) [0.8%] (n=127) [0.5%] (n=22) [0.4%] (n=32) [0.5%] (n=41) (n=127) (n=22) (n=32) (n=41) x 4 (0.885) (0.924) (0.006) (0.013) (0.006) (0.010) (0.006) (0.009) (0.002) (0.009) (0.006) (0.013) (0.006) (0.010) (0.006) (0.009) (0.002) (0.009) [1.8%] [1.2%] [0.7%] [0.9%] [1.8%] [1.2%] [0.7%] [0.9%] (n=122) (n=25) (n=32) (n=37) (n=122) (n=25) (n=32) (n=37) 3 x (0.849) (0.884) (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 (0.802) (0.814) (0.008) (0.008) (0.020) (0.020) (0.005) (0.005) (0.015) (0.015) (0.011) (0.011) (0.040) (0.040) (0.007) (0.007) (0.020) (0.020) [2.9%] [2.0%] [2.2%] [1.6%] (n=136) (n=23) (n=33) (n=40)

21 On-Site Dosimetry Review Audit Discrepancies Discovered (Jan. 05 April 13) Discrepancies Regarding: Number of Institutions Receiving 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%) This is a beam measurement issue and TPS beam modeling challenge.

22 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

23 TG-155 Small Field Dosimetry Huge Resource of Published data: 239 references 5 Tables 15 Figures 13 Conclusions and recommendations

24 New IROC Houston audit OSLD

25 Commissioning and Testing Three dosimeters were used to estimate the dose for each field size: Exradin A16 Exradin D1V diode Exradin D1V diode PTW CC04 Exradin W1

26 First Test Irradiation away from RPC

27 All of the following are current methods to provide a verification of your small field dosimetry data except: 1% 1. Use of published data 8% 86% 4% 2% 2. Your 2 nd set of measurements 3. Rely on manufacturer s data 4. Remote audit 5. Indep. meas. from another physicist

28 All of the following are current methods to provide a verification of your small field dosimetry data except: The answer is: Reliance on the manufacturers data Ref: Das et al, AAPM TG 155 report, submitted to Med Phys

29 The challenge continues for the <2 x 2 cm 2 field sizes The correct dosimeter is essential. There are independent audits and standard data to help. We will defeat this dragon! Summary Conclusion

30 Thank You Questions?

31 References Francescon 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, Followill DS: Erratum, J Appl Clin Med Phys 15 (2): /2014.

32 Acknowledgements Work supported by PHS grant CA10953 awarded by NCI, DHHS