IMRT QA using an EPID

1. IMRT QA using an EPID  �Randy Holt, Ph.D., DABR North Valley Radiation Oncology Enloe Medical Center, Chico, CA

2. Holt EPI Dosimetry ACMP 2005 Orlando 2 Advantages of EPID 1) No Film Processing 2) Immediate feedback 3) No heavy awkward cassettes 4) Large Fields are a problem � Solution coming? 5) DIGITAL IMAGE PROCESSING 6) Review at any workstation

3. Holt EPI Dosimetry ACMP 2005 Orlando 3 Amorphous Silicon

4. Holt EPI Dosimetry ACMP 2005 Orlando 4 Amorphous Silicon EPID

5. Holt EPI Dosimetry ACMP 2005 Orlando 5 EPID as a physics tool No film to take or store No film scanner to warm up No cumbersome film scanning software Linear Dose response Programmable via R&V New analysis tools Can be used for both commissioning and long term QA Can�t completely replace film

6. Holt EPI Dosimetry ACMP 2005 Orlando 6 EPID as a physics tool Flatness/Symmetry Collimator radiation Light Field Radiation Field MLC position tests Does it have a role in IMRT???

7. Holt EPI Dosimetry ACMP 2005 Orlando 7 EPID relative dose linearity

8. Holt EPI Dosimetry ACMP 2005 Orlando 8 Predicting the EPI Dose Meausrements at cassete are not �fluence� Cassette has inherent scatter. Must measure and model scatter kernel. McCurty, et.al.

9. Holt EPI Dosimetry ACMP 2005 Orlando 9 EPI Dose Kernel Assume convolution holds, start with known f(t). Measure output, h(t) Compute smearing function, g(t) = scatter kernel Direct solution, g(t) = IFT(FT(h(t))/FT(g(t))) Noise sensitive � use iterative de-convolution

10. Holt EPI Dosimetry ACMP 2005 Orlando 10 EPI Dose Kernel Method Errors No tolerance for �delay� error (spatial delay is a simple translation). Cassette must be perfectly centered, and at correct distance. Point source model based, so non-focal output elements will introduce error at gradients. Output not a pure radial function, pure symmetry and axis-variable flatness.

11. Holt EPI Dosimetry ACMP 2005 Orlando 11 Commissioning: dMLC Stress Test OF course you can do this stuff on film too.OF course you can do this stuff on film too.

12. Holt EPI Dosimetry ACMP 2005 Orlando 12 dMLC Commish: Fluence Edge vs. Gantry Angle Series of six gantry angles. When I originally did this series of tests with film, it took me almost two days with film and scanner to get believable results. With the EPID, I did this series in an evening. How do you quickly analyzeSeries of six gantry angles. When I originally did this series of tests with film, it took me almost two days with film and scanner to get believable results. With the EPID, I did this series in an evening. How do you quickly analyze

13. Holt EPI Dosimetry ACMP 2005 Orlando 13 Dose Gradient Analysis TG-23, TG-40, TG-53 How to apply to IMRT? But a group from St. Louis came up with a nice metric, which they have recently improvedBut a group from St. Louis came up with a nice metric, which they have recently improved

14. Holt EPI Dosimetry ACMP 2005 Orlando 14 Gamma Analysis Gamma is a Difference-Distance Function with thresholds, e.g. 3mm, 3% Gamma <= 1 ? Pass Gamma > 1 ? Fail Poor GammaPoor Gamma

15. Holt EPI Dosimetry ACMP 2005 Orlando 15 Marginal Gamma By the way, this was artificially poor. Good gammaBy the way, this was artificially poor. Good gamma

16. Holt EPI Dosimetry ACMP 2005 Orlando 16 Better Gamma So while I didn�t have this marvelous tool when I originally commissioned, I wouldn�t have been nearly as ill-tempered by the time I did get the program rolling. A bit later I�ll tell you about how we are beginning to use this device for ongoing QA. The next item in our commissioning checklist is , So while I didn�t have this marvelous tool when I originally commissioned, I wouldn�t have been nearly as ill-tempered by the time I did get the program rolling. A bit later I�ll tell you about how we are beginning to use this device for ongoing QA. The next item in our commissioning checklist is ,

17. Holt EPI Dosimetry ACMP 2005 Orlando 17 EPI-Dosimetry: Some caution Careful continuous calibration is required. Also, the cassette sags, and CAX adjustment is a constant Careful continuous calibration is required. Also, the cassette sags, and CAX adjustment is a constant

18. Holt EPI Dosimetry ACMP 2005 Orlando 18 dMLC Wedge: Morning Warmup Wedge Gamma FasterWedge Gamma Faster

19. Holt EPI Dosimetry ACMP 2005 Orlando 19 dMLC Wedge Gamma Monthly testsMonthly tests

20. Holt EPI Dosimetry ACMP 2005 Orlando 20 Monthly QA: dMLC H-stress test Other QA reduction - Dose and transfer verificationOther QA reduction - Dose and transfer verification

21. Holt EPI Dosimetry ACMP 2005 Orlando 21 Current �Standard� for IMRT QA Test each fluence map for agreement Take phantom measurements of each beam and composite Take composite fluence map film and compare to TPS prediction

22. Holt EPI Dosimetry ACMP 2005 Orlando 22 Proposed Standard for IMRT QA Prove that MLCs can deliver a fluence map Prove that your TPS can predict the dose delivered by that fluence map Implement and conduct daily, weekly and monthly QA on the MLC and TPS system Run �Hand Calcs� and Fluence Map verification using an Independent Algorithm (IMSure, RadCalc, MU Check) or Portal Dosimetry Take additional measurements if warranted

23. Holt EPI Dosimetry ACMP 2005 Orlando 23 Summary PDI shows promise at reducing effort for ongoing QA PDI can be a strong, and rapid tool for acquiring valuable physics data. Some time, effort and understanding of the physics behind the method is required � but the payoff is reduced overall physics effort. When they ask you when they can start the first patient tell them�When they ask you when they can start the first patient tell them�

24. Holt EPI Dosimetry ACMP 2005 Orlando 24 Acknowledgments