Patient DVH based QA metrics using the Planned Dose Perturbation Algorithm

Transcription

1 Session V: New Therapy Technologies Patient DVH based QA metrics using the Planned Dose Perturbation Algorithm March 31, 2012 Hosang Jin, Ph.D. Assistant Professor University of Oklahoma

2 Conflict of Interest This talk mainly deals with a commercial product 3DVH from the Sun Nuclear Corporation. The speaker has not received any research funding from the company and has no disclosures.

3 Outline Conventional IMRT QA metric Patient DVH based QA Planned Dose Perturbation (PDP) Algorithm DVH based QA using PDP (clinical cases) Limitations of PDP Other per patient dose QA methods Conclusions

4 Conventional IMRT QA methods Per patient phantom measurement (dose in phantom) Ion chamber+film or 2D/3D diode/ic array test Proposed by Dan Low in 1998 Based on tolerance of dose difference and DTA (distance to agreement) Tolerance: 95% passing rate with 3% and 3 mm (most commonly used; very site and machine specific) ( r m ) 2 m c m c min 2 2 r( r, r ) d ( r, r ) D 2 r c

5 Gamma test (SNC MapCHECK2) Measurement Plan

6 Gamma test with a wrong beam? Measurement Plan

7 Clinically approved original plan Error induced plan by changing optimization constraints

8 Insensitivity of the test Acceptable plan Unacceptable plan IBA I mrt Matrixx 2D measured dose distribution Binary gamma plots: red pixels are points that failed a 2%/2 mm gamma analysis Kruse, Med Phys, 37, 2010

9 Insensitivity of the test There is no clear distinction in overall gamma score between the acceptable and unacceptable IMRT plans. While planar dosimetry may comprise one facet of an effective IMRT QA protocol, gamma scores could not reliably identify a plan with poor dosimetric accuracy. Kruse, Med Phys, 37, 2010

10 + IMRT QA performed in phantom geometry not in patient geometry

11 Patient DVH based QA Step 1: Conventional per beam QA or arc QA (ArcCHECK or MapCHECK) Dose error per each beam (2D) Step 3: Comparison (Reference (original) vs. Comparison (perturbed)) Treatment planning system Independent workstation (3DVH) Step 2: Planned dose perturbation (PDP) Perturb ed dose (3D)

12 What is PDP? Any errors detected by the conventional per beam planar dose QA method is used to perturb the original 3D patient dose. PDP uses perturbation methodology designed specifically for Compton effects of high energy photons. PDP alters dose only if and where dose differences are detected in conventional dosimetry array systems. PDP does not require secondary dose calculation that is a new source of error. Conventional IMRT QA 2D error mask [Dose difference + local percentage errors] Note: the test and passing rates are NOT stored/used. A built in PDP model Error mask Dose along the beam perturbed by error Accumulating the total dose perturbation over all voxels and beams CMF: contribution modifying function Zhen et al, Med Phys, 38, 2011

13 Accuracy of PDP White paper Error free plan Error induced plan 2D simulated measurements 2D calculation distribution 2D error masks PDP 3D errorfree plan Compa rison 3D PDPcorrected plan 3D errorinduced plan

14 NOTE: No actual QA measurements were not performed; free from measurement induced errors

15 15 clinical IMRT cases Varian Eclipse (AAA) and 2100C EDR2+a Farmer chamber

16 Results Chamber: Difference: ~1% (when corrected for 1% diode array offset, 3DVH vs. chamber is statistically same.) Film Conclusions

17 Clinical case I: Prostate boost Reference: Planning Comparison: PDP Prostate boost: 2160 cgy Total number of fields: 7; Total MUs: 387 D test passing rate (3%/3 mm): composite 98.3%, perbeam QA: %

18 Dose difference histogram 3D gamma test 1%/1 mm: 83.9% 2%/2 mm: 99.5% 3%/3 mm: 100.0%

19 Clinical case II: Head and Neck Reference: Planning Comparison: PDP Tongue: 7200 cgy Total number of fields: 9, Total MUs: 1077 D test passing rate (3%/3 mm): composite 99.8%, perbeam QA: %

20 Dose difference histogram 3D gamma test 1%/1 mm: 62.2% 2%/2 mm: 96.0% 3%/3 mm: 99.8%

21 Limitation I Low resolution High resolution

22 Limitation I cont d Low resolution High resolution

23 Limitation II Sun Nuclear: White paper What if the heterogeneity correction of the TPS is inaccurate?

24 Limitation III Conventional QA procedure Treatment planning (Plan approval) IMRT QA Pass Treatment Fail (e.g.) 95% pass rate with 3%/3 mm New QA procedure Treatment planning (Pre Plan approval) QA delivery DVH based QA (Post plan approval) Pass Treatment Fail Acceptable tolerances? Action levels? Physician s review? Zhen et al, Med Phys, 38, 2011

25 Dose QA method I: Direct fluence map Fluence map EPID 2D array (IC) Step 2: 3D dose calculation using measured fluence maps and patient CT data Step 3: Comparison/Analysis Step 1: Measurement of direct fluence map Patient geometry QA Clinically relevant comparison (DVH and 3D dose difference) More sources of error: de convolution of fluence map and independent dose calculation Need of commissioning of systems Commercially available: OSL Dosimetry Check (EPID) and IBA Compass (IC array) systems

26 Dose QA method II transmission dosimetry EPID Step 1: Measurement during treatment Step 2: 3D dose reconstruction using backprojection algorithm Step 3: Comparison/Analysis Wendling et al, Med Phys, 36, 2009 Actual patient geometry Potentially adaptive treatment Dose reconstruction: source of error Need of pre treatment QA

27 Conclusions It should be noted that high passing rates in conventional QA do not alone imply accurate dose calculation and/or delivery The PDP algorithm was shown to accurately predict the DVH impact and clinically relevant dose using conventional planar QA results. However, it could introduce more complex and inefficient QA in the busy clinic. Most importantly, acceptable tolerances, action levels, and potential changes in QA procedures should be explored extensively. Limitations of the PDP should be further investigated.

28 Acknowledgement Imad Ali, Ph.D. Salahuddin Ahmad, Ph.D. Vance Keeling Stacey Geier (Sun Nuclear; providing 3DVH materials)