Measurement Guided Dose Reconstruction (MGDR) Transitioning VMAT QA from phantom to patient geometry

Transcription

1 Measurement Guided Dose Reconstruction (MGDR) Transitioning VMAT QA from phantom to patient geometry Raj Varadhan, PhD, DABMP Minneapolis Radiation Oncology

2 Conflict of interest None Acknowledgement: Jim Ernsberger, SNC.

3 Transitioning from Phantom geometry to patient geometry for VMAT H & N QA: Purpose: The purpose of this study is to a) investigate the accuracy of patient DVH based QA using volumetric measurement guided dose reconstruction (MGDR) for head & neck Volumetric Modulated Arc Therapy (VMAT) delivery by benchmarking against RPC Head and Neck Phantom results, b) investigate the correlation of 3D detector phantom based QA using local gamma and global gamma results when compared against patient DVH based QA. Methods: The RPC head and neck phantom was planned and irradiated using 2 arc VMAT delivery method. The results of point doses from RPC TLDs at eight locations and dose profiles from radiochromic film through center of primary PTV inside the phantom were compared against the treatment planning system (TPS) and the results from the MGDR analysis based on 3D detector phantom QA. Also, the 3D gamma value was calculated using the MGDR analysis which compares the TPS dose to the predicted dose distribution. Five head and neck patients were planned using VMAT delivery method and the QA was performed using both 3D detector phantom based QA and patient DVH based analysis using MGDR. The correlation between gamma pass rates using both global and local gamma criteria was compared against the gamma pass rates using MGDR analysis. Results: The ratio of MGDR calculated dose to primary and secondary planning target volume (PTV) compared with both RPC TLD and Eclipse TPS doses was The ratio of estimated cord dose from MGDR analysis was 1.09 and 1.03 when compared to RPC TLD and TPS doses respectively. The displacement between calculated dose gradient in the region between primary PTV and organ at risk (OAR) in all 3 planes from MGDR was within 3mm and 1mm when compared to RPC film and TPS profiles respectively. The average 3D local gamma pass rate for the five clinical cases using MGDR was 97.5% and 92% when using 3% 3mm and 2%2mm analysis criteria respectively. The average gamma pass rate using global gamma 3%3mm criteria was 99.8%, compared to an average local gamma pass rate of 87.1% using 3D detector phantom based QA with a maximum increase of 18%.

4 Conclusion: Benchmarking the accuracy of patient DVH based QA results to the RPC head and neck phantom established a baseline accuracy and confidence in use of MGDR analysis for VMAT delivery in a realistic patient geometry. For the 5 clinical test cases studied, the low pass rates obtained using the local gamma evaluation criteria in phantom based QA had no significant clinical impact for the patient when evaluated using DVH based QA.

5 Objectives Background Status Quo adequate? Review of Physics behind MGDR Benchmarking accuracy of MGDR & comparison with traditional phantom QA Conclusions

6 Background: Traditional VMAT/RapidArc QA employs detector based geometry using the ubiquitous 3%/3mm dose and DTA threshold for gamma QA pass rates. The traditional QA detects errors (sensitivity) However no information on the size and spatial location of errors in patient geometry. (specificity) Is status quo enough or accurate to detect clinically meaninful errors?

7 VMAT QA Every patient receives a patient specific QA. ACR/ASTRO Practice guideline definition for IMRT/VMAT:. accuracy of dose delivery should be documented by irradiating a phantom containing a calibrated dosimetry system to verify dose delivered is same as dose planned MGDR refers to a solution where errors in QA can be meaningfully correlated to patient specific geometry and structures

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9 Gamma QA a) Per beam, planar IMRT QA passing rates do not predict clinically relevant patient dose errors, B. Nelms et al Med Phys, 38 January 2011:

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11 Moving from gamma passing rates to patient DVH based QA metrics in pretreatment dose QA, Zhen H. et al., Med. Phys Oct;38(10):

12 Is current QA/methodology adequate?

13 What does conventional QA results mean?

14 How to reconstruct 3Ddose? Commercially available ArcCheck 3DVH Delta 4 DVH anatomy EPID Math Resolutions etc.

15 ArcCHECK based MGDR In BEV mode, the detector geometry is invariant of gantry angle

16 PDP or 3DVH

17 3DVH

18 Physics of MGDR Basic premise is to use the errors measured in phantom geometry to perturb the TPS dose by a correction factor that faithfully represents the ACTUAL dose delivered to the patient.

19 6 step process going from phantom to patient in 3DVH Step 1 AC Phantom measurement MUST make the measurement with PMMA plug

20 Step 2 Synchronize angle with time AC dose is updated every 50 ms ( in its own clock) RT Plan control points are a function of gantry angle NOT time In VMAT gantry angle is not linear with time (variable speed) Need to synchronize CP with Dose(t) SNC does with virtual gantry angle inclinometer

21 Step 2 Virtual Inclinometer

22 Step 3 Calculate RELATIVE dose in uniform PMMA phantom Divide the dynamic RT plan(cp) into sub beams Process the time stamped sub beams to create the fluence Perform convolution type calculation for each sub beam to derive RELATIVE dose

23 Step 4 Convert sub beam dose to absolute dose Use entrance and exit measured absolute dose Convert relative dose to semi empirical absolute dose Called morphing because calibration factor changes along the ray Larger weight given near the entrance diode vs exit diode

24 Step 5 Sum the sub beams in phantom

25 Step 6 Make the jump to patient geometry

26 PDP Model Is it accurate? NOT interested in solid water and film Need validity in actual patient geometry We used RPC H & N Phantom

27 VMAT Head & Neck Treatment Delivery Benchmarking Accuracy of Patient DVH based QA with Radiological Physics Center (RPC) Head and Neck Phantom Results.

28 RPC HEAD & NECK PHANTOM

29 TLD Results comparison Dose units in cgy Location Eclipse Mean RPC TLD(Mean) 3DVH(MEAN) (RPC/3DVH) (RPC/TPS) Primary PTV sup. ant Primary PTV inf. ant Primary PTV sup. post Primary PTV inf. post Secondary PTV sup Secondary PTV inf Organ at risk sup Organ at risk inf

30 RPC Film vs Eclipse TPS

31 MGDR (green) vs Eclipse TPS Right Left Profile

32 RPC Film vs Eclipse TPS

33 MDGR vs TPS Anterior Posterior Profile

34 RPC Film vs Eclipse TPS

35 MGDR vs Eclipse TPS Superior Inferior Profile

36 3DVH analysis

37 Comparison of QA for Patient H & N VMAT plans Patient Arc check QA local gamma pass rate Arc check Global gamma 3D DVH local gamma pass 3D DVH 2% 2mm % 100% 98.20% 93.80% % 99.50% 97.5% 92% % 100% 99.40% 96.20% % 99.80% 99.20% 96.80%

38 ArcCHECK Phantom QA Patient 2

39 3DVH QA Patient 2

40 3DVH analysis

41 ArcCHECK Phantom QA Patient 3

42 3DVH Patient 3

43 CONCLUSIONS For VMAT, phantom geometry based gamma QA metrics have a weak correlation with actual dose delivered to patient. Need to take a step back and think what is the goal of QA? Are you checking the TPS model indirectly or errors that have patient specific clinical impact? If the answer is latter, then MGDR analysis is the likely solution.

44 Is current QA/methodology adequate?