Measurement Guided Dose Reconstruction (MGDR) Transitioning VMAT QA from phantom to patient geometry Raj Varadhan, PhD, DABMP Minneapolis Radiation Oncology
Conflict of interest None Acknowledgement: Jim Ernsberger, SNC.
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 1.05. 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%.
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.
Objectives Background Status Quo adequate? Review of Physics behind MGDR Benchmarking accuracy of MGDR & comparison with traditional phantom QA Conclusions
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?
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
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: 1037 44
Moving from gamma passing rates to patient DVH based QA metrics in pretreatment dose QA, Zhen H. et al., Med. Phys. 2011 Oct;38(10):5477 89
Is current QA/methodology adequate?
What does conventional QA results mean?
How to reconstruct 3Ddose? Commercially available ArcCheck 3DVH Delta 4 DVH anatomy EPID Math Resolutions etc.
ArcCHECK based MGDR In BEV mode, the detector geometry is invariant of gantry angle
PDP or 3DVH
3DVH
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.
6 step process going from phantom to patient in 3DVH Step 1 AC Phantom measurement MUST make the measurement with PMMA plug
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
Step 2 Virtual Inclinometer
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
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
Step 5 Sum the sub beams in phantom
Step 6 Make the jump to patient geometry
PDP Model Is it accurate? NOT interested in solid water and film Need validity in actual patient geometry We used RPC H & N Phantom
VMAT Head & Neck Treatment Delivery Benchmarking Accuracy of Patient DVH based QA with Radiological Physics Center (RPC) Head and Neck Phantom Results.
RPC HEAD & NECK PHANTOM
TLD Results comparison Dose units in cgy Location Eclipse Mean RPC TLD(Mean) 3DVH(MEAN) (RPC/3DVH) (RPC/TPS) Primary PTV sup. ant. 671 682 672.9 1.01 1.02 Primary PTV inf. ant. 680 704 679.8 1.04 1.04 Primary PTV sup. post. 692 707 671.7 1.05 1.02 Primary PTV inf. post. 691 725 686.7 1.06 1.05 Secondary PTV sup. 552 567 547.5 1.04 1.03 Secondary PTV inf. 552 568 553.2 1.03 1.03 Organ at risk sup. 282 299 275.4 1.09 1.06 Organ at risk inf. 270 282 263.4 1.07 1.04
RPC Film vs Eclipse TPS
MGDR (green) vs Eclipse TPS Right Left Profile
RPC Film vs Eclipse TPS
MDGR vs TPS Anterior Posterior Profile
RPC Film vs Eclipse TPS
MGDR vs Eclipse TPS Superior Inferior Profile
3DVH analysis
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 1 92.70% 100% 98.20% 93.80% 2 81.50% 99.50% 97.5% 92% 3 83.30% 100% 99.40% 96.20% 4 90.80% 99.80% 99.20% 96.80%
ArcCHECK Phantom QA Patient 2
3DVH QA Patient 2
3DVH analysis
ArcCHECK Phantom QA Patient 3
3DVH Patient 3
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.
Is current QA/methodology adequate?