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Evaluation of Patient DVH-Based QA Metrics for Prostate VMAT: Correlation Between Accuracy of Estimated 3D Patient Dose and MLC Position Error
N Kadoya1*, M Saito1 ,Y Fujita2 , M Ogasawara1 , K Ito1 , K Sato3 , K Kishi3 , S Dobashi4 , K Takeda4 , K Jingu1 , (1) Tohoku University School of Medicine, Sendai, Miyagi, (2) Tokai University School of Medicine, Isehara, Kanagawa, (3) Tohoku University Hospital, Sendai, Miyagi, (4) Tohoku University, Sendai, Miyagi
Presentations
TU-G-BRD-9 (Tuesday, July 14, 2015) 4:30 PM - 6:00 PM Room: Ballroom D
Purpose:The purpose of this study was to evaluate the accuracy of patient DVH-based QA metrics and test the hypothesis: measurement-guided 3D dose reconstruction (MGDR) captures the induced dose error.
Methods:We used 3DVH software with an ArcCHECK to estimate 3D patient dose by MGDR. Two different calculating modes of MGDR were used: “Normal Sensitivity” and “High Sensitivity”. Ten prostate cancer patients treated with hypo-fractionated VMAT (67.6 Gy/26 Fr) were studied. For the baseline plan, we induced MLC errors (-0.75, -0.5, -0.25, 0.25, 0.5 and 0.75 mm for each single bank), generated by in-house software. We compared the 3D patient dose estimated by 3DVH and that calculated by the treatment planning system. We evaluated the correlation between dose estimation error and MLC position error.
Results:Slopes of linear fit to dose estimation error versus MLC position error for mean dose and D95 to the PTV were 1.76 and 1.40% mm-1, respectively, for “Normal Sensitivity” and -0.53 and -0.88% mm-1, respectively, for “High Sensitivity”, showing better accuracy for “High Sensitivity” than “Normal Sensitivity”. On the other hand, the slopes for mean dose to the rectum and bladder and V55 to the rectum and bladder were -1.00, -0.55, -3.53 and -1.85% mm-1, respectively, for “Normal Sensitivity” and -2.89, -2.39, -6.24 and -4.11% mm-1, respectively, for “High Sensitivity”, showing significant better accuracy for “Normal Sensitivity” than “High Sensitivity”.
Conclusion:Our results showed that 3DVH had some residual error for both sensitivities, indicating the MGDR could capture a part of the induced error but not all of the induced error.Furthermore, we found that “Normal Sensitivity” might have better accuracy for the DVH metric for PTV and that “High Sensitivity” might have better accuracy for DVH metrics for the rectum and bladder. We must be willing to tolerate this residual error in clinical care.
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