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Investigating VMAT Planning Technique to Reduce Rectal and Bladder Dose in Prostate Cancer Treatment Plans

S Rana

S Rana1*, C Cheng2, (1) ProCure Proton Therapy Center, Oklahoma City, OK, (2) Vantage Oncology, West Hills, CA

SU-E-T-611 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The purpose of this study was to compare the dosimetric quality of new partial-double arc (partial-DA) technique in volumetric-modulated arc therapy (VMAT) planning of prostate cancer.

Methods:In this retrospective study, twelve cases of low-risk prostate cancer were included. For each case, a full-double arc (DA) plan was created using two arcs in Varian Eclipse (version 11). The partial-DA plans were then created using identical dose-volume optimization values and beam parameters as in full-DA plans except for arcs which included anterior (30degree) and posterior (60degree) avoidance sectors. All plans were calculated by analytical anisotropic algorithm. For planning target volume (PTV), maximum dose (Dmax), mean dose (Dmean), and conformity index (CI) were evaluated. For bladder and rectum, volumes that received 70,50,40,and 20Gy (V70,V50,V40 and V20, respectively),and Dmean were compared. For femoral heads, V20 and Dmean were evaluated. Additionally, monitor units (MUs) and integral dose were compared.

Results:Doses to PTV were comparable in partial-DA and full-DA plans with an average difference ±0.2%. The CI (1.24; p=0.960) was same for both sets of plans. Dose to rectum was always lower in partial-DA plans by average differences of 1.8%(p=0.183) for V70, 9.2%(p=0.014) for V50, 22.6%(p<0.000) for V40, 19.4%(p=0.028) for V20, and 9.5%(p<0.000) for Dmean. Similarly, dose to bladder was always lower in partial-DA plans by average differences of 10.2%(p=0.002) for V70, 31.0%(p=0.001) for V50, 40.9%(p=0.001) for V40, 21.8%(p=0.001) for V20, and 18.5%(p<0.000) for Dmean. In contrast, partial-DA technique delivered higher dose to femoral heads by average differences of 52.2%(p<0.000) for V20 and 21.2%(p<0.000) for Dmean. Partial-DA plans produced higher number of MUs (7.5%,p=0.005) and integral dose (5.5%,p<0.000).

Conclusion:In comparison to full-DA technique, our purposed partial-DA technique reduced rectal and bladder dose significantly but with higher femoral head dose, which was nonetheless within planning criteria, for the same PTV coverage.

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