SU-E-T-477: IMPT Vs. VMAT: A Treatment Planning Study for High-Risk Prostate Cancer.

PURPOSE: The purpose of this study was to compare the dosimetric results between intensity modulated proton therapy (IMPT) and volumetric modulated arc therapy (VMAT) plans for a high-risk prostate cancer.

METHODS: Six high-risk prostate cancer cases were included in this retrospective study. For each case, IMPT plans were generated using multiple field optimization (MFO) technique (2 fields) with XiO treatment planning system (TPS), whereas VMAT plans were generated using RapidArc technique (2 full arcs) with Eclipse TPS. IMPT and VMAT plans were optimized for a total prescription dose of 79.2 Gy[RBE] and 79.2 Gy, respectively. IMPT and VMAT plans were compared using physical dose-volume parameters. The absolute differences in a dosimetric parameter between IMPT and VMAT plans were calculated. The results were averaged over six cases.

RESULTS: For the planning target volume (PTV), the mean dose was comparable between IMPT (80.65 Gy[RBE]) and VMAT (80.33 Gy) plans, whereas the PTV coverage was slightly better in IMPT (97.41%) than in VMAT (95.00%) plans. The mean dose to the rectum and bladder were lower in IMPT (17.68 Gy[RBE] and 15.70 Gy[RBE], respectively) when compared to VMAT (42.17 Gy and 30.29 Gy, respectively) plans. The rectal V₇₀, V5₀, and V₃₀ were lower in IMPT plans than in VMAT plans by 4.91%, 28.34%, and 46.74%, respectively. For bladder, V₇₀ was comparable (8.92 Gy(RBE) vs. 9.03 Gy), whereas V5₀ and V₃₀ were lower in IMPT plans by 6.25% and 25.92%, respectively, when compared to VMAT plans. The mean doses to the left and right femoral heads were lower in VMAT plans (18.08 Gy and 21.91 Gy, respectively) than in VMAT plans (31.35 Gy[RBE] and 29.52 Gy[RBE], respectively)

CONCLUSION: Both IMPT and VMAT plans provided excellent target coverage and acceptable femoral head dose. IMPT provided better tissue sparing for the rectum and bladder compared to VMAT.