Developing a class solution for Prostate Stereotactic Ablative Body Radiotherapy (SABR) using Volumetric Modulated Arc Therapy (VMAT)

Background and purpose: To develop a class solution for prostate Stereotactic Ablative Radiotherapy (SABR) using Volumetric Modulated Arc Therapy (VMAT). Materials and methods: Seven datasets were used to compare plans using one 360 degrees arc (1FA), one 210 degrees arc (IPA), two full arcs and two partial arcs. Subsequently using 1PA, fifteen datasets were compared using (i) 6 mm CTV-PTV margins, (ii) 8 mm CTV-PTV margins and (iii) including the proximal SV within the CTV. Monaco (TM) 3.2 (Elekta (TM)) was used for planning with the Agility (TM) MLC system (Elekta (TM)). Results: Highly conformal plans were produced using all four arc arrangements. Compared to 1FA, 1PA resulted in significantly reduced rectal doses, and monitor units and estimated delivery times were reduced in six of seven cases. Using 6 mm CTV-PTV margins, planning constraints were met for all fifteen datasets. Using 8 mm margins required relaxation of the uppermost bladder constraint in three cases to achieve adequate coverage, and, compared to 6 mm margins, rectal and bladder doses significantly increased. Including the proximal SV required relaxation of the uppermost bladder and rectal constraints in two cases, and rectal and bladder doses significantly increased. Conclusions: Prostate SABR VMAT is optimal using IPA. 6 mm CIV-PTV margins, compatible with daily fiducial-based IGRT, are consistently feasible in terms of target objectives and OAR constraints. (C) 2013 The Authors. Published by Elsevier Ireland Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).