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

Abstract

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° arc (1FA), one 210° arc (1PA), two full arcs and two partial arcs. Subsequently using 1PA, fifteen datasets were compared using (i) 6mm CTV-PTV margins, (ii) 8mm CTV-PTV margins and (iii) including the proximal SV within the CTV. Monaco 3.2 (Elekta) was used for planning with the Agility MLC system (Elekta).

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 6mm CTV-PTV margins, planning constraints were met for all fifteen datasets. Using 8mm margins required relaxation of the uppermost bladder constraint in three cases to achieve adequate coverage, and, compared to 6mm 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 1PA. 6mm CTV-PTV margins, compatible with daily fiducial-based IGRT, are consistently feasible in terms of target objectives and OAR constraints.

Keywords: Image Guided RadioTherapy (IGRT); Prostate cancer; Stereotactic Ablative Body Radiotherapy (SABR); Volumetric Modulated Arc Therapy (VMAT).