Enhancing jawless VMAT plan quality for hypofractionated left breast cancer with the avoidance structure tool

Abstract

Introduction: The Avoidance Structure (AvS) tool shields a structure by maintaining the multileaf collimator leaves over the structure's projection in the beam's eye view. In this work, we tested the dosimetric impact of using part of the outlines of the heart and lungs as avoidance structure when creating knowledge-based left breast volumetric modulated arc therapy treatment plans. Methods: Dose-volume histogram estimates were calculated using a RapidPlan model for 30 patients who received whole breast radiation therapy in five fractions. Then, two sets of plans were generated for a Halcyon v2 linear accelerator by alternating the use of the AvS tool. Dose metrics for both procedures were compared. Results: Coverage on the target volume remained similar regardless of the use of AvS. The mean absorbed dose received by the heart, as well as the relative volumes receiving 1.5, 3.5 and 7.8 Gy, decreased with effect sizes (ESZ) 0.98, 1.43, and 0.63 respectively. Homolateral lung volumes receiving 1.5 and 3.5 Gy were also lowered using AvS. However, the dose received by 0.03 cc of the contralateral breast increased with an ESZ of 0.52. Conclusions: Except for the contralateral breast, the AvS tool improved DVH metrics for organs at risk without compromising target coverage or worsening hotspots.