Improved Dosimetry and Plan Quality for Accelerated Partial Breast Irradiation Using Online Adaptive Radiation Therapy: A Single Institutional Study

Abstract

PurposeAccelerated partial breast irradiation (APBI) is an attractive treatment modality for eligible patients as it has been shown to result in similar local control and improved cosmetic outcomes compared to whole breast radiotherapy. The use of online adaptive radiotherapy (OART) for APBI is promising as it allows for a reduction of PTV margins since breast motion and lumpectomy cavity volume changes are accounted for in daily imaging. Here we present a retrospective, single-institution evaluation on the adequacy of kV-cone beam computed tomography (CBCT) OART for APBI treatments. Methods and Materials19 patients (21 treatment sites) were treated to 30 Gy in 5 fractions between January of 2022 and May of 2023. Time between simulation and treatment, change in gross tumor (i.e. lumpectomy cavity) volume (GTV), and differences in dose volume histogram (DVH) metrics with adaption were analyzed. The Wilcoxon paired, non-parametric test was utilized to test for DVH metric differences between the scheduled plans (initial plans recalculated on daily CBCT anatomy) and delivered plans, either the scheduled or adapted plan, which was re-optimized using daily anatomy. ResultsMedian (inter-quartile range) time from simulation to first treatment was 26 days (21-32 days). During this same time, median GTV volume reduction was 16.0% (7.3-23.9%) relative to simulation volume. Adaptive treatments took 31.3 min (27.4-36.6 min) from start of CBCT to treatment session end. At treatment, the adaptive plan was selected for 86% (89/103) of evaluable fractions. In evaluating plan quality, 78% of delivered plans met all target, OAR, and conformity metrics evaluated, compared to 34% of scheduled plans. ConclusionsUse of OART for stereotactic linac-based APBI allowed for safe, high-quality treatments in this cohort of 21 treatment courses. Although treatment delivery times were longer than traditional stereotactic body treatments, there were notable improvements in plan quality for APBI using OART.