Abstract
BACKGROUND AND PURPOSE: Daily differences in breast contour and arm positioning can result in frequent re-positioning and offline re-planning in postoperative breast radiotherapy (RT). Online adaptive radiotherapy (oART) enables daily adaptation of the radiotherapy treatment plan. This study aimed to implement oART in breast and axillary target volumes to eliminate the need for repositioning and replanning. MATERIALS AND METHODS: Patients referred for postoperative partial breast irradiation (PBI) and whole breast irradiation (WBI)/post-mastectomy radiotherapy (PMRT) with or without axillary levels I-II (5x5.2 Gy) or I-IV (15x2.67 Gy) were included. On-couch treatment times were evaluated as well as dosimetry, target volume, acute toxicity, and in-house satisfaction questionnaires. RESULTS: Seventy-nine patients (n = 66 PBI/WBI/PMRT, n = 13 WBI + axillary levels I-II/I-IV) were treated with postoperative RT (465 fractions). On-couch median time ranged from 14.5 (range: 10.1-29.2) minutes for right-sided WBI to 25.8 (range: 22.6-36.8) minutes for left-sided WBI + axillary levels I-II. All adapted treatment plans met the D98% ≥ 95 % coverage criteria for the target PTV. Relative breast CTV volumes for the adapted plan decreased in fraction 1 and increased in fractions 2-5 compared to the CTV volume on the planning CT. Toxicity was mainly grades 0 and 1 and patient satisfaction was good. CONCLUSION: Implementation of oART was feasible in all breast target volumes with acceptable treatment duration, only mild acute toxicity, favorable patient-reported comfort, and dosimetry that met clinical standards. Future work will focus on the implementation of oART in RT boost indications, and selection of breast cancer patients who will benefit most from oART.