CBCT-Guided Online Adaptive Radiation Therapy for Accelerated Partial Breast Irradiation: A Single-Institution Experience

Purpose/Objectives: Accelerated partial breast irradiation (APBI) using stereotactic body radiation therapy (SBRT) offers shorter treatment, comparable local control, and improved cosmesis for early-stage breast cancer (ESBC). Cone beam CT (CBCT)-guided online adaptive radiation therapy (OART) enhances target coverage and minimizes normal tissue exposure. This retrospective study evaluates clinical characteristics and outcomes in patients treated with APBI using OART. Materials/Methods: From January 2022 to May 2024, 40 patients (including 2 with bilateral disease) received SBRT with OART. Targets included the lumpectomy cavity (GTV), a 1 cm CTV margin, and a 3 mm PTV margin. Patients received 30 Gy in 5 fractions. Daily CBCT guided alignment and adaption of targets and organs-at-risk was performed. Either the scheduled or adapted plan was selected each day. Outcomes were assessed at 1 month and most recent follow-up using CTCAE criteria. Results: Patients (ages 50-83) had predominantly right-sided (59.5%) and upper outer quadrant tumors (61.9%), with a median size of 0.9 cm. The median interval from lumpectomy to radiation was 62 days; treatment lasted a median of 9 days. The adapted plan was used in 90% of fractions. At 1 month, 48% had no toxicity; all reported events were grade 1. At a median 14.3-month follow-up, most toxicities remained grade 1; 3 patients had grade 2 effects. No recurrences were observed. Conclusions: This study, the largest APBI with OART cohort to date, demonstrates excellent cosmesis, minimal toxicity, and promising early oncologic outcomes for ESBC patients.