CBCT-Guided Online Adaptive Ultrahypofractionated Whole-Breast Radiotherapy for Early-Stage Breast Cancer: Dosimetric Outcomes and Acute Toxicity

Ultrahypofractionated whole-breast radiotherapy reduces treatment burden but increases the importance of accurate dose delivery at each fraction. This retrospective single-center study evaluated cone-beam computed tomography-guided online adaptive radiotherapy in 55 patients with early-stage breast cancer treated with 26 or 27 Gy in five fractions. Dosimetric analysis was available for 52 patients and 259 paired scheduled and adapted fractions. Linear mixed-effects models with a patient-specific random intercept were used to account for repeated fractions within individual patients. The adapted plan was selected for 242 out of 260 delivered fractions (93.1%). Mean planning target volume V95% increased from 93.52 ± 2.94% with the scheduled plan to 98.13 ± 1.04% with adaptation (p < 0.001), with reduced interfraction variability. Ipsilateral lung V30% increased modestly from 13.94% to 14.96% (p < 0.001). In left-sided cases, no significant differences were observed for heart V25% (1.400% vs. 1.395%; p = 0.3825) or left anterior descending coronary artery D0.03 cm3 (1.771 vs. 1.873 Gy; p = 0.1390). Acute grade 1 toxicity occurred in five patients (9.1%), with no grade ≥ 2 events. Mean treatment-session duration was 28 min. Online adaptation significantly improved daily target coverage and was technically feasible within a five-fraction whole-breast workflow. Acute toxicity was limited in this cohort, while longer follow-up and cumulative dose analyses are needed to further define its impact on organ-at-risk exposure and clinical outcomes.