Tolerance of Adjuvant Ultrahypofractionated Whole-Breast Radiation Therapy Employing Moderately Hypofractionated Sequential Boost: A Single Institution Analysis.

Purpose: This analysis evaluates early and intermediate treatment tolerance in a prospective observational cohort study of patients who underwent adjuvant ultrahypofractionated whole-breast radiation therapy (uhWBRT), with or without moderately hypofractionated sequential boost, following breast-conserving surgery. Methods and Materials: uhWBRT was introduced in our department in March 2020. Data from 436 patients with breast tumors not requiring lymphatic irradiation were analyzed, including 376 with invasive carcinomas (pT1-pT3) and 60 with ductal carcinoma in situ. The mean age was 62 years (range, 26-85). Acute reactions (Common Terminology Criteria for Adverse Events v4.03) were assessed at radiation therapy completion and after 2 to 3 weeks. Late effects and patient-reported outcomes (Late Effects in Normal Tissues-Subjective, Objective, Management and Analytic and Harvard for Cosmesis) were evaluated at 6 months after radiation therapy and annually thereafter. The prescribed uhWBRT dose was 26 Gy in 5 daily fractions. A sequential boost of 10.0 to 12.5 Gy in 4 to 5 daily fractions was administered to 338 patients (77.5%), while 98 (22.5%) did not receive a boost. Results: Acute toxicity grades 0, 1, and 2 were observed in 29.8%, 59.9%, and 10.3% of patients, respectively, at radiation therapy completion and 52.1%, 40.8%, and 7.2% of patients at 2 to 3 weeks after radiation therapy. Grade 2 late effects were identified in 5.3%, 2.0%, 1.8%, 1.1%, and 0%, and grade 3 late effects were identified in 1.5%, 2.3%, 0.9%, 0%, and 0% of patients at 6 months, 1 years, 2 years, 3 years, and 4 years. Patient-reported outcomes for cosmesis were rated as good or excellent in 97.7% of patients. After a mean follow-up of 18 months (median 14, range, 0-48), 1 local failure, 2 nodal failures, and 9 distant relapses were detected. Three deaths were reported, all nontumor-related. Conclusions: Early and intermediate results indicate that the treatment schedules, including the moderately hypofractionated boost, are safe and well tolerated, with acute toxicity rates comparable to those in the FAST-Forward trial. Although our study follow up is relatively short, our findings indicate that uhWBRT, with or without a moderately hypofractionated boost, is safe and well tolerated.