Acute toxicity of hypofractionated radiotherapy for Japanese breast cancer patients after surgery: a single center prospective observational study (HyPORT-BC).

Background: This single center prospective observational study was conducted to evaluate the acute toxicity of hypo-fractionated radiotherapy for Japanese breast cancer patients after surgery.

Methods: This study recruited patients who were scheduled for moderately hypo-fractionated radiotherapy including supraclavicular lymph node area (Cohort M) or ultra-hypo-fractionated radiotherapy for the conserved breast (Cohort U) as postoperative treatment for breast cancer. Radiotherapy plans were generated using automated planning system. Irradiation of 42.5 Gy/16 fractions (Cohort M) or 26 Gy/5 fractions (Cohort U) was delivered, and boost irradiation of 10 Gy/5 fractions was added as needed. The primary endpoint was the proportion of grade ≥ 2 acute adverse events within 90 days. The toxicities were evaluated using CTCAE ver 5.0.

Results: Between January 2023 and December 2023, 123 patients (81 in Cohort M and 42 in Cohort U) were enrolled. All the included patients were Japanese and completed their planned radiotherapy and were also able to be evaluated for acute adverse events. Grade 1/2/3-5 acute adverse events were observed in 67/12/0 for Cohort M and 31/4/0 for Cohort U. The proportion of grade ≥ 2 acute adverse events within 90 days was 15% (95% confidence interval 8-24%) for Cohort M and 10% (95% confidence interval 3-23%) for Cohort U.

Conclusions: The proportion of acute toxicity of hypo-fractionated radiotherapy for Japanese breast cancer patients after surgery was shown to be acceptable in this study.