Are We Missing Acute Toxicities Associated with Hypofractionated Breast Irradiation? A Report from a Large Multi-Center Cohort Study.

PURPOSE/OBJECTIVE(S): The efficacy and long-term safety of hypofractionated whole breast irradiation (HF-WBI) has been established through multiple randomized trials, yet data about acute toxicities remains more limited. Since 2013, our group has prospectively collected acute toxicity data from weekly treatment evaluations and additional assessment after completion. In 2016, we intentionally shifted the post-treatment assessment follow-up (f/u) visit from 1 month to 2 weeks in order to evaluate for missed acute toxicity occurring in that immediate post-treatment window. Here we report whether 2-wk f/u has resulted in increased detection of acute toxicities as compared with 4-wk f/u.

MATERIALS/METHODS: We prospectively compared acute toxicity for patients treated with HF-WBI between 1/1/2013 and 8/31/2015 ("4 wk f/u cohort") to patients treated between 1/1/2016 - 8/31/2018 ("2 wk f/u cohort"). Analyses included a multivariable model that adjusted for other factors known to correlate with toxicity. We prospectively defined acute toxicity as maximum breast pain (moderate or severe rating) and/or occurrence of moist desquamation reported 7 days prior to the completion of RT until 42 days following completion.

RESULTS: 2689 patients who received post-lumpectomy radiation and boost were analyzed, 1862 patients in the 2-wk f/u cohort and 827 in the 4-wk f/u cohort. All acute toxicity measures assessed were statistically similar between follow-up cohorts when compared in an unadjusted fashion. Overall acute composite toxicity was 26.4% and 27.7% for patients during the 4-week follow-up and 2-week follow-up cohorts, respectively. Overall acute composite toxicity remained similar between follow-up cohorts in a multivariable, adjusted model and was significantly related to patient's age, BMI, smoking status, and with treatment technique (IMRT vs 3D-CRT) but not follow-up cohort.

CONCLUSION: An earlier post-treatment follow-up for HF-WBI patients did not reveal a significant increased incidence of acute toxicities at 2 weeks compared to 4 weeks. This study provides physicians and patients with additional data on the safety and tolerability of HF-WBI for early stage breast cancer.