Abstract: Background and purpose: The long-term safety and efficacy of hypofractionated whole-breast irradiation (HFWBI) have been well established in randomized trials, but little is known about the effect of HF-WBI with simultaneous boost of the tumor bed. This study aimed to assess the safety and efficacy of HF-WBI with simultaneous tumor bed boost after breast-conserving surgery in Chinese population. Methods: Patients with pT_1-2N₀M₀ invasive breast cancer, negative margins, clips marked tumor bed after breast-conserving surgery were prospectively and consecutively enrolled in this study. All patients received total dose of 40 Gy/15 Fx/3 weeks for whole breast, combined with simultaneous boost of tumor bed, total dose of 48 Gy/15 Fx/3 weeks. The acute radiotherapy-induced toxicities, cosmetic effect and prognosis were followed up after the treatment. Results: From Jan. 2015 to Aug. 2016, 358 patients were enrolled in the study. The median age was 45 years (range, 25-71 years), and 71.2% patients were premenopausal young women. A total of 276 (77.1%) patients had T1 tumors, and 82 (22.9%) patients had T2. During the treatment, 53.6% and 8.1% patients had Grade 1 and Grade 2 radiation dermatitis, respectively, mainly manifested as skin erythema (38.8%) or desquamation (41.3%). Thirteen patients experienced moist desquamation mainly in the nipple and areola area. No grade≥3 radiation dermatitis was observed. Four patients had Grade 1 and one patient had Grade 3 radiation pneumonitis. 37.0% and 44.8% patients’ self-assessed cosmetic effects were “Excellent” and “Good”, respectively. With a median follow-up time of 28.3 months (range: 6.0-40.7 months), three patients experienced locoregional recurrence and four patients had distant metastasis (two of them with locoregional recurrence). The 2-year disease-free survival was 98.6%. Conclusion: Hypofractionated whole-breast irradiation combined with simultaneous boost of the tumor bed has few acute toxic effects and can be well tolerated in patients with early stage breast cancer after breast-conserving surgery. Long-term follow-up is needed to confirm its late toxic effects and efficacy of disease
control.

Key words: Breast cancer, Hypofractionated whole breast irradiation, Tumor bed boost, Side effects