Abstract PS4-08-22: Hypofractionated Whole Breast Irradiation with Concomitant Boost following Breast Conservation Surgery in Early-Stage Breast Cancer

Abstract Background: Hypo-fractionated radiation therapy (HFRT) for breast cancer has been shown to have comparable local control and toxicity to conventional fractionation (CF) whole breast irradiation (WBI). Tumor bed boost improves local tumor control and can be delivered either sequentially (SB) or concomitantly (CB) with WBI. Initial results from NRG RTOG 1005 reported CB with HFRT over 15 fractions (F) was non-inferior to SB after HFRT or CF WBI in terms of in-breast recurrence (IBR), adverse events (AE), and cosmesis, with the added benefit of reduction of overall treatment time. We report outcomes of HFRT with CB in patients with invasive breast cancer (IBC) and ductal carcinoma in situ (DCIS), with majority treated in prone position at our institution. Methods: We retrospectively identified patients with early-stage, clinically node-negative IBC and DCIS treated with HFRT WBI with CB to the lumpectomy cavity from 2013 to 2025. All underwent breast-conserving surgery followed by WBI 40 Gy, plus a CB to lumpectomy cavity of 48 Gy, delivered in 15 F over 3 weeks using 3D conformal (3DCRT). Patient demographics, clinical characteristics, and treatment details were collected from electronic medical records. Ipsilateral breast recurrence (IBR), Disease-free survival (DFS), overall survival (OS), breast cancer specific survival, radiation toxicities, and cosmesis outcomes were recorded. Radiation toxicities were graded using CTCAE v4.0, and physician-reported cosmetic outcomes were categorized as excellent/good versus fair/poor according to the NRG-RTOG Global Cosmetic Score. Results: A total of 407 patients (343 IBC, 64 DCIS) met inclusion criteria; 74 were