ESTRO 2020 Abstract Book

S518 ESTRO 2020

Oncology, Malaga, Spain ; 2 Hospital Universitario Virgen de la Victoria, Medical Physicist, Malaga, Spain Purpose or Objective Breast conservation surgery (BCS) followed by adjuvant whole breast radiotherapy (WBI) is the standard of care for early-stage breast cancer, with a proven local control benefit and overall survival. A sequential radiotherapy boost to the tumor cavity further reduces local recurrences. Consequently, boost RT is an accepted practice in patients at higher risk for local recurrence. If an RT boost is administered, 10 to 14 Gy in either 2 to 2.5Gy fractions is usually administered, Currently, hypofractionated radiotherapy is the standar for WBI. It is unclear how best to incorporate a boost into this hypofractionated schema. The aim of this study is to determine the safety and local control of hypofractionated whole breast radiation (HFRT) with simultaneous integrated boost (SIB) in women with breast cancer after BCS. Material and Methods Between May 2013 and December 2017, 469 women with DCIS or invasive carcinoma were treated with HFRT after BCS. Patients had at least one indication for a boost: age ⩽ 50 years, surgical margins ⩽ 2 mm, or DCIS>25% and grade III. RT was delivered using 3D-CRT to the whole breast with a boost dose to the tumor bed area. All patients were treated with opposing tangential fields in the supine position, arm above the head. The main fractionation scheme used was 15 daily fractions at 2.67Gy per fraction, with a total dose of 40Gy to the whole breast. For negative lumpectomy margins, we added a SIB of 0.53Gy per fraction, for a total dose of 48 Gy to the tumor bed. If the margins were positive, we used a SIB of 0.86Gy per fraction, for a total dose of 53Gy to the tumor bed. If the surgical margins ⩽ 2 mm but negative, RT boost was 51 Gy in 15 fractions. Results Median age was 47 years (range 23-75 years). All patients had ECOG 0–1. The patient- tumor and treatment-related characteristics are summarized in tables 1 and 2. Clinical stage was 0is in 32 patients (6.9%), I in 242 (51.6%), II in 171 (36.5%) and III in 24 (5%). 119 patients (25.4%) underwent regional RT. Estrogen-receptor status was positive in 364 (77.6%) patients. 314 patients (67%) received chemotherapy and 350 (74.5%) hormonotherapy. Median PTV boost volume was 61cm3 (range 12-822cm3) y median PTV breast volumen was 747 cm3 (range 173-2686cm3).The acute skin toxicity was mild in 346 patients (73.8%), moderate in 98 (20.9%) and severe in 1 patient (0.2%). 53 patients (11.3%) had grade 1 or 2 late toxicity (fibrosis) to the boost area. One patient (0.2%)had grade 3 fibrosis and underwent mastectomy. At a median follow-up of 47 months, local control was 97.5%. In the multivariate analysis, fibrosis was significantly associated with the PTV breast volume (p=0.01) greater than 747cm3 and PTV boost volume (p=0.01) greater than 61cm3.