ESTRO 2025 - Abstract Book

S496

Clinical - Breast

ESTRO 2025

Material/Methods: The phase II study was designed using the 2-stage Simon method. A total of 148 patients have been enrolled between February 2022 to June 2024. Eligibilities include ≥50 years of age with early stage (pT1-T2 pN0) invasive ductal carcinoma (IDC), and ≥70 years of age with lobular carcinoma or pN1a invasive ductal carcinoma. A single dose of 17.5 Gy was delivered under stereotactic localization to the 95% of the PTV using the GammaPod system. Imaging, planning and dose delivery were mostly completed within a 1 hour session. The primary endpoint was grade 2 or higher radiation fibrosis and radionecrosis (RN) according to the Common Toxicity Criteria (v4.0). Secondary endpoints included: local control (LC), regional control (RC), metastasis-free survival (MFS) and cancer specific survival (CSS). The Kaplan-Meier method was used to estimate outcomes and cumulative incidence. Results: Median follow-up was 19.6 months (std 8.5; IQR13.1-28.2). The tumor phenotypes were as follows: 140 (94.5%) ER/PgR positive-Her2 negative, 4 (2.7%) ER/PgR positive-Her2 positive and 4 (2.7) triple negative. Twenty-three (15.5%) were T1a, 62 (41.8%) T1b, 59 (39.8%) T1c and 4 (2.7%) T2. 143 (96.6%) were N0 and 5 (3.3%) were N1a. 119 (80%) patients received hormonal therapy, 3 (2%) received chemotherapy. Grade 1 fibrosis occurred in 54/148 patients (36.5%), Grade 2 fibrosis occurred in only 5 patients (3.4%). The incidence rate of Grade 2 fibrosis at 6-12-18 months were 2.1%, 3.6% and 3.6% respectively. No patients experienced grade 3 or higher treatment-related toxicity. Symptomatic RN occurred in 8 patients (5.4%). The incidence rate of RN at 6-12-18 months were 0.7%, 4.5% and 5.5%, respectively. LC, RC and MFS at 18 months were 97.3%, 98.7%, and 100% respectively. Only two patients had a LR: one was a ductal carcinoma in-situ developed in the index quadrant, one was IDC outside the radiation field together with regional failure. CSS at 18 months was 100%.