ESTRO 2025 - Abstract Book

S479

Clinical - Breast

ESTRO 2025

1320

Digital Poster Stereotactic body radiotherapy versus conventional external beam radiotherapy as progression-directed therapy in oligoprogressive breast cancer Jeffrey Chun Hin Chan 1 , James Chung Hang Chow 1 , Ka Man Cheung 1 , Kelvin King Hei Bao 1 , Cindy Wai Yin Pang 1 , Connie Hoi Man Ho 2 , Gavin Tin Chun Cheung 3 1 Department of Clinical Oncology, Queen Elizabeth Hospital, Kowloon, Hong Kong. 2 Department of Clinical Oncology, Tuen Mun Hospital, NT, Hong Kong. 3 Department of Oncology, United Christian Hospital, Kowloon, Hong Kong Purpose/Objective: Oligoprogressive metastases (OPM) develop in around 20% of patients with metastatic breast cancer (MBC) while on systemic therapy (ST). Progression-directed radiation therapy (RT) may help delay ST change but data is limited. We aim to evaluate the efficacy of stereotactic body radiotherapy (SBRT) versus conventional external beam RT (EBRT) in delaying ST change in MBC patients with OPM. Material/Methods: All MBC patients (n=996) from two tertiary oncology centres who were treated with RT between 2013-2023 were identified from a prospective database. Patients who met both criteria of 1) developed no more than 3 sites of extracranial OPM during ST and 2) had SBRT or EBRT to all OPM while remaining on the same ST were included. The primary endpoint was time to next systemic therapy (TTNST) after RT. We evaluated the association of TTNST with RT modality and clinicopathological characteristics using log-rank test and multivariate Cox regression. Results: Fifty-one female MBC patients were eligible for analysis. Median age was 49 (range 27-76), among which 39 (76.5%) had OPM detected by PET and 38 (74.5%) were on first-line ST. The median time of developing OPM from the start of the current ST was 18.5 months (mth). A total of 73 OPM were irradiated, 23 (31.5%) by SBRT and 50 (68.5%) by EBRT. The number of patients with 1, 2 and 3 sites of OPM was 39 (76.5%), 9 (17.6%) and 3 (5.9%), respectively. The most common sites of OPM were bone (n=48, 65.8%) and distant lymph nodes (n=11, 15.1%). With a median follow up of 41.9mth, the median TTNST was 12.1mth. Five patients (9.8%) received more than one course of RT for repeated OPM episodes. All changes of ST after RT were due to distant failure. The median overall survival was not reached. In univariate analysis, patients who underwent SBRT had significantly longer TTNST compared with patients who underwent EBRT (median, 36.5mth vs. 9.3mth, p=0.021). After adjusting for molecular subtype, line of ST, site of RT, time to OPM development and extent of disease outside RT field, the use of SBRT remained to be independently associated with longer TTNST (HR 0.35; 95% CI, 0.15-0.83, p=0.017). No grade 3 or higher toxicities were observed.