Metastasis-directed stereotactic radiotherapy and systemic treatment continuation for patients with oligoprogressive metastatic breast cancer.

Background: About 15-20 % of patients with metastatic breast cancer (mBC) can experience oligoprogressive disease (OPD) in ≤ 5 sites of disease. Patients with OPD may benefit from metastasis-directed stereotactic radiotherapy (SBRT) to all sites of cancer progression while maintaining the same systemic treatment, aiming to prolong the time to next systemic treatment (NEST). This study aims to assess the outcomes provided by this multimodal strategy.

Methods: Prospective-retrospective, single-center, cohort study including consecutive patients who received SBRT to all extracranial OPD sites (≤ 5), from January 2011 to June 2023, without changing systemic therapy, according to the multidisciplinary tumor board's indication. The primary endpoint was post-radiotherapy progression-free survival (pRT-PFS). A sample size of 130 patients was needed to estimate a median pRT-PFS of 8 months with a 95 % confidence interval (95 %CI) ranging from 5.4 (considered clinically significant) to 10.6 months.

Results: 129 patients were included: 99 (77 %) had hormone receptor-positive/HER2-negative (HR+/HER2-) disease, 116 (90 %) had ≤ 2 oligoprogressive lesions, 118 (91 %) presented with non-visceral OPD involving bones or lymph nodes. Patients experienced OPD after a median PFS on systemic therapy (pre-OPD PFS) of 11.3 months (95 % CI, 8.7-13.0). Median pRT-PFS was 11.3 months (95 % CI, 9.1-13.5) and median NEST was 13.6 months (95 % CI, 11.5-15.2). Only 19 (15 %) patients experienced a subsequent PD in the OPD sites treated with SBRT.

Conclusion: Patients with oligoprogressive mBC, especially with HR+/HER2- disease and non-visceral OPD after a durable pre-OPD PFS, benefit from OPD-directed SBRT while maintaining the same systemic treatment, suggesting its broader implementation in clinical practice.