The role of stereotactic body radiotherapy in oligoprogressive breast cancer: A site-specific analysis of the prospective, phase-II RADIANT trial.

[BACKGROUND] Standard-of-care management for patients with progressive metastatic breast cancer is changing systemic therapy lines. For patients with limited disease progression ('oligoprogression'), there is interest in treating progressive sites with stereotactic body radiation therapy (SBRT) whilst maintaining the current systemic therapy. Here we report on the clinical, quality of life (QOL) and adverse event findings for a cohort of patients with oligoprogressive breast cancer enrolled on the prospective, phase-II RADIANT clinical trial.

[METHODS] RADIANT (NCT04122469) was a single-arm, phase-II basket trial which included patients with oligoprogressive metastatic breast cancer. Patients on systemic therapy for ≥ 3 months received SBRT over 1-5 fractions, targeting up to 5 metastases with radiographic progression. The primary endpoint was cumulative incidence of change in systemic therapy. Secondary endpoints included local control, progression-free survival, overall survival, adverse events and health-related (HR) QOL. Analysis by disease histology was planned .

[RESULTS] Thirty patients were enrolled and analyzed; the median age was 60.0 years, 80% had invasive ductal carcinoma and 90% were estrogen-receptor (ER)-positive. Most patients had recurrent metastatic disease (63.3%), while 36.7% had metastatic disease. Most patients were on first-line (66.7%) systemic therapy. Median follow-up time was 33.7 months (range 2.5-57.2 months). The cumulative incidence of change in systemic therapy at 1-year was 30.0% (95% CI, 17.2-52.4%) and at 2-years was 50.4% (95% CI, 34.9-72.8%). At 1-year, local control rate was 90.0% and distant control rate was 56.7%. There were no grade ≥ 3 adverse events attributable to SBRT. HRQOL was maintained throughout the follow-up period.

[CONCLUSION] Among this cohort of patients with oligoprogressive breast cancer, SBRT is a safe and promising intervention, with potential to delay next-line systemic therapy. However, as a significant cohort of patients do require a change in systemic therapy within 1-2 years of SBRT, biomarkers are needed to best select patients who would benefit clearly from this approach.