2852 - Post-Hoc Subgroup Analysis of Patients with Breast Cancer Experiencing Polymetastatic Progression on CURB Oligoprogression Trial
Presenter(s)
S. J. Rosenzweig1, C. J. Tsai2, N. Riaz1, P. Iyengar1, A. J. Khan1, S. N. Powell1, and A. J. Xu1; 1Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, 2Radiation Medicine Program, Princess Margaret Cancer Centre, University Health Network, Toronto, ON, Canada
Purpose/Objective(s): In the phase II CURB trial of patients with =5 progressive lesions after =1 prior systemic therapy, randomizing patients to receive stereotactic body radiotherapy (SBRT) vs continued standard of care (SOC) did not improve PFS in the breast cancer cohort (Tsai, Lancet 2024). We hypothesized that SBRT may nevertheless delay subsequent polymetastatic progression in patients with oligoprogressive breast cancer.
Materials/Methods: We performed a post-hoc analysis of 47 patients with oligoprogressive breast cancer enrolled on the CURB Oligoprogression trial. Post-progression PET and/or CT imaging were reviewed to characterize polymetastatic progression, defined as >5 progressing sites. Polymetastatic PFS (PM-PFS) was defined as time to radiographic polymetastatic progression or death, estimated using Kaplan–Meier methods with log-rank testing. Outcomes were compared by receipt of SBRT and change in systemic therapy, which was not mandated on the SOC arm.
Results: Polymetastatic progression after oligoprogression was not observed in 7 (29%) patients receiving SBRT vs 4 (17%) in the SOC arm. Median PM-PFS was 13.8 [95% CI 5.9-31.3] vs 5.5 [CI 4.5-17.8] months for SBRT vs SOC arm, respectively (HR 1.5, CI 0.9-2.9, p=0.34). Among the surviving patients (9 patients), with median follow up of 45 months, 4 patients remain alive without polymetastatic progression and 3 without any progression. Non-progressing patients were younger (median age 33), had non-TNBC, limited metastatic burden (1-2 sites), and no history of polymetastatic disease. 12 patients in the SOC arm received neither SBRT nor systemic therapy change at enrollment and had significantly shorter PM-PFS than those who received SBRT or systemic therapy change (4.5 [CI 2.3-NA] vs 14.3 [CI 7.8-24.3] months, p=0.00012; HR 4.0 [CI 1.9-8.6]).
Conclusion: A greater proportion of patients treated with SBRT remained without polymetastatic progression, with a trend towards prolonged PM-PFS. Improved outcomes were primarily observed in patients with favorable baseline characteristics and limited disease burden. Lack of any treatment modification at oligoprogression (no SBRT and no systemic therapy change) was associated with worsened PM-PFS. SBRT may offer a clinical benefit in delaying PM-PFS, especially for patients unable to change systemic therapy and warrants further investigation.