Outcome of patients with an ultralow risk 70-gene signature in the MINDACT trial.

500 Background: Gene signatures have proven successful in identifying patients with a low risk of distant recurrence who could forego chemotherapy (CT) and are currently included in international treatment guidelines for breast cancer. For the 70-gene signature (MammaPrint) an additional threshold was established within the low risk category to identify patients with an ultralow risk of distant recurrence. In independent cohorts, these patients had excellent breast cancer specific survival at 15 years, suggesting that ultralow risk cancers represent indolent disease (Esserman, JAMA Oncol 2017, Delahaye, BC Res Treat 2017). Here we evaluate survival of patients with an ultralow risk 70-gene signature who participated in the randomized phase 3 MINDACT trial (Piccart, Lancet Oncol 2021). Methods: Of the 6,693 patients enrolled in the MINDACT trial (EORTC 10041/BIG 3-04) between 2007-2011, profiling revealed an ultralow risk 70-gene signature in 1,000 patients (15%). We assessed 5- and 8-year distant metastasis free interval (DMFI) and breast cancer specific survival (BCSS) in patients stratified by 70-gene signature result (high, low, ultralow), and within the ultralow risk group stratified by clinical risk. For these exploratory analyses, we used Kaplan-Meier estimates for time to event endpoints and Cox-regression models to calculate hazard ratio’s (HR). Results: Median follow-up was 8.7 years. Among the ultralow risk patients (n = 1,000), 67% were ≥50 years, 81% had tumors < 2cm, 80% were lymph node negative, 96% had grade 1 or 2 tumors and 99% were ER-positive. Systemic therapy was received by 83% of patients (69% endocrine therapy (ET), 14% ET + CT) and 16% received no adjuvant systemic treatment (AST). Survival estimates for all endpoints are shown in the table; 8-year DMFI was 97.0% (95% CI 95.8-98.1) for ultralow risk. The 8-year DMFI in ultralow risk patients who received no AST or ET only was 97.8% (95% CI 95.3-100) and 97.4% (95% CI 96.1-98.7), respectively. The HR for DMFI was 0.66 (95% CI 0.46-0.95) for ultralow vs low risk, after adjusting for tumor and treatment characteristics (preliminary results). Conclusions: In this prospective study, patients with an ultralow risk 70-gene signature have an excellent prognosis with 8-year BCSS above 99% regardless of clinical risk status, and with an 8-year DMFI of 95-98%.[Table: see text]