Additional prognostic value of the BCT score in ER+HER2- breast cancer patients receiving a 21-gene assay-guided adjuvant treatments

Background: The 21-gene recurrence score (RS)-guided decision-making for adjuvant treatment has been utilized as a standard of care for early ER+HER2- breast cancer. We investigated a prognostic value of the Breast Cancer Test (BCT) score, a multigene assay incorporating clinical risk, in estrogen receptor (ER)+HER2- breast cancer patients receiving RS-guided adjuvant treatments, specifically focusing on chemotherapy-untreated patients with low RS. Methods: This multicenter cohort study included 759 patients who received RS-guided adjuvant treatment. The primary endpoint was recurrence-free survival (RFS), and the secondary endpoint was distant recurrence-free survival (DRFS). Results: At a median follow up of 85 months, the 7-year RFS was 92.9% (95% CI, 90.9%-94.9%). Among the 592 chemotherapy-untreated patients with low RS, the RFS differed significantly according to the BCT score (P=.014); the 7-year RFS was 95.5% (95% CI, 93.4%-97.7%) in the BCT-low group, while it was 89.9% (95% CI, 84.9%-95.1%) in the BCT-high group. The BCT score was an independent prognostic factor for both RFS and DRFS. In addition, the RFS of the low-BCT score group was superior to that of the high-BCT group in women aged 50 years or younger, with an RS of 16 to 25. Conclusions: Our study suggests the utility of the BCT score in stratifying the relapse risk among chemotherapy-untreated patients with a low RS, particularly in young women with an RS of 16–25 who are at risk for long-term recurrence.