Impact of 21-Gene Expression Assay on Clinical Outcomes in Node-Negative ≤ T1b Breast Cancer

Background Prior to the advent of Oncotype DX 21-gene recurrence score (oDX) assay, the National Comprehensive Cancer Network (NCCN) guideline supported omission of adjuvant chemotherapy in patients with ≤ 1 cm (T1b) hormone receptor-positive (HR +), human epidermal growth factor receptor 2 (HER2−) node tumors. However, around 30% of these patients would have an oDX recurrence score that warrants consideration of adjuvant chemotherapy. To clarify the potential benefit of oDX in these patients, we performed a retrospective analysis comparing clinical outcomes of women with T1a or T1b, N0 HR + HER2− according to performance of oDX. Patients and Methods After receiving institutional review board (IRB) approval, an institutional database was queried to identify patients with HR + HER2− ≤ T1bN0 tumors ( n = 2307) diagnosed between 2009 and 2018. Patients were further stratified by recurrence score (RS) defined as low (< 18), intermediate (18–30), or high (> 30). Log-rank, Kaplan–Meier, and inverse probability of treatment weighting (IPW) analyses were used to compare disease-free survival (DFS) and overall survival (OS) across groups. Results Performance of oDX ( n = 1149, 49.8%) was associated with larger tumors, younger age, and White race. On univariate analysis, performance of oDX was associated with improved OS ( P < 0.01). On multivariate IPW analysis, performance of oDX lengthened DFS by an average of 16.5 months, while OS was similar between groups ( P < 0.01 and P = 0.73). The improved DFS was mainly driven by those with tumors ≥ T1b. Conclusions Overall, outcomes were excellent regardless of oDX testing. Performance of oDX testing was associated with improved DFS in patients with tumors ≥ T1b. Our results support routine use of oDX testing in patients with tumors ≥ T1b.