Benefit of systemic therapy in MINDACT patients with small, ER-positive, HER2-negative breast cancers.

Small, hormone receptor-positive (HR+), HER2-negative (HER2-), lymph node-negative breast cancers are associated with relatively low rates of disease recurrence and have therefore been underrepresented in clinical trials assessing the effects of systemic therapy. Consequently, it remains uncertain if this patient population derives benefit from these treatments. For this exploratory analysis, we selected MINDACT (NCT00433589) patients with a HR+, HER2-, T1ab (≤1 cm) tumor and negative lymph nodes. Patients with discordant clinical risk and MammaPrint genomic risk classification were randmomized to receive chemotherapy based on either the clinical or the genomic risk assessment. Endocrine therapy treatment was based on local guidelines. 715/6693 (10.7%) MINDACT patients had HR+, HER2-, T1abN0 breast cancer and were included in this analysis. All were clinically low-risk, 124/715 (17.3%) were genomic high-risk. For genomic high-risk tumors, 8-year distant metastasis-free survival (DMFS) was 92.9% (95% CI 86.2-96.4%) compared to 95.0% (95% CI 92.8-96.6%) for genomic low-risk tumors. For genomic high-risk tumors treated with or without chemotherapy, 8-year DMFS was 89.2% (95% CI 73.6-95.8%) and 94.1% (95% CI 82.9-98.1%), respectively. For genomic low-risk tumors, the 8-year DMFS and disease-free survival (DFS) were 96.1% (95% CI 93.4-97.6%) and 89.3% (95% CI 85.5-92.2%) when treated with endocrine therapy and 92.9% (95% CI 87.9-95.9%) and 79.4% (95% CI 72.5-84.8%) without. In conclusion, although the number of randomized patients is small, patients with small, genomic high-risk breast cancer did not seem to derive benefit from chemotherapy. Endocrine therapy was associated with improved outcomes even in genomic low-risk breast cancers.