Top Line: Breast cancer is a heterogeneous disease, particularly among the ER+ variety.
The Study: When you think about it, some of these cancers need chemo, while plenty of them would be fine without any systemic therapy whatsoever. Opened in Stockholm back in 1976, the STO-3 clinical trial randomized 1780 postmenopausal women with pN0 breast cancer of any receptor status to +/- adjuvant tamoxifen. Here’s a secondary analysis of the 565 women with evaluable ER+/HER2- tumors. What we’ve learned after 25+ years of follow-up on distant recurrence rates is that those with larger, low-risk tumors benefitted the most from tamoxifen. While there was no benefit for T1a/b tumors, there was for T1c (HR 0.53) and T2 (HR 0.34). There was also benefit with tamoxifen for grade 1 (HR 0.24) and grade 2 (HR 0.50) tumors but not for grade 3. Finally, there was a benefit for PR+ tumors (HR 0.38) but not PR-. This all suggests worse biology (read: higher grade and PR-) likely needs cytotoxic therapy while small indolent tumors can get by with nothing.
TBL: ER+ breast cancer with more indolent biology and >1 cm in size appear to benefit most from endocrine therapy. | Dar, JAMA Netw Open 2021