CanAssist Breast-based prognostication in low-intermediate estrogen receptor positive (ER) early-stage breast cancer patients helps guide treatment decisions.

Background: The estrogen receptor (ER) is one of the key biomarkers in breast cancer (BC), and therapy decisions are based on ER expression levels. However, the benefit of endocrine therapy in patients with ER expression (ER) is debatable. Owing to aggressive tumor biology, like triple-negative BC patients, many ER patients are considered to have worse outcomes and may benefit from additional drugs. This treatment dilemma in ER patients can be addressed by prognostication for risk of recurrence, which remains underexplored.

Objective: The study aims to assess whether CanAssist Breast (CAB), an immunohistochemistry-based prognostic test validated globally in ER+/PR+/HER2- early-stage breast cancer (EBC) patients, would help prognosticate ER patients and thereby aid in treatment planning.

Design: We conducted secondary data analyses of previously published retrospective studies to evaluate CAB prognostication in ER and ER subgroups across different clinical parameters.

Methods: Analysis of CAB-based risk stratification was conducted on 2896 ER+/PR+/HER2- EBC patients with a known percentage of ER staining in both ER and ER subgroups. Kaplan-Meier survival curves were used to evaluate distant recurrence-free interval (DRFI).

Results: ER patients constituted 6% of the total cohort. Overall, CAB significantly identified 65% of ER patients as low risk (LR) with acceptable DRFI of 91% and 35% as high risk (HR) with worse DRFI of 61% ( < 0.0001; hazard ratio (HR/LR), 5.175). ER patients are mostly younger, with T2, grade 3, lymph node positive tumors, and have a twofold higher incidence of distant recurrence than ER patients. CAB-based prognostication was significant in these subgroups analyzed with acceptable DRFI in LR patients of ~90% and a drop in DRFI in HR patients to ⩽66% ( = 0.01 to  < 0.0001).

Conclusion: CAB-based risk stratification of ER patients is significant and would add value in treatment decisions for additional targeted treatments to HR patients.