To determine whether tumor grade, molecular subtype and hypoxia predict response to hypofractionated versus standard radiotherapy (RT) following breast-conserving surgery (BCS) for node-negative breast cancer in a randomized controlled trial (RCT).

Formalin-fixed paraffin-embedded (FFPE) tumor blocks were available on 989 of 1234 patients enrolled in the Hypofractionation Whole Breast Irradiation (HWBI) Trial. A central pathology review and assessment of tumor grade using the Nottingham grading system was carried out. Tumors were classified by molecular subtype as luminal A, luminal B, HER2 enriched, basal-like or unclassified using a six-biomarker panel; ER, PR, HER-2, Ki67, CK5/6 and EGFR. Tumors were also classified as hypoxic based on the expression of HIF1α, CAIX or GLUT-1. The primary end point was local recurrence (LR).

Median follow-up was 12 years. In the multivariable Cox model, molecular subtype was the only factor predictive of LR, the 10-year cumulative incidence was 4.5% for luminal A and basal-like, 7.9% for luminal B and 16.9% for HER-2 enriched tumors (P < 0.01). Tumor grade, molecular subtype or hypoxia did not predict response to hypofractionation.

In women enrolled in the HWBI trial following BCS tumor molecular subtype predicted LR. However tumor grade, molecular subtype and hypoxia did not predict response to hypofractionation suggesting that patients with node-negative breast tumors of all grades and molecular subtypes may be safely treated with hypofractionated RT regimens.