The prevalence of luminal B subtype is higher in older postmenopausal women with ER+/HER2- breast cancer and is associated with inferior outcomes.

To establish whether clinicopathologic and genomic characteristics may explain the poor prognosis associated with advanced age in ER+/HER2- breast cancer. The cohort included 271 consecutive post-menopausal patients with ER+/HER2- invasive breast cancer ages 55 years and older. Patients were categorized as "younger" (ages 55- < 75) and "older" (ages ≥75). The Kaplan-Meier method was used to estimate locoregional recurrence (LRR), recurrence-free interval (RFi), and overall survival (OS). Gene expression of tumor samples was assessed with Affymetrix Rosetta/Merck Human RSTA microarray platform. Differential gene expression analysis of tumor samples was performed using R package Limma. 271 breast cancer patients were identified, including 186 younger and 85 older patients. Older patients had higher rates of Luminal B subtype (53% vs 34%) and lower rates of Luminal A subtype (42% vs 58%, p = 0.02). Older patients were less likely to receive chemotherapy (9% vs 40%, p < 0.001) and hormone therapy (71% vs 89%, p < 0.001). For cases of grade 1-2 disease, older patients had a higher proportion of the luminal B subtype (49% vs. 30%, p = 0.014). Age ≥ 75 predicted for inferior OS (HR = 3.06, p < 0.001). The luminal B subtype predicted for inferior OS (HR = 2.12, p = 0.014), RFi (HR 5.02, p < 0.001), and LRR (HR = 3.12, p = 0.045). There were no significant differences in individual gene expression between the two groups. Women with ER+/HER2- breast cancer ≥75 years old had higher rates of the more aggressive luminal B subtype and inferior outcomes. Genomic testing of these patients should be strongly considered, and treatment should be intensified when appropriate.

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Declaration of Competing Interest Kamran A. Ahmed has received research funding from Bristol-Myers Squibb and Genentech. Loretta Loftus reports that she owns stock in Johnson & Johnson, Elidilly, Amgen, Pfizer, and Gilead. She serves on the advisory board for Cardinal Health, AthenaX, Daichi, Castle, Biotheranostics, and Targeted Oncology, and on the development committee for Tyme inc.