Background: Newer multigene molecular profiling assays for breast carcinoma rely heavily on the quantification of genes of proliferation, whereas traditional histological grading reports the mitotic count. The mitotic activity of invasive breast carcinomas may be undervalued; therefore, an evaluation of the prognostic significance of mitotic score in predicting prognosis was performed. Methods: Retrospective analysis of a single institutional cohort of newly diagnosed estrogen receptor positive (ER+), HER2 negative (HER2−) unilateral invasive breast carcinomas was performed. Mitotic scores from the 3-part Nottingham combined histological grade were compared with clinical parameters. Mitoses were counted on Olympus BX50 microscopes and assigned scores of 1–3 based on observed mitoses. Results: A total of 1292 ER+, HER2− invasive breast carcinoma patients were identified, with a median follow-up time of 2.6 years (range 0–14 years). Higher mitotic score was significantly associated with younger age, larger tumor size, angiolymphatic invasion, node-positive disease, higher stage, and the use of hormonal and cytotoxic chemotherapy. Mitotic score was significant in modeling time to local/regional recurrence (p = 0.02), recurrence-free survival/RFS (p < 0.001), and overall survival/OS (p = 0.01) with higher mitotic scores associated with worse outcomes. Higher mitotic score correlated significantly with intermediate/high risk Oncotype Dx recurrence scores (p = 0.009). Conclusions: First-generation molecular profiling assays for estrogen receptor positive invasive breast carcinomas derive much of their predictive power from quantifying genes of proliferation into a single score. Sometimes overlooked in the profusion of molecular data, the time-tested, mitotic count in the Nottingham combined histological grade is a good single-parameter predictor of survival.
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