The menstrual cycle is an under-appreciated factor in premenopausal breast cancer diagnosis and treatment

Abstract Premenopausal breast cancer is a complex disease with poorer outcomes compared to postmenopausal breast cancer. Despite the well-known impact of estrogen and progesterone on the biology of hormone responsive breast cancers, the effect of menstrual cycle phase on diagnosis, treatment, and survival outcomes has been under-studied. Evidence is now emerging that menstrual cycle-associated hormonal fluctuations affect expression of clinically-employed biomarkers and impact upon surgical and adjuvant treatment outcomes. As we engage in a new era of precision medicine, there is an ongoing effort to improve prediction of treatment response. For premenopausal breast cancer, this must include incorporation of menstrual cycle data into treatment recommendations. We advocate that menstrual cycle phase at the time of diagnosis and treatment be routinely recorded. This will enable establishment of robust datasets to support research on how best to incorporate menstrual cycle-associated changes in breast cancer biology into breast cancer care.