What Is Known about Breast Cancer in Young Women?

SIMPLE SUMMARY: Breast cancer is the most common cancer affecting women under 40 years of age worldwide, with an increasing number of cases diagnosed each year. Despite this, breast cancer in young women is poorly understood as they are often underrepresented in clinical trials. Breast cancers in young women tend to be more aggressive and present at later stages as young women often do not meet screening age criteria. Recommended treatment may also be different due to unique fertility and psychosocial considerations. We herein summarize the unique challenges faced by young women, including risk factors, diagnosis, treatment, and survivorship issues, and draw attention to areas where further research is needed. ABSTRACT: Breast cancer (BC) is the second leading cause of cancer-related death in women under the age of 40 years worldwide. In addition, the incidence of breast cancer in young women (BCYW) has been rising. Young women are not the focus of screening programs and BC in younger women tends to be diagnosed in more advanced stages. Such patients have worse clinical outcomes and treatment complications compared to older patients. BCYW has been associated with distinct tumour biology that confers a worse prognosis, including poor tumour differentiation, increased Ki-67 expression, and more hormone-receptor negative tumours compared to women >50 years of age. Pathogenic variants in cancer predisposition genes such as BRCA1/2 are more common in early-onset BC compared to late-onset BC. Despite all these differences, BCYW remains poorly understood with a gap in research regarding the risk factors, diagnosis, prognosis, and treatment. Age-specific clinical characteristics or outcomes data for young women are lacking, and most of the standard treatments used in this subpopulation currently are derived from older patients. More age-specific clinical data and treatment options are required. In this review, we discuss the epidemiology, clinicopathologic characteristics, outcomes, treatments, and special considerations of breast cancer in young women. We also underline future directions and highlight areas that require more attention in future studies.