Purpose: Young-onset breast cancers tend to be more aggressive than later-onset tumors and may have different risk factor profiles. Among young-onset cases, there may also be etiologic differences between ductal carcinomas in situ (DCIS) and invasive breast cancer, particularly if some factors promote malignant transformation.
Methods: We evaluated the association between several potential risk factors and young-onset breast cancer in the Two Sister Study (2008-2010), a sister-matched case-control study involving 1,406 women diagnosed with breast cancer before age 50 (1,185 invasive, 221 DCIS) and 1,648 controls.
Results: Older age at menarche, younger age at menopause, premenopausal hysterectomy, early age at first-term pregnancy, obesity, and consumption of alcohol were associated with reduced risk of young-onset breast cancer. These patterns remained when we limited analysis to invasive breast cancers. In general, effect estimates were similar for young-onset invasive breast cancer and DCIS, although the number of DCIS cases was small.
Conclusions: In this sister-matched case-control study of young-onset breast cancer, many of the studied risk factors were associated with young-onset invasive breast cancer. There were few discernable differences in risk factors for young-onset DCIS versus young-onset invasive breast cancer.
Keywords: Breast cancer; Ductal carcinoma in situ; Etiologic heterogeneity; Young-onset breast cancer.