Young women have poorer survival after breast cancer than older women. It is unclear whether this survival difference relates to the unique distribution of hormone receptor (HR) and human epidermal growth factor receptor 2 (HER2) defined molecular breast cancer subtypes among adolescent and young adult (AYA) women aged 15--39 years. The purpose of our study was to examine associations between breast cancer subtypes and short-term survival in AYA women, as well as determine whether the distinct molecular subtype distribution among AYA women explains the unfavorable overall breast cancer survival statistics reported for AYA women compared to older women.

Data for 5,331 AYA breast cancers diagnosed between 2005 and 2009 were obtained from the California Cancer Registry. Survival by subtype (triple-negative; HR+/HER2-; HR+/HER2+; HR-/HER2+) and age-group (AYA versus 40 to 64 year olds) was analyzed with Cox proportional hazards regression with follow-up through 2010.

With up to 6 years of follow-up and a mean survival time of 3.1 years (SD = 1.5 years), AYA women diagnosed with HR-/HER + and triple-negative breast cancer experienced a 1.6-fold and 2.7-fold increased risk of death, respectively, from all causes (HR-/HER + hazard ratio: 1.55; 95% confidence interval (CI): 1.10, 2.18; triple negative HR: 2.75; 95% CI: 2.06, 3.66) and breast cancer (HR-/HER + hazard ratio: 1.63; 95% CI: 1.12, 2.36; triple negative hazard ratio: 2.71; 955 CI: 1.98, 3.71) than AYA women with HR+/HER2- breast cancer. AYA women who resided in lower socioeconomic status neighborhoods, had public health insurance and were of Black, compared to White, race/ethnicity experienced worse survival. This race/ethnicity association was attenuated somewhat after adjusting for breast cancer subtypes (hazard ratio: 1.33; 95% CI: 0.98, 1.82). AYA women had similar all-cause and breast cancer-specific short-term survival as older women for all breast cancer subtypes and across all stages of disease.

Among AYA women with breast cancer, short-term survival varied by breast cancer subtypes, with the distribution of breast cancer subtypes explaining some of the poorer survival observed among Black, compared to White, AYA women. Future studies should consider whether distribution of breast cancer subtypes and other factors, including differential receipt of treatment regimens, influence long-term survival in young compared with older women.