Background: Breast cancer (BC) is the most common malignancy in AYA (15-39 years) females and is more aggressive than in adult women (40-69 years). However, an updated overview on BC in AYAs and how it differs from BC in adult women is lacking.

Methods: We used data from the EUROCARE-6 study to calculate incidence rates (IRs) of BC in AYA and adult women over the period 2001-2013 in Europe. We estimated 5-year and 10-year relative survival (RS) using period analysis. Stage at diagnosis distribution and conditional RS (CS) were used to investigate survival differences. We also reported cumulative incidence of multiple primary neoplasms (MPNs) in AYAs.

Results: The IR of BC in Europe was ∼18 per 100,000 in AYA females and more than ten-fold higher in adult women. Most BC cases were diagnosed at Stages I-II (74 % in AYAs vs 77 % in adults). Overall, AYAs had lower 5-year RS than adult women (85.0 % vs 89.5 %, respectively); the survival gap slightly decreased in CS. AYAs had worse survival than adult women for early stage and Stage III but better RS for Stage IV (5-year RS 31.5 % vs. 26.3 %). Cumulative incidence of all MPNs in AYAs was ∼6 % at 14 years from BC diagnosis.

Conclusions: Our findings support the more aggressive behavior of BC in AYAs. The high proportion of early stage at diagnosis is reassuring considering that organized BC screening is not available for AYAs in Europe. Conversely, the incidence of MPNs in AYAs is relevant and might deserve a revision of follow-up strategies.