Impact of fertility concerns on endocrine therapy.

631 Background: Adolescent and young adult (AYA) women with breast cancer who have not completed their desired childbearing pre-diagnosis may be more likely to decline adjuvant endocrine therapy (ET) and/or to temporarily or permanently stop ET to pursue pregnancy. Fertility counseling and use of fertility preservation strategies (e.g., oocyte/embryo cryopreservation) might impact these decisions. Methods: In the Valuing Opinions and Insights from Cancer Experiences (VOICE) Study, we investigated whether use of fertility preservation strategies were associated with ET initiation and early discontinuation amongst AYAs with fertility concerns. Eligible females were diagnosed with stage 1–3 estrogen-receptor positive breast cancer at age 15-39 during 2013-2022 and responded to a survey in 2023-2024. Those who reported that they had not completed their desired family size or had been unsure about having completed their desired family size at the time of their cancer diagnosis were categorized as having had fertility concerns. ET initiation was defined as 2+ ET prescription fills within 18 months after diagnosis. Among ET initiators, we assessed early discontinuation (defined as lack of ET prescription fill over any 180 day period until 5 years after their first ET fill date). Due to inadequate power, we did not model the relationship between oocyte/embryo cryopreservation and ET initiation. However, we did have power to assess the association between oocyte/embryo cryopreservation and ET discontinuation using multivariable Cox regression to calculate Hazard Ratios (HR) and 95% Confidence Intervals (CI). HRs for discontinuation were adjusted for age at diagnosis, stage, study site, diagnosis year, income, and children before diagnosis (yes/no). Results: Among 518 AYA breast cancer survivors, 56% (N = 290) had fertility concerns at diagnosis. Of these 290, 41% had 1+ child before diagnosis, 90% received fertility counseling, 27% froze eggs or embryos, and 10% had 1+ child after diagnosis. The majority (89%) initiated ET. Among the 32 who did not initiate ET, 7 (22%) reported that fertility concerns impacted that decision. Oocyte/embryo cryopreservation was used by 26% of ET initiators and 34% of ET non-initiators. Among the 258 ET initiators, 89 (35%) discontinued ET early. Compared to those who did not use fertility preservation strategies, patients who froze oocytes/embryos were more likely to discontinue ET (HR 1.89; 95% CI 1.18–3.05). Among those who discontinued ET, 19% had 1+ child after diagnosis (compared to 2% among those who did not discontinue ET). Conclusions: This study confirmed prior studies showing that many AYAs with breast cancer have fertility concerns, and these concerns may impact decisions about ET. In our analysis, those who cryopreserved oocytes/embryos were more likely to discontinue ET early, and a higher proportion who discontinued ET had 1+ live birth after breast cancer.