Abstract 7321: Second primary cancer risks following breast cancer in BRCA1/2 pathogenic variant carriers

Abstract Background: Second primary cancer (SPC) risk estimates in breast cancer (BC) survivors carrying germline BRCA1/2 pathogenic variants (PVs) remain uncertain. We estimated relative and absolute SPC risks following BC in BRCA1/2 PV carriers using genetic testing laboratory data in England, linked for the first time to population-scale electronic health records data from the National Disease Registration Service and Hospital Episode Statistics. Material and methods: The cohort contained 27, 154 women diagnosed with primary BC and tested for germline BRCA1/2 PVs between 1995-2019. Follow-up began at the latest of the genetic test dates and one year following the BC diagnosis and continued until the first SPC diagnosis, death, migration, relevant surgery, and the end of 2020, whichever came first. We estimated overall and site-specific standardized incidence ratios (SIRs) by comparing observed and expected SPC counts, with expected counts calculated using incidences for the general English population accounting for calendar year, SPC site, age, and gender. We stratified the SIRs by age, estrogen-receptor status, and cancer site. We further assessed associations using Cox regression adjusted for age at BC diagnosis and estrogen-receptor status of the first BC, and estimated 10-year absolute SPC risks with Kaplan-Meier analysis. Results: There were 1909 BRCA1 (138 with SPCs) and 1926 BRCA2 (101 with SPCs) PV carriers, with mean follow-up lengths of 4. 2 and 4. 4y and median follow-up lengths of 3. 4 and 3. 6y. In BRCA1 PV carriers, there were elevated contralateral BC (SIR: 15. 3, 95%CI: 11. 5-19. 8), ovarian (SIR: 43. 3, 95%CI: 30. 9-58. 9), non-breast and non-ovarian combined (SIR: 2. 23, 95%CI: 1. 63-2. 98), colorectal (SIR: 4. 72, 95%CI: 2. 58-7. 92) and uterine (SIR: 3. 42, 95%CI: 1. 25-7. 44) SPC risks. In BRCA2 PV carriers, there were increased contralateral BC (SIR: 7. 53, 95%CI: 5. 33-10. 3), ovarian (SIR: 16. 5, 95%CI: 10. 1-25. 5), non-breast and non-ovarian (SIR: 1. 72, 95%CI: 1. 27-2. 27) and pancreatic (SIR: 6. 57, 95%CI: 2. 63-13. 5) SPC risks. Consistently, when we compared BC survivors carrying a BRCA1 PV to BC survivors without a PV in either gene, they were at elevated contralateral BC (HR: 3. 94, 95%CI: 2. 91-5. 33), ovarian (HR: 35. 7, 95%CI: 21. 3-60. 0), non-breast and non-ovarian (HR: 1. 53, 95%CI: 1. 11-2. 12) and colorectal (HR: 2. 95, 95%CI: 1. 56-5. 55) SPC risks, whereas BRCA2 PV carriers were at increased contralateral BC (HR: 2. 59, 95%CI: 1. 83-3. 65), ovarian (HR: 12. 3, 95%CI: 6. 93-21. 9) and pancreatic (HR: 5. 32, 95%CI: 2. 30-12. 3) SPC risks. We estimated 10-year cumulative contralateral BC, ovarian, and non-breast and non-ovarian SPC risks of 15%, 6. 3%, and 7. 8% in BRCA1 PV carriers and 12%, 2. 9%, and 6. 2% in BRCA2 PV carriers. Conclusions: Cancer risks were increased, at combined and specific sites, following BC in BRCA1/2 PV carriers. These findings could aid the clinical management of BC survivors carrying BRCA1/2 PVs. Citation Format: Isaac Allen, Hend Hassan, Yvonne Walburga, Catherine Huntley, Lucy Loong, Tameera Rahman, Beth Torr, Andrew Bacon, Craig Knott, Sophie Jose, Sally Vernon, Margreet Lüchtenborg, Joanna Pethick, Francesco Santaniello, Shilpi Goel, Sophie Allen, Katrina Lavelle, Fiona McRonald, Diana M. Eccles, Eva Morris, Steven Hardy, Clare Turnbull, Marc Tischkowitz, Paul Pharoah, Antonis C. Antoniou. Second primary cancer risks following breast cancer in BRCA1/2 pathogenic variant carriers abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 7321.