Second primary breast cancer in young breast cancer survivors.

10503 Background: Prior data suggests that the risk of second primary breast cancer (SPBC) is higher among women diagnosed with primary breast cancer (BC) before age 40. There is evidence that risk differs by primary tumor characteristics, cancer treatments, and genetic predisposition. However, studies investigating risk of SPBC by clinical risk factors are often limited by low numbers of young women and lack of complete patient data, particularly regarding germline genetic risk. Methods: Using the Young Women’s BC Study (YWS), a prospective cohort of 1,302 women ≤40 years diagnosed with BC from 2006-2016, we sought to compare clinical and treatment characteristics among women who developed an SPBC vs. those who did not. The analytic cohort included women diagnosed with Stage 0-III BC who did not undergo bilateral mastectomy (N = 685). Demographics, genetic testing and results, clinical events, and treatment data were collected through self-report on baseline and follow-up surveys (every 6 months for 3 years, then annually). Detailed tumor and treatment information, confirmation of genetic testing results, and new and recurrent cancer events were obtained through regular and triggered medical record review. Univariate and multivariate Cox proportional hazards models were used to examine associations of age, race, genetic testing results, hormone receptor status, stage, and grade of primary tumor, and cancer treatments with SPBC. Participants were censored at the time of local recurrence, metastasis, death, or loss of follow-up. Results: At a median follow-up time of 9.9 years [inter-quartile range (IQR) = 6.6-12.0 y], 17 patients developed an SPBC (2.5%), in whom 3 occurred in the ipsilateral breast after lumpectomy as primary BC surgery. Median time from primary BC diagnosis to SPBC diagnosis was 4.2 years (IQR = 3.3-5.6 y). Compared to those who did not develop an SPBC, a greater percentage of women who developed an SPBC had in situ disease as their primary BC (24% v. 6%) and were of Ashkenazi descent (12% v. 5%). In the 577 women who had had genetic testing, 544 (94%) were non-carriers of pathogenic variants. Risk of SPBC was 2.2% (N = 12) in non-carriers and 9.1% (N = 3) in the 33 carriers. In Cox univariate analyses, carriers had a 4.1 times higher risk of SPBC compared to non-carriers (HR = 4.14, 95% confidence interval (CI) = 1.67-14.66, p = 0.03), and primary diagnosis with in situ BC was associated with a 5-fold increased risk of SPBC (HR = 5.02, 95% CI = 1.63-15.41, p = 0.005). Both factors remained strongly associated with SPBC when tested together in a multivariate model. Conclusions: This prospective study demonstrated a low risk of SPBC in the first 10 years after diagnosis of early BC in young women who are not germline pathogenic variant carriers. The finding that risk of SPBC was higher after in situ primary BC at a young age warrants further investigation. These findings have important implications for the treatment and follow-up care of young women with BC.

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