Impact of timing of diagnosis and breastfeeding on postpartum breast cancer outcomes.

e12537 Background: Pregnancy-associated and postpartum breast cancers (PABC/PPBC) have a worse prognosis than other breast cancers. We hypothesized that patients diagnosed earlier postpartum and those breastfeeding (BF) at diagnosis would have inferior outcomes compared to patients diagnosed later or non-BF patients. Methods: We identified PPBC patients diagnosed at our institution 2 years (2-year PP) or 2-5 years (2-5 year PP) following a live birth. Clinicopathologic details, BF status at diagnosis, mastitis in the 6 months prior to diagnosis, and whether symptoms of PPBC were initially attributed to BF or lactational change by the patient or a provider were ascertained via chart review. Characteristics were compared between groups using chi-square or Kruskall-Wallis tests. Kaplan-Meier analysis was used to assess overall survival (OS) and metastasis-free survival (MFS). Results: From 2002 to 2024, 60 2-year PP patients and 101 2-5 year PP patients were identified. The 2-5 year PP group was older (median age 39.6 vs 37.4 years, p<.001) and less likely to be BF (7% vs 60%, p<.001) at the time of diagnosis. Patients in the 2-5 year PP group were diagnosed at a lower clinical T-stage (50% vs 20% T1, 36 vs 17% T3-4, p<.001) and N-stage (61% vs 37% N0, 11% vs 29% N2-3, p=.01) compared to the 2-year PP group. There was a trend toward decreased metastasis (3% vs 12%, p=.06) and lower grade (13% vs 5% Grade 1, 55% vs 65% Grade 3, p=.2) at diagnosis for 2-5 year PP vs 2-year PP patients. Estrogen and progesterone receptor positivity and triple negative disease did not differ between groups, but HER2+ disease was more common among 2-year PP patients (36% vs 17%, p=.01). Among 2-year PP patients, BF at diagnosis showed a trend toward increased clinical nodal involvement (71% vs 50%, p=.17), metastasis at diagnosis (17% vs 4%, p=.3), and HER2 positivity (44% vs 22%, p=.13). BF patients were significantly more likely to have a diagnosis of mastitis in the 6 months preceding PPBC diagnosis (31% vs 4%, p=.03) and to have their PPBC symptoms initially attributed to BF or lactational change (58% vs 4%, p<.001). There was no difference in months postpartum at diagnosis between BF and non-BF patients (median 11.8 vs 13.5 months, p=.7). Median follow-up was 54 months. The 2-year PP group had inferior OS (5-year OS 79% [95% CI 67-93] vs 97% [95% CI 93-100], p<.001) and MFS (5-year MFS 74% [95% CI 60-90] vs 93% [95% CI 87-99], p=.003) compared to 2-5 year PP patients. Among the 2-year PP group, there was a trend toward worse OS (5-year OS 74% [95% CI 59-93] vs 86% [95% CI 70-100], p=.08) and significantly worse MFS (5-year MFS 62% [95% CI 45-86] vs 91% [95% CI 75-100], p=.032) for those BF vs not BF at diagnosis. Conclusions: Breast cancer diagnosed in the first two years postpartum has a worse prognosis compared to later PPBC. Breastfeeding at diagnosis is associated with inferior outcomes and may lead to delayed diagnosis due to misattribution of symptoms to lactational change.