Metastatic phenotype at diagnosis for prediction of survival in de novo stage IV neuroendocrine carcinoma of the breast: A SEER Research Plus analysis.

e13106 Background: Neuroendocrine carcinoma of the breast (NECB) is a rare malignancy, and population-level data describing metastatic patterns and outcomes in de novo stage IV disease remain limited. Identification of high-risk metastatic phenotypes at diagnosis may inform prognostication in this understudied population. Methods: We performed a retrospective population-based analysis using SEER Research Plus to identify patients diagnosed with de novo stage IV NECB between 2010 and 2021, defined as Combined Summary Stage (2004+) = distant. Metastatic sites at diagnosis (bone, liver, lung, brain) were summarized among cases with known status. Metastatic phenotypes were categorized as visceral (liver and/or lung and/or brain involvement) versus non-visceral, and bone-only versus any visceral involvement. Overall survival (OS) was estimated using Kaplan–Meier methods and compared using log-rank tests. Results: Fifty patients with de novo metastatic NECB were identified. Among cases with known metastatic site data, bone was the most frequent site of metastasis (59.2%), followed by liver (34.0%), lung (30.4%), and brain (16.7%). Among patients with complete metastatic site information (n = 45), 46.7% presented with single-site metastasis and 40.0% with ≥2 metastatic sites at diagnosis. Median OS for the overall cohort was 25.0 months, with estimated 1-, 3-, and 5-year OS rates of approximately 55%, 32%, and 25%, respectively. Visceral metastases at diagnosis were associated with significantly inferior survival compared with non-visceral disease (median OS ~7 vs ~38 months; p≈0.01). Liver involvement identified a particularly high-risk subgroup, with markedly shorter survival compared with absence of liver metastases (median OS ~3 vs ~32 months; p≈0.002). In contrast, patients with bone-only metastatic disease demonstrated more favorable survival compared with those with any visceral involvement (p < 0.05). Survival outcomes among patients with brain metastases were poor, though analyses were limited by small sample size. Conclusions: In this contemporary population-based analysis of de novo stage IV NECB, metastatic phenotype at diagnosis strongly influenced survival. Visceral involvement, particularly liver metastases, identified a high-risk subgroup with poor outcomes, while bone-only metastatic disease was associated with more favorable survival. These findings provide contemporary survival benchmarks for metastatic NECB and support risk stratification in this rare breast cancer subtype.