An Individual Patient Data Meta-Analysis on the Characteristics, Treatments, and Outcomes of the Patients With Neuroendocrine Neoplasm of the Breast Reported in Literature Until August 2024.

Neuroendocrine neoplasm (NEN) of the breast is a rare type of breast cancer with limited knowledge about its characteristics and optimal management. The diagnostic criteria have been constantly updated by the World Health Organization (WHO) with increasing understanding, most recently in 2019. Focusing on characteristics, treatments and outcomes, we performed a comprehensive individual patient data (IPD) meta-analysis of the cases reported in literature between 2003 and August 2024. PubMed and Web of Science were searched systematically for case reports using predefined search terms. The protocol was prospectively registered with the international prospective register of systematic reviews (PROSPERO CRD42022356345). 185 publications, consisting of case reports or small case series published between 2003 and 2024, were eligible. They reported on 202 patients. Median age at diagnosis was 54 years (range 13-86). The 5-year overall survival proportion was 77%. On univariable analysis, overall survival was significantly influenced by regional and distant metastases, histologic grading, receptor status, Ki-67 index and surgical intervention. We did not observe any significant outcome improvement over time. Only poorly differentiated neuroendocrine carcinoma and lack of surgery were found to be prognostic factors for poor overall survival in multivariable analysis. This rare subtype of breast cancer is associated with a poor prognosis with no meaningful treatment progress in the last 2 decades. The present study identified key prognostic factors. However, the available data are insufficient to determine the optimal treatment approach. Establishing uniform diagnostic criteria and consistently reporting cases is crucial to generate a larger pool of data, thereby advancing our understanding and management of this rare disease.