Neuroendocrine carcinoma (NEC) of the breast is a rare distinct clinicopathological entity, which, clinically, tends not to be aggressive. Mucinous carcinoma of the breast is also a rare distinct entity and is classified into types A (paucicellular) and B (hypercellular). It is well known that type B mucinous carcinoma frequently shows neuroendocrine differentiation. However, the coexistence of NEC and mucinous carcinoma within the same breast tumor is extremely rare. In the present study, we report a case of solid NEC of the breast with a mucinous carcinoma component and discuss the tumorigenesis of this extremely rare lesion. A 37-year-old Japanese female presented with a right breast tumor. Mastectomy and removal of the right axillary lymph nodes were performed. The resected breast tumor was composed of solid NEC (approximately 85% of the tumor) and type B mucinous carcinoma components. The right axillary lymph nodes had metastatic solid NEC. Three years later, local recurrence of mucinous carcinoma occurred in the operation scar of the right thoracic wall. In addition, seven years after the first surgery, metastatic solid NEC in the liver was observed. Recent molecular studies clearly revealed that no differences in gene expression are present between type B mucinous carcinoma and NEC. Therefore, the present case may represent solid NEC and type B mucinous carcinoma as part of a spectrum with the same genetic background. Moreover, the prognosis of solid NEC is thought to be good, and metastasis is extremely rare. However, metastasis of solid NEC of the breast may occur at a later stage. Therefore, long-term follow-up is required for patients with solid NEC of the breast.

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