Background: Metastatic neuroendocrine carcinoma often presents as carcinoma of unknown primary. Although most cases display immunohistochemical positivity for neuroendocrine markers, subsets of cases display reduced or negative expression for some of these proteins. The identification of metastatic neuroendocrine carcinomas is even more complicated by the occurrence of unrelated tumor types with focal neuroendocrine differentiation.
Case presentation: Our patient was a 74-year-old man of Middle Eastern ethnicity. An initial biopsy of a soft tissue metastasis displayed a neuroendocrine profile indicative of a metastatic neuroendocrine carcinoma, positive for CD56 and synaptophysin, and focally for ISL LIM homeobox 1 and insulinoma-associated protein 1. The Ki-67 index was 50%. Chemotherapy was initiated, but our patient progressed. Scrapings from a pathological hip fracture 3 months later revealed focal synaptophysin immunoreactivity and widespread melanoma antigen, human melanoma black 45, and SOX10 positivity, which are indicative of metastatic malignant melanoma with focal neuroendocrine differentiation.
Conclusions: Malignant melanoma may display neuroendocrine differentiation, and the entity should be considered a rare differential diagnosis when assessing biopsies of suspected neuroendocrine carcinomas.
Keywords: Carcinoma of unknown primary; Malignant melanoma; Metastasis; Neuroendocrine cancer; Neuroendocrine differentiation; Synaptophysin.