Malignant Melanoma With Neural Marker-Positive Distant Organ Cancers.

Malignant melanoma is a melanocyte-derived tumor known for its aggressive clinical behavior. Melanocytes originate from the neural crest, which also gives rise to neural tissues. Malignant melanoma can occasionally exhibit neural differentiation. We report a case of a 70-year-old male with malignant melanoma exhibiting neural marker positivity in the absence of typical melanoma markers. The patient initially presented with a dark nodule on his left heel, which was confirmed as malignant melanoma through cytology. Surgical resection and lymph node dissection were performed, revealing atypical melanocytes. Despite postoperative nivolumab treatment, metastases in the brain and lungs were observed. Histological examination of the brain tumor showed neural differentiation markers (thyroid transcription factor 1 (TTF-1), cytokeratin 7 (CK7), AE1/AE3, and epidermal growth factor receptor (EGFR)) with negative melanoma markers. The patient eventually succumbed to the disease despite multiple treatments. An autopsy revealed multiple organ tumors (brain, duodenum, stomach, liver, and bile duct) negative for melanoma markers but positive for neuroendocrine markers (CD56, synaptophysin, and chromogranin A). This case suggests two possibilities: the coexistence of malignant melanoma with neuroendocrine tumors or a transformation of melanoma into a neuroendocrine phenotype. This case highlights the need for clinicians to consider the potential for melanoma to lose typical markers and transform into neuroendocrine cancer.