Melanocytic medulloblastoma with ganglioneurocytomatous differentiation: A case report

Melanotic or melanocytic medulloblastoma is a rare variant of medulloblastoma, especially when the tumor shows advanced neuronal differentiation. We report a case of this tumor, which developed in the cerebellar vermis in an 8‐year‐old girl. Initial biopsy specimens were identified as classical medulloblastoma with a high MIB1 index. Surgical removal of the tumor was performed after chemo‐radiotherapy, and black pigments were noticed on the tumor surface. Histologically, the tumor was composed of classical medulloblastoma with the presence of pigmented epithelial cells forming tubules and clusters. Immunohistochemically, the pigmented tumor cells were positive for S100 protein, HMB45, and MART1, indicating that the pigments were derived from melanosomes, and these features were compatible with melanocytic medulloblastoma. Interestingly, some of the non‐pigmented or amelanotic tumor cells were also positive for HMB45 and S100 protein. Although the tumor showed an unusual cell combination, it was distinguished from atypical teratoid/rhabdoid tumor (AT/RT) by nuclear expression of INI1/BAF45 protein. The tumor also possessed ganglion‐like cells within the neuropil matrix, which resembled small mature ganglion cells, and was consequently designated as ganglioneurocytoma. The melanotic medulloblastoma and part of the ganglioneurocytomatous area were fused with each other. Hence, the present case provides new information indicating that melanocytic medulloblastoma differs from AT/RT, and that it can exhibit advanced neuronal differentiation. In addition, reduction of the tumor MIB1 index was observed after chemo‐radiotherapy.