Medulloblastoma with myogenic differentiation showing double immunopositivity for synaptophysin and myoglobin

Reported herein is a case of medulloblastoma with myogenic differentiation in a 3‐year‐old girl who died 1 year after appearance of clinical signs. Magnetic resonance imaging indicated a mass lesion in the cerebellar vermis. She underwent total resection of the tumor, followed by chemotherapy and radiotherapy in the brain and spinal cord. The resected specimen mainly consisted of densely packed cells with round‐to‐oval highly chromatic nuclei surrounded by scanty cytoplasm and focally of long spindle‐shaped cells with elongated nuclei and eosinophilic cytoplasm showing discernible cross‐striations. Immunohistochemistry indicated partial expression of synaptophysin in the former area and focal expression of desmin in the latter area. The diagnosis was medulloblastoma with myogenic differentiation, also known as medullomyoblastoma. Autopsy indicated disseminated proliferation of immature neuroglial cells with highly chromatic nuclei and scanty cytoplasm showing partial expression of synaptophysin, neurofilaments, and GFAP, and focal proliferation of round‐to‐oval immature cells showing immunoreactivity of myoglobin. The tumor cells had large nuclei, frequent mitoses, apoptoses, nuclear molding, and cell wrapping, indicating moderate anaplasia. Their Ki‐67 labeling index was 54%. In addition, some tumor cells had double immunopositivity for synaptophysin or neurofilament and myoglobin, suggesting that the neuroectodermal cells may undergo differentiation into rhabdomyoblasts.