メラニン形成を伴う小脳の髄芽細胞腫様腫瘍

MEDULLOBLASTOMA-LIKE TUMOR OF THE CEREBELLUM WITH MELANIN FORMATION (MELANOTIC MEDULLOBLASTOMA)

I．はじめに

　melanocyteは神経櫛起原の細胞と考えられており，脳脊髄軟膜に生狸的に存在し，その増生又は腫瘍が脳軟膜に原発することがある^4,17）（leptomeningeal melanosis，primary malignant melanoma）。神経系腫瘍にはこの種の悪性黒色腫，メラノーシスのほか，脳膜腫，神経鞘腫，神経細胞腫でメラニン色素の形成を伴う腫瘍も知られている^6,7）。1962年，Fowler and Simpson^3）は2.5歳の男児に発生した小脳のmedulloblastoma様腫瘍で，メラニン形成を伴う症例をmalignant melanin-formingtumor of the cerebellumとして報告し，以後，この種の症例は稀ではあるが，pigmented papillary medullo—blastoma^11），medulloblastoma-like tumor with melaninformation^1），melanotic medulloblastoma of the cere—bellum^16），などの腫瘍名で報告されている。ここに報告する症例もこの型の症例である。

A case of medulloblastoma-like tumor with mela-nin formation was presented in a 17 year-old man. At the age of 16, this patient was admitted to the hospital because of increasing headache, frequent vomiting and staggering gait for a period of seve-ral months. A clinical diagnosis of a tumor in the cerebellum was made clinically and a posterior fossa craniotomy was carried out. At surgery a mass in the midline of the cerebellum was notedand this was removed partially. A suspected dia-gnosis of pigmented papillary medulloblastoma was made histologically. Despite intensive postopera-tive radiotherapy the patient died 16 months after the start of symptoms. Autopsy disclosed a re-current tumor mass occupying the cerebellar vermis with diffuse infiltration of the cerebellar leptomen-inges by tumor tissue speckled with black spots. Pigmented deposits were also found over the sur-face of the brain stem and the cerebellum. Micro-scopically both the cerebellar tumor and the metastatic deposits were consisted of two types of cells, pigmented and non-pigmented. Non-pigmen-ted cells constituted the bulk of the cerebellar tumor and the histology of the tumor most re-sembled that of the classical medulloblastoma. The cells were small and appeared poorly differentiated. There were rosettes of Homer Wright type. Some pleomorphism was present with occasional forma-tion of giant cells. This was considered to be probable radiation effect. The pigmented cells were found chiefly in the leptomeningeal infiltrates, but occasionally they were present within the main tumor mass forming papillary structure around core of vasculo-connective tissue stroma. In areas they formed a single-layered epithelial lining of roughly tubular structures associated with an abundant fibrous stroma. The pigmented granules were found to have histochemical and ultrastructural features of melanin. There was no ample proof that small poorly differentiated cells produce melanin granules. This case is considered to belong to a rare variant of medulloblastoma reported first by Fowler and Simpson as malignant melanin-forming tumor of the cerebellum.