[Ossified choroid plexus papilloma--case report].

While the calcification has been documented radiologically in 4 - 25 percent of the cases with choroid plexus papilloma, the ossification of choroid plexus papilloma has been reported only in 3 cases on literature. In this paper, we present a case of large ossified choroid plexus papilloma in the right lateral ventricle. A 35-year-old man was admitted with left hemiparesis and headache. Skull X-rays showed an abnormal calcified mass (25 mm x 23 mm x 14 mm) in the right temporal region. CT scan revealed an abnormal high density area protruding into the right lateral ventricle, and a low density area at the right corona radiata. Operation was carried out with a right temporal craniotomy. The mass was yellowish-elastic hard and protruded into the right inferior horn of the lateral ventricle. The histological examination of surgical specimen showed an ossified choroid plexus papilloma with some bone marrow formation and numerous calcification. Calcification of choroid plexus papilloma is found frequently, but ossification is a very rare neuropathological abnormality. Bone and/or cartilage formation in neuroepithelial neoplasms is an unusual event histopathologically. Several mechanisms have been proposed, such as (1) metaplasia of connective tissue, (2) transformation of neuroepithelial cells to mesenchymal tissue, (3) teratomatous nature, (4) heteroplasia, (5) mixed mesenchymal-neuroepithelial nature, and (6) ossification as an end stage of mucoid degeneration. In our case, it was considered that the bone formation resulted from metaplasia of connective tissue of choroid plexus papilloma.