A rare, mostly benign, primary tumor of the meninges (arachnoid cap cells), usually located in the supratentorial compartment, commonly appearing in the sixth and seventh decade of life, clinically silent in most cases or causing hyperostosis close to the tumor and resulting in focal bulging and localized pain in less than 10% of cases. Additional features may include headache, seizures, gradual personality changes (apathy and dementia), anosmia, impaired vision, exophthalmos, hearing loss, ataxia, dysmetria, hypotonia, nystagmus, and rarely spontaneous bleeding.

A Malignant mesothelioma originating from cells of the peritoneum (the thin layer of mesothelium lining the abdomen). Peritoneal mesothelioma is the second most common form of mesothelioma after pleural mesothelioma.