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.

Malignant mesothelioma is a form of cancer that originates from the cells of the mesothelium, a thin tissue layer surrounding the body's internal organs. Malignant mesothelioma is almost exclusively caused by asbestos exposure, pleural mesothelioma beeing the most common form, affecting the lining of the lungs called the pleura. Other forms such as perioneal-, percardial- or testicular- mesothelioma are much rarer.