[A huge idiopathic brain stone in the posterior fossa].

We report a case of idiopathic brain stone occurring in the posterior fossa. A 72-year-old woman with a huge calcified mass in the posterior fossa on plain craniogram was referred to us. She had suffered from intractable tinnitus and mild cerebellar ataxia for twenty years. CT scan revealed a dense bony mass in the posterior fossa. Angiogram only showed an avascular huge mass in the posterior fossa. MRI clearly revealed the shape, heterogenous composition and anatomical relation to the surrounding structure of the mass. The mass was totally removed uneventfully under ABR and SEP monitoring. The patient was discharged with improvement of subjective symptoms. The mass was histologically diagnosed as an idiopathic brain stone because its components were mainly calcified tissues and collagen fiber of unknown etiology; no viable cells were found in any of the specimens. Idiopathic brain stones are very rare and only eight cases have been reported in the literature. The etiology and clinical symptoms of this condition are briefly discussed.