Arachnoid cyst of the quadrigeminal plate cistern: report of two cases.

Arachnoid cysts of the quadrigeminal plate cistern are rare , probably congenital , cystic infratentorial lesions that are located between the collicular plates and the incisural notch of the tentorium. Twenty-nine cases have been reported in the literature since 1940 [1-18] , the great majority before the advent of CT. The current report describes two cases in which the location and precise extent of an arachnoid cyst of the quadrigeminal plate cistern were clearly shown by CT in conjunction with reformatted imaging in the sagittal plane. Additional data concerning the CSF communications were provided by radionuclide or metrizamide CT ventriculography or concomitant metrizamide CT and radionuclide cisternography. Both cases also showed significant supratentorial extension of the cyst. The finding of ectopic choroid plexus in the wall of the cyst in one patient was decidedly rare .