[Preoperative diagnosis of posterior fossa tumors (author's transl)].

The wide-scale use of the CT Scanner has completely modified the radiological approach in the diagnosis of infratorial tumors. In the cerebello pontine angle tumors, computerized tomography offers enough information for surgery (at least in those cases where enlargement of porus is noticeable on the skull film). On the other hand, if clinical picture is incomplete or CT scann atypical (spontaneous hyperdensity), a vertebral angiography is necessary to recognize a meningioma or a neurinoma of the firth nerve. A low density suggesting a cyst (epidermoid or other) is a good indication for pneumoencephalography. In cerebellar tumors of the hemispheres, vertebral angiography should be performed in the case of multiple tumors (except if there is a good chance of metastases) and also with a single tumor showing a high enhancement with contrast; in a solitary cyst, angiography is recommanded when clinical signs suggest hemangioblastoma. Isotopic scanning is only indicated when there is a suspicion of metastases. In midline tumors, one has to consider the relationship with the 4th ventricle; no other radiological exams are necessary if tumor is likely located in the cerebellar vermis; on the contrary, if the ventricular cavity is barely visible positive ventriculography is helpful. In the anteriorly located neoplasms, pneumoencephalography with tomography may precise the exact situation in relation with the brain stem. Regardless of the anteroposterior location, a highly contrasted tumor should be explored by angiography.