[Anatomo-radiological classification of meningioma of the posterior skull base].

Between 1972 and february 1993, 140 posterior skull base (clivus and posterior aspect of the petrous bone) meningiomas were seen and treated in our department. Since 1982, we use an anatomo-radiological classification system which is suggested here as a standard classification method for comparison among series of these tumors. Once a meningioma of this region has been discovered the problem is to establish as precisely as possible: its site of origin which will provide evidence of the likely direction of displacement of the blood vessels and nerves of the region. Its exact area of attachment is of prime importance in selecting the best surgical approach. Its size and tumoral extensions which will often dictate the addition of other approaches to allow complete removal. THE SITE OF ATTACHMENT (140 CASES): The posterior skull base may be subdivided into four zones: the clival zone: which comprises the clivus itself and that part of the petrous apex situated medial to the trigeminal impression; the posterior surface of the petrous bone itself subdivided into three roughly equal zones; *an anterior zone (zone A) situated between the trigeminal impression and the anterior lip of the IAM, *a median zone (zone M) from the anterior lip of the IAM to a line immediately posterior to the labyrinthine block, *a posterior zone (zone P) extending from this line to the sigmoïd sinus. In our series, we find meningiomas of the posterior skull base with a discrete site of attachment in the clival zone (10 cases), zone A (29 cases), zone M (31 cases), zone P (19 cases) others have a much wider attachment encompassing two or even three zones, zone A. M (29 cases), zone M. P (15 cases), zone A. M. P (7 cases). THE STAGE OF THE TUMOR: The tumor's base of implantation having been precisely defined, the stage of the tumor is then determined by measuring the distance between the tumor surface and the line of attachment. In our series, 131 tumors were measured: 13 tumors were stage I (tumors with an encroachment into the angle of less than 1 cm), 38 tumors were stage II (1-1.9 cm), 37 tumors were stage III (2-2.9 cm), 43 tumors were stage IV (more than 3 cm). TUMOR EXTENSIONS: These are appreciated on axial, coronal and sagittal images (MRI). Meningiomas of the clivus may extend [8 cases]: very commonly to the petrous apex (zone A) [7 cases], almost always to the tentorium [7 cases], often to the cavernous sinus [5 cases].(ABSTRACT TRUNCATED AT 400 WORDS)