Insular Gliomas with Exophytic Extension to the Sylvian Cistern: A Glioma Growth Pattern That Has Gone Previously Unnoticed.

Background: An exophytic tumor is defined as a tumor that has its epicenter in the nervous tissue but grows outside the anatomical superficial boundaries of the brain within an adjacent space. Exophytic extension of hemispheric gliomas is extremely rare. The object of this study is to describe the exophytic growth pattern of insular gliomas.

Methods: A series of 28 insular gliomas operated on consecutively were analyzed. The definition of exophytic glioma included these 2 criteria: 1) preoperative magnetic resonance imaging with evidence of exophytic local tumor extension outside the anatomical superficial boundaries of the brain; and 2) surgical identification of piamater and arachnoid invasion, with tumor growth to the adjacent cisterns.

Results: A series of 6 exophytic gliomas (21.4%) are reported, among a series of 28 consecutive insular gliomas operated. The exophytic component originated from the posterior portion of the basal frontal lobe, with extension to the sphenoidal compartment of the sylvian cistern, reaching the temporal pole. All exophytic tumors were type 5A in Yasargil classification. The histologic diagnosis was World Health Organization grade II glioma in 3 cases and anaplastic glioma in 3 cases. All patients underwent surgery, and the exophytic component was removed completely.

Conclusions: Radiologic features that define the exophytic growth pattern in insular gliomas are the posterior displacement of the middle cerebral artery and a sharp subarachnoid margin that separates the exophytic tumor from the temporal pole. Contrary to the tumor that infiltrates the anterior perforating substance, the exophytic tumor is amenable for safe and complete resection.