Predominance of pilocytic histology in dorsally exophytic brain stem tumors

We report the magnetic resonance imaging (MRI) and clinicohistologic characterization of dorsally exophytic brain stem gliomas (DEBSGs). Between 1983 and 1991, 12 of 51 patients evaluated for the diagnosis of brain stem glioma were found to have DEBSGs emanating from the pons, pontomedulla- ry junction or medulla. Eleven of the 12 patients had classic juvenile pilocytic astrocytomas. Unlike most other brain stem tumors, these patients were young (median 38 months, range 17-75), had a relatively long duration of symptoms (median 7 months, range 2-24) and displayed signs of increased intracranial pressure with limited cranial nerve paresis, absence of pyramidal tract findings, and near normal brain stem auditory-evoked potentials. MRI characteristically showed sharply demarcated lesions with decreased signal intensity on Tl, and increased intensity on T2 sequences. Except for cystic areas, these tumors showed bright, uniform enhancement after gadolinium-DTPA. In all patients, 50-100% of the tumor volume could be resected. Three of 10 patients who received no immediate postoperative treatment eventually demonstrated disease progression, and 2 patients with subtotal resections who were treated with radiation and/or chemotherapy post operatively remain disease-free for extended periods of time. The only death occurred in the 1 patient treated with chemotherapy who died of secondary leukemia. The overall and progression-free survival of these patients at 2 years is 100 and 67% as compared to 18 and 21%, respectively, for other concomitantly treated non- exophytic brain stem gliomas. The excellent prognosis for this unusual group of brain stem tumors may be associated with the generally benign biology of pilocytic tumors and the ability to achieve significant degrees of resection.