Retromastoid Approach for Biopsy of Brain Stem Tumors

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Abstract
Brain stem tumors constitute about 10 to 20% of all pediatric brain tumors. The clinical manifestations are protean and may be produced by malignant as well as benign lesions. Although the computed tomographic (CT) scan has contributed considerably in the diagnosis of these lesions, it is not possible to differentiate these tumors from viral encephalitis of the brain stem, vascular malformations, and other benign lesions. Over the years, biopsy of brain stem lesions has been considered a low yield and high risk procedure. In one of the recent series, the incidence of nondiagnostic biopsy approached 20% and major deterioration of cranial nerve function also occurred in 20% of the cases. Traditionally, suboccipital craniectomy and biopsy of the floor of the 4th ventricle has been used. This is usually associated with aggravation of cranial nerve deficits. Review of pathological specimens indicates that brain stem gliomas most often involve the pons, with predominantly unilateral involvement, and frequently fill in part of the cerebellopontine angle. On this basis, we have recently used a retromastoid approach on five patients with brain stem tumors to obtain biopsies from the ventrolateral pons and the cerebellopontine angle. A diagnosis was made in each case without complication. Minimal morbidity might be anticipated from this approach, as all cranial nerve nuclei are far away from the biopsy site. The biopsy of brain stem lesions is extremely important because aggressive therapeutic decisions may be made only after a histological diagnosis has been made. In one of our cases, a diagnosis of medulloblastoma was made in a patient with a clinical and CT scan diagnosis of brain stem glioma; this diagnosis changed the treatment plan. The retromastoid approach for the biopsy of brain stem tumors is a safe procedure with minimal morbidity and a high diagnostic yield.

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