Imaging of the Brainstem Tumors

Abstract

Brainstem tumors are diverse, with a pathology closely related to their topography. Accordingly a precise evaluation of the lesional anatomy is requested for diagnosis, as well as a detailed structural study. This includes conventional T1, T2 and FLAIR, DWI/ADC for cellular density, T2* imaging for blood, and contrast administration for vascularity and BBB leakage. DTI may be helpful in assessing location and involvement of the longitudinal tracts. The brainstem is divided craniocaudally into midbrain, pons, medulla, and medullo-cervical junction and ventrodorsally into basal and tegmental segments. The most common, and devastating, brainstem tumor is the diffuse infiltrating pontine glioma (DIPG): it develops in the ventral pons, as an infiltrative often anteriorly exophytic mass. Developing nodules with restricted diffusion point to high cellularity often associated with contrast enhancement, necrosis, micro-bleeds, and dilated arterial and venous perforators. Perfusion imaging would show high blood flow and MR spectroscopy high lactate and lipids. By contrast, the tegmental tumors are typically pilocytic astrocytomas, dorsally exophytic into the ventricular lumen. They may be solid, cystic, or both, infiltrate the brainstem (which ependymomas do not do), and usually enhance; still perfusion is low, but lactate may be high. The medullo-cervical tumors may be gangliogliomas. Ventral tumors of the medulla or midbrain may be benign or malignant, diffuse infiltrative tumors being fibrillary astrocytomas. Tectal tumors (actually not really of the brainstem) usually are low-grade gliomas. Other than PNETs and the highly vascular hemangioblastomas, differential includes cavernomas, inflammation, and abscesses.