Neuropathology for the neuroradiologist: Rosettes and pseudorosettes

The neuropathologic diagnosis of brain tumors entails the microscopic examination of conventional formalin-fixed paraffin-embedded tissue samples surgically removed from a radiographically defined lesion. A preliminary diagnosis is often rendered with frozen sections, though the final or definitive diagnosis usually requires more elaborate studies. Typically, the tissue is first fixed for a minimum of several hours in 10% neutral buffered formalin, processed through a series of dehydrating and clearing reagents, and embedded in a hardening wax, such as paraffin. The latter enables the tissue to be thinly sliced with a microtome, transferred to a glass slide, and then stained with dyes such as hematoxylin-eosin (H&E) that contrast the different cellular elements. Pathologists rely on visual clues such as pattern recognition when examining the stained tissue with a microscope, much as radiologists rely on gray-scale patterns of densities and intensities on images. Some histologic patterns of cellular architecture are distinctive if not pathognomonic, whereas others are less specific, but nevertheless considerably narrow the differential diagnosis. The precise biologic bases for some of the observed microscopic patterns are poorly understood, though their recognition remains useful nonetheless. Although more advanced methods of tissue examination-such as histochemical and immunohistochemical profiling, genetic analysis, and electron microscopy-have been developed, the microscopic review of H&E-stained material remains a critical component of tumor diagnosis.