Pathology of pineal parenchymal tumors.

Pinea l parenchymal tumors (PPTs) are neuroepithelial tumors that arise from pineocytes or their precursors. According to the currently revised WHO classification of tumors of the central nervous system, PPTs are subdivided into well-differentiated pineocytoma, poorly differentiated pineoblastoma, and PPT with intermediate differentiation (PPTID). Pineocytomas are slow-growing neoplasms composed of small mature cells resembling pineocytes. Large pineocytomatous rosettes are the most characteristic appearance. Pineoblastomas are the most primitive form and have a highly malignant biological behavior. PPTIDs show an intermediate histological grade of malignancy between pineocytomas and pineoblastomas. Immunohistochemically, PPTs are positive for several neuronal markers, including synaptophysin, neurofilaments, class III beta-tubulin, and chromogranin A. Photosensory differentiation is associated with immunoreactivity for retinal S-antigen and rhodopsin. Ultrastructurally, dense core vesicles and clear vesicles are present in both cytoplasm and cellular processes, the latter showing occasional synapse-like junctions. In some cases, ultrastructural evidence of photoreceptor differentiation, such as synaptic ribbons, microtubular sheaves, and cilia, is observed. Little is known about the genetics responsible for the development of PPTs. Several chromosomal abnormalities have been identified frequently in pineoblastomas and PPTIDs but less commonly in pineocytomas. Pineoblastomas are known to occur in patients with RB1 gene abnormalities, and these tumors also develop in patients with familial bilateral retinoblastomas (trilateral retinoblastoma syndrome). However, specific gene abnormalities involved in the tumorigenesis of PPTs have not been identified.