PINEAL REGION TUMORS

            Tumors in the pineal region can be classified into three major groups based on their origin: germ cell, pineal parenchyma, and parapineal. Germinoma is the least differentiated of the germ cell group. It occurs in children and young adults and accounts for more than 50% of all pineal region tumors. The other germ cell tumors include embryonal carcinoma, yolk-sac tumor, and choriocarcinoma. Differentiation along three germ layers results in a teratoma. The true pinealomas consist of pineoblastoma and pineocytoma. Pineoblastoma is an embryonal tumor of neuroectoderm, related to neuroblastoma and medulloblastoma, and is found primarily in young children. Pineocytomas are less cellular and exhibit benign behavior. The parapineal lesions include gliomas of the tectum and posterior third ventricle, meningiomas arising within the quadrigeminal cistern, and developmental cysts (epidermoid, dermoid, arachnoid cyst). Endnote

            The clinical expression of these tumors is usually related to mass effect upon adjacent brain structures. Hydrocephalus secondary to aqueductal obstruction is a common presentation. Compression of the tectum of the midbrain can produce paralysis of upward gaze, the classic Parinaud's syndrome. Germinomas and gliomas have a propensity to grow into the third ventricle and compress the hypothalamus, resulting in endocrine dysfunction. Dissemination through the CSF pathways is a known complication of pineoblastoma and germinoma.

            Pineal germinomas and primary pineal tumors are most often isointense with the brain on T1- and T2-weighted images. A few lesions exhibit long T1 and T2, which may correlate with embryonal cell elements. Despite this relative lack of contrast, with multiplanar imaging plain MR delineates pineal region masses better than CT, showing the relationships of the tumor to the posterior third ventricle, vein of Galen, and aqueduct. Endnote These tumors are well defined and enhance to a moderate degree, usually without central necrosis, cystic change, or hemorrhage. Enhanced scans are essential to assess CSF spread of tumor. In young patients with germinoma, the difficulty of visualizing calcium is a disadvantage of MR, as this may be the only evidence of tumor. Endnote

            Meningiomas can appear very similar on plain scan, but their intense enhancement may set them apart from other lesions. Gliomas infiltrate the tectum and posterior walls of the third ventricle. They tend to be poorly circumscribed and produce symptoms earlier. Edema is not a consistent finding, and enhancement is variable. Larger gliomas in the splenium of the corpus callosum may present as pineal region masses.

            Teratomas are of mixed signal intensity, frequently with calcification. They may also have cystic components and fat. Arachnoid cysts, epidermoid and dermoid tumors can usually be distinguished from other pineal region tumors by their increased signal on T2-weighted images.

            Pineal cysts were visualized in 4.3% of normal patients in one MR study. Endnote These apparently benign lesions are seen best as areas of high signal on intermediate T2-weighted images. They are not associated with hydrocephalus or a pineal mass and are not clinically significant. 

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