Primary Neurohypophyseal Glial Tumors

Abstract

The pituitary stalk and the posterior pituitary gland are composed of specialized glial cells called pituicytes. Hence, it is understandable that the neurohypophysis can be a host to all of the neoplastic processes that originate from the glial cell series. These primary neurohypophyseal glial tumors are rare. We distinguish astrocytoma, tanycytoma, and granular cell tumor (GCT). Tanycytoma and astrocytomas, which include pituicytoma and pilocytic astrocytoma, are specific to the neurohypophysis, whereas GCT is less specific as it has been described elsewhere in the CNS. In addition, very few cases of neurohypophyseal ependymoma, pleomorphic xanthoastrocytoma, and primitive neuroectodermal tumors have been reported. Pituicytoma, formerly called infundibuloma, is considered grade I glial neoplasm by the 2007 WHO classification. Because it arises from pituicytes, it is highly specific to neurohypophysis. When arising from the infundibulum it is suprasellar, whereas when arising from the posterior lobe or both it is intrasellar. Suprasellar extension may induce optic chiasm and hypothalamus compression. Pituicytoma is seen in adults with a mean age of 50 years, without gender predilection. Only a few cases have been described in children. The main clinical signs are visual impairment, headache, hypopituitarism, and fatigue. Paradoxically, diabetes insipidus is an uncommon symptom in pituicytoma. It can also be incidentally discovered in asymptomatic patients. On MRI, the normal bright spot on T1WI of the posterior pituitary is typically absent. The pituicytoma appears as a solid, well-circumscribed mass with an average size of 16 mm, ranging between 5 and 30 mm. It can be suprasellar, intra- and suprasellar, or, rarely, purely intrasellar in location, with occasional cavernous invasion. This tumor is hyperintense on T2WI, and iso- or hypointense to gray matter on T1WI (Figs. 36.1 and 36.2). The presence of a cystic component is uncommon. After gadolinium administration, the tumor will typically show an intense and homogeneous enhancement (Fig. 36.2). Uncommonly, it will show moderate and heterogeneous enhancement (Fig. 36.1). A nonspecific pre- and postsellar dural enhancement can be seen (Fig. 36.2). When seen, the anterior displacement of the adenohypophysis by the tumor is an affirmative sign suggesting the neurohypophyseal origin (Fig. 36.1). Sellar enlargement, though uncommon, can help to differentiate pituicytoma from pituitary adenoma. Diagnosis may be improved by better morphological characterization but most importantly by clinical correlation (visual impairment and headache without diabetes insipidus). However, the final diagnosis is made by immunohistochemistry and electron microscopy. A total surgical resection is curative, with no recurrence or need for adjuvant therapy. Pituitary pilocytic astrocytoma is indistinguishable from pituicytoma on imaging. The differential diagnosis is made on pathology. Tanycytoma is a hypothalamic–suprasellar tumor that can also involve the pituitary stalk. It is clinically aggressive with a high recurrence rate, and is characterized by its large size. Tumor encasement of the arteries of the circle of Willis with possible infarction, extension beneath the frontal lobes, and third ventricle obstruction are common features in children. Tanycytoma appears hyperintense on T2WI and hypointense on T1WI with cystic and solid components. Intense enhancement of the solid portion is seen after contrast administration. The prognosis in children is worse than in adults. GCT is a rare and benign tumor of the neurohypophysis, considered a WHO grade I glial neoplasm. Previously called granular cell myoblastoma or choristoma, it arises from granular cell type pituicytes, although structurally identical tumors have been described elsewhere in the CNS. This tumor is usually diagnosed in the fifth decade and twice as often in females than in males. In most cases, GCT consists of small nests of tumor cells that do not have any space-occupying effects, and patients are asymptomatic. When symptomatic, symptoms are related to very large size and mass effect. These signs consist of visual disturbances, hypopituitarism, hyperprolactinemia, and headache. The appearance of GCT on MRI is nonspecific, with an enhancing suprasellar or supra- and intrasellar mass which is isointense to gray matter on both T2 and T1 WIs. Homogeneous or heterogeneous but intense enhancement is possible and reflects its high vascularity. Calcifications may be seen. Absence of the normal pituitary bright spot may be a clue that the tumor is of neurohypophyseal origin, but this finding is not specific. Treatment for symptomatic GCT is surgical; postoperative irradiation is controversial. Germ cell tumors are described in Chap. 31.