Posterior Pituitary Astrocytoma: A Rare Tumor of the Neurohypophysis: A Case Report

Case 1

A 36-year-old female patient presented with uncontrollable chronic headaches and amenorrhea. The clinical examination did not reveal any abnormality. Laboratory tests revealed mild prolactinemia. On MR imaging, a posterior pituitary mass, isointense on T1-weighted images (T1WI) and hyperintense on T2-weighted images (T2WI), was identified, without significant involvement of the stalk. Postcontrast scans showed attenuated heterogeneous enhancement of the mass. The patient underwent trans-sphenoidal surgery (TSS), with some residual tumor being seen on postoperative imaging. A progressive increase in the bulk of the residual tumor was noted on follow-up MR examinations during the next 5 years. The imaging study performed before this admission demonstrated a bulky pituitary mass, as well as a bulky stalk with suprasellar extension and attenuated nonhomogenous enhancement. In the meantime, she also had developed fatigue and unbearable headaches. The patient underwent a second TSS, and histology of the specimen revealed neoplastic proliferation of astrocytes and spindle-shaped cells with fascicles. The nuclei had generally maintained their ovoid shapes, with stippled to hyperchromatic chromatin and fusiform piloid cytoplasm. There were foci of eosinophilia, stellate cytoplasm, and occasional gemistocytic morphology, and many cells were positive for glial fibrillary acidic protein (GFAP). There was no significant mitotic activity. This histology was interpreted as consistent with a posterior pituitary astrocytoma.

Case 2

A 45-year-old female patient presented with uncontrollable chronic bitemporal, occipital, and frontal headaches. She also complained of nausea and episodes of blurry vision. Neurologic examination was normal. The MR imaging revealed a small pituitary mass with suprasellar extension, isointense on T1WI and slightly hypointense on T2WI. Postgadolinium-enhanced scans demonstrated nonhomogeneous enhancement. This mass was resected via a transspenoidal approach, and histology revealed a moderately cellular astrocytomas with compact fibrillated cells in a fascicular pattern. GFAP highlighted the fibrillary aspect. This was also interpreted as consistent with a posterior pituitary astrocytoma.

Discussion

The anterior and posterior lobes constitute 2 distinct parts of the pituitary gland and have distinct embryologic backgrounds. The posterior pituitary, being part of the neurohypophysis, harbors specialized glial cells named “pituicytes.” They are modified glial cells of empendymal cell lineage located in the neurohypophysis and pituitary stalk. Five different types of pituicytes have been identified on the basis of their cytoplasmic characteristics (1), and attempts are presently underway to characterize and classify each type as a progenitor of a separate type of neurologic tumor. The cell types are as follows (2, 3):

Major cells: These cells contain electron lucent cytoplasm with oval or irregular nuclei. There are distinctive nucleoli and variable numbers of organelles.

Dark cells: These cells have the same ultrastructure but with an electron-attenuated matrix. Most pituicytomas are believed to arise from the two cell types mentioned above.

Oncocytic cells: These cells have a high mitochondrial content.

Ependymal cells: These cells show rudimentary ependymal patterns.

Granular cells: These cells contain numerous electron-attenuated granules, which are periodic acid shift–positive and are otherwise known as brown cytoplasmic cells. It is believed that they give rise to granular cell tumors or choristomas (4).

Although sometimes clinically indistinguishable from other tumors of the region, pituicytomas are histologically distinctive because of the spindled appearance of the cells on light microscopy as well as lack of immunoreactivity for neuroendocrine markers and pituitary hormones. In the largest series of these tumors, reviewed by Brat et al (5), most pituicytomas of the posterior pituitary have been found to demonstrate the presence of GFAP immunoreactivity as well as intermediate filaments on ultrastructural analysis.

Although much rarer than the tumors of the adenohypophysis, commonly described posterior pituitary tumors include metastases, choristomas, hamartomas, craniopharyngiomas, germinomas, gangliogliomas, teratomas, schwannomas (6), and astrocytomas. The tumor previously described as a pituitary myoblastoma is now considered to have been inappropriately identified, because no myoblasts are present in the pituitary gland. Choristoma is also probably a misnomer, this term being more appropriate for granular cell tumors found outside the pituitary. The term has been largely replaced by granular cell tumor, on the basis of a microscopic picture of cytoplasm, which is rich in granules (4). Needless to say, adenoma still remains the most common tumor of the pituitary gland as a whole. Tumors of the posterior pituitary should always be considered in the list of differential diagnosis in an appropriate clinical setting with suggestive imaging features. Because of their rarity, it is difficult to ascertain the exact incidence of these tumors. The usual presentation is that of a young to middle-aged woman with signs of an intracranial focal lesion causing headaches and endocrine hypofunction. Visual symptoms as a result of compression of the optic chiasm are probably less common because of the anatomic location of the mass in the posterior pituitary. They may also be asymptomatic and discovered incidentally.

MR imaging findings are usually that of a focal well-circumscribed solid posterior intrasellar mass with suprasellar extension without a bulky pituitary stalk. These masses usually manifest with low signal intensity on T1WIs, low to intermediate intensity on T2WIs, and fairly homogenous enhancement on postcontrast T1WIs. Meticulous attention to the posterior pituitary is the key to diagnosing these rare tumors, and the orientation and bulk of the stalk should be carefully assessed.

Contrast-enhanced CT probably has little role in imaging of the pituitary astrocytoma unless MR is contraindicated. Pituitary astrocytoma is a distinct and important entity to be considered in the differential diagnosis of posterior pituitary masses. Radiologic/pathologic correlation is critical. It is important that a precise histologic diagnosis be established by immunohistochemistry and electron microscopy as cure can be achieved by means of total excision, whereas, if untreated, these tumors can grow to become extremely large.