PITUITARY TUMORS & MORPHOLOGICAL FEATURES
Pituitary tumors of the anterior pituitary are more common than those of the posterior pituitary and hypothalamus. The most common of the anterior pituitary tumours are basically adenomas; primary and metastatic carcinomas hence being rare. Craniopharyngioma and granular cell tumour (choristoma) are the other benign pituitary tumours presenting occasionally. All pituitary tumours, whether benign or malignant, cause symptoms by following 2 ways:
1. Pressure effects. These cause by the expansion of the lesion resulting in the destruction of the surrounding glandular tissue by pressure atrophy. This causes erosion and enlargement of sella turcica, upward extension of the tumor damaging the optic chiasma, optic nerves, neurohypophysis, and adjacent cranial nerves, and rarely, downward extension into the nasopharynx.
2. Hormonal effects. Depending upon their cell types, pituitary adenomas produce an excess of pituitary hormones and the corresponding clinical syndromes of hyperpituitarism. Infarction and destruction of adenoma may cause symptoms of hypopituitarism.
Pituitary Adenomas( Common pituitary tumors):
Adenomas are the most common pituitary tumors. They are generally classified according to their H & E staining characteristics of granules into acidophil, basophil, & chromophobe adenomas. However, this morphologic classification considers as quite inadequate because of the significant functional characteristics of each type of adenoma including the chromophobe adenoma, which on H & E staining does not show visible granules. As a result of advances in the ultrastructural and immunocytochemical studies, a functional classification of pituitary adenoma has emerged.
Grossly, pituitary adenomas range in size from foci of < 10 mm in size (termed microadenoma) to large adenomas up to many centimeters in diameter. They are spherical, soft, and encapsulated.
Histologically, by light microscopy of H & E stained sections, an adenoma composed predominantly of one of the normal cell types of the anterior pituitary i.e. acidophil, basophil, or chromophobe cells. These cells may have the following 3 types of patterns:
1. Diffuse pattern composes of polygonal cells arranged in sheets with scanty stroma.
2. Sinusoidal pattern consists of columnar or fusiform cells with fibrovascular stroma around which the tumor cells arrange.
3. Papillary pattern composes of columnar or fusiform cells arranged about fibrovascular papillae.
Functionally, most common pituitary adenomas, in decreasing order of frequency, are: lactotroph (PRL-secreting) adenoma, somatotroph (GH-secreting) adenoma and corticotroph (ACTH-secreting) adenoma. Infrequently, mixed somatotroph-lactotroph (GH-PRL-secreting) adenoma, gonadotroph (FSH-LH-secreting) adenomas, and null-cell (endocrinologically inactive) adenomas or oncocytoma are present. Pleurihormonal-pituitary adenoma can have multiple hormone elaborations on another hand.
Functional classification of pituitary adenoma done by carrying out specific immunostains against the hormone products.
Pituitary adenoma may also occur as a part of multiple endocrine neoplasia type I (MEN-I) in which adenomas of pancreatic islets, parathyroids, and the pituitary are found. Clinically, the patients are characterized by a combination of features of Zollinger-Ellison’s syndrome, hyperparathyroidism, and hyperpituitarism.
Craniopharyngioma(benign pituitary tumors):
Craniopharyngioma is a benign tumor appearing from remnants of Rathke’s pouch. It is much more occuring in children and young adults. The tumor, though benign, compresses as well as invades the adjacent structures extensively.
Grossly, the tumor is encapsulated, adherent to surrounding structures, and is typically cystic, reddish-grey mass.
Histologically, craniopharyngioma closely resembles ameloblastoma of the jaw. There are 2 distinct histologic features i.e.:
1. Stratified squamous epithelium frequently lining, a cyst and containing loose stellate cells in the center; and
2. Solid ameloblastous areas.
Granular Cell Tumour (Choristoma)
Though tumours of the posterior pituitary are rare, granular cell tumour or choristoma is the most common tumour of the neurohypophysis. It is generally composed of a mass of cells having granular eosinophilic cytoplasm similar to the cells of the posterior pituitary. It arises as a result of the developmental anomaly and hence the name choristoma. Generally, it remains asymptomatic and is discovered as an incidental autopsy finding.