Neoplasms in the region of the pituitary fossa

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44

Neoplasms in the region of the pituitary fossa

Neoplasms in the region of the pituitary fossa may arise in the pituitary gland itself, the sphenoid bone that surrounds the fossa, or the suprasellar region (Figs 44.1, 44.2). Structures in the suprasellar region and adjacent to the pituitary fossa include the hypothalamus, optic chiasm, nasal sinuses, and cavernous sinuses and their contents.

Common neoplasms in this region are:

Uncommon neoplasms in this region are:

Other neoplasms are rare.

image LOCALIZATION IN THE DIFFERENTIAL DIAGNOSIS OF PITUITARY REGION LESIONS

image Review of imaging or imaging reports is helpful in generating a differential diagnosis.

image Pituitary adenomas are usually centered on the sella but can have suprasellar, clival, or cavernous sinus components.

image Cystic suprasellar masses are most often craniopharyngiomas or Rathke cleft cysts. Occasionally, pituitary adenomas are cystic. Rarely, adenomas may contain Rathke cleft cysts.

image A diffusely expanded pituitary gland should also prompt consideration of lymphocytic hypophysitis, other inflammatory processes, lymphoma, and pituitary hyperplasia.

image The differential diagnosis of a suprasellar mass in a young person should include germ cell tumors and craniopharyngioma, in addition to pituitary adenoma.

image A destructive clival lesion is most often a chordoma or chondrosarcoma, but can rarely be a pituitary adenoma.

image Carcinomas from the salivary glands or sinonasal cavities may infiltrate up through the skull base to involve the pituitary region.

image When a potential ‘adenoma’ is hormone-negative and synaptophysin-positive, correlation with imaging can be helpful, localizing tumor to sella or cribriform plate, but cribriform plate involvement requires exclusion of olfactory neuroblastoma.

image Optic chiasm masses suggest pilocytic astrocytoma or meningioma.

The pituitary gland weighs about 600 mg and consists mainly of the adenohypophysis (anterior lobe) and the neurohypophysis (posterior lobe) (Figs 44.3–44.6); the pars intermedia is poorly developed in man.

Adenomas are the commonest neoplasms in the pituitary gland and are derived from cells in the adenohypophysis. Neoplasms of neurohypophyseal origin are very rare.

PITUITARY ADENOMAS

Pituitary adenomas are derived from secretory cells in the adenohypophysis. Some adenomas secrete peptides in an unregulated manner and may therefore produce abnormal endocrine effects in addition to causing mass effects in the region of the pituitary fossa (Fig. 44.7).

image PITUITARY PEPTIDES

Secretory cells in the adenohypophysis synthesize six major peptide hormones:

TSH, FSH, and LH are glycoproteins composed of a common α-subunit and different β-subunits. The cross-reactivity sometimes seen with antibodies to these peptides is due to the considerable homology between the β-subunits.

The neurohypophysis is continuous with the infundibular stalk and the median eminence. Its two major peptide hormones are synthesized in the supraoptic and paraventricular nuclei of the hypothalamus and are:

MACROSCOPIC APPEARANCES

Pituitary adenomas are soft and have a beige or cream color. Discussion between the neurosurgeon and pathologist at the time of operation sometimes raises the possibility of an alternative diagnosis (e.g. a tough mass is more likely to be a meningioma).

By convention, macroadenomas and microadenomas have diameters above and below 10 mm, respectively. Most microadenomas diagnosed in life are ACTH-cell adenomas and PRL-cell adenomas presenting early with endocrine effects. Histopathologic assessment of surgically treated microadenomas requires a thorough examination of all submitted tissue.

image GENETICS OF SPORADIC PITUITARY TUMORS

image Nearly all pituitary adenomas are monoclonal.

image Downregulation of tumor suppressor genes, such as RB1, CDKN2A, CDKN1A, and other cyclin-dependent kinase inhibitors, is often due to epigenetic changes such as promoter hypermethylation or histone modification. Somatic mutations of these genes are uncommon in most adenomas.

image Oncogenes including GNAS, CREB1, and PTTG1 are associated with adenomas. The most consistent finding is GNAS mutation, which occurs in up to 40% of sporadic GH or somatotroph adenomas (Fig. 44.8).

image The pituitary tumor transforming gene (PTTG1) is overexpressed in most adenomas. A postulated mechanism is promoter hypomethylation, as somatic mutations are generally not identified.

image Oncogene-induced senescence (largely irreversible cell cycle arrest) is hypothesized to explain the relative indolence of pituitary adenomas. Animal studies suggest that PTTG1 overexpression when combined with DNA damage may lead to p21-mediated tumor cell senescence in GH adenomas.

image Mutations of TP53, RB1, and NME1 as well as amplification of HRAS and MYC are observed in aggressive tumors. HRAS mutations have been documented in pituitary carcinoma.

MICROSCOPIC APPEARANCES

image PITUITARY ADENOMAS

image The commonest neoplasms in the region of the pituitary fossa, and represent 10–15% of intracranial neoplasms.

image Have an incidence ranging from 1–15/100 000 in different series.

image Most common in the third to sixth decades.

image Show a female:male ratio of approximately 2:1 in younger patients.

image May be an incidental finding in approximately 10% of elderly people according to some necropsy series (40% of these neoplasms are prolactinomas).

image May present with the effects of increased peptide production (i.e. acromegaly or gigantism due to excess GH, Cushing syndrome due to excess ACTH, amenorrhea, galactorrhea, or impotence due to excess PRL, hyperthyroidism due to excess TSH).

image May present with mass effects (i.e. headache, compression of the optic chiasm, (other) cranial nerve palsies, compression of the othalamus, hypopituitarism, pituitary infarction). Pituitary peptide deficiencies preceding complete hypopituitarism tend to occur in sequence (i.e. GH → FSH/LH → TSH → ACTH).

image Can compress the pituitary stalk and thereby compromise transport of PRL inhibitory factor (dopamine) to the adenohypophysis, resulting in an elevated PRL level (‘stalk effect’), but PRL concentrations seldom exceed 200 mg/L in these circumstances.

image May present acutely with mass effects when they undergo infarction or hemorrhage.

image PITUITARY ADENOMAS WITH INCREASED RISK FOR AGGRESSIVE BEHAVIOR

aIncludes increased risk of metastasis (carcinoma).

The histology of pituitary adenomas is varied (Table 44.1). Many adenomas consist of small, oval, or polyhedral cells. These may be arranged in monotonous sheets or show a variety of acinar, papillary, trabecular, or other patterns (Figs 44.9–44.11). The nuclei of neoplastic cells are generally round or oval and contain the stippled chromatin typical of neuroendocrine neoplasms. Tiny nucleoli may be evident. Cytologic pleomorphism and mitotic figures may be present, but do not necessarily signify aggressive biologic behavior (Fig. 44.12). Invasion of local structures (Fig. 44.13), particularly the dura, is not infrequent, even by adenomas with bland cytologic features.

Dystrophic calcification and eosinophilic (amyloid) bodies are strongly associated with prolactinomas (Fig. 44.14). Long-term treatment of prolactinomas with bromocriptine before surgical resection produces fibrosis.

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