NEUROPATHOLOGY FOR MEDICAL STUDENTS
Presented by William I. Rosenblum, MD
CHAPTER 9--TUMORS OF THE NERVOUS SYSTEM
Section 4: Tumors Arising from Intracranial Structures Other Than Brain Tissue or Its Investments
This chapter contains four interrelated sections. The other three sections are:
Pituitary adenomas make up approximately 8% of all intracranial tumors. They predominate in adolescence and young adulthood. They are usually benign and grow slowly. Historically, they have been divided into chromophobe, eosinophilic and basophilic varieties, depending on their tinctorial properties.
This specially stained image (shown above) of a normal pituitary gland illustrates from left to right, E-red cytoplasm (eosinophilic), B-blue (basophilic cytoplasm which is NOT well shown in this reproduction) and C-colorless cytoplasm which is appreciated in this low power figure only as a lot of empty space around nuclei (chromophobic).
The adenomas are thought to be derived from one of these normal cell types. Previously, it was taught that basophilic adenomas produced ACTH, that eosinophilic adenomas produced growth hormone and that chromophobe adenomas produced no hormone at all.
The basophilic adenomas were thus responsible for producing Cushing's syndrome, the eosinophilic adenomas for producing gigantism (in juveniles) or acromegaly (in adults whose epiphyses are closed), and the chromophobe adenoma for producing pressure alone. Without a hormone to belie its presence, chromophobe adenomas could remain undetected longer and grow larger until they had compressed the optic nerves (to produce bitemporal hemianopsia) or the normal pituitary gland itself. This results in pituitary insufficiency (hypogonadism and/or amenorrhea, etc.). Of course, such tumors would tend to occur in older age groups because of the longer period of growth.
In many respects, the above statements are true. But a number of pituitary adenomas refuse to obey the rules. For example, some chromophobe adenomas secrete hormones while eosinophilic and basophilic adenomas may produce clinical effects other than those predicted. At present, thanks to specific immunohistochemical stains, we can precisely define the type of hormone secreted by a given pituitary adenoma. It is no longer necessary to rely on the older, imprecise histochemical stains.
We now know that prolactinomas comprise the majority of pituitary adenomas (35-45%). There is a 4:1 female preponderance. These tumors occur typically in young women as microadenomas (defined as 1 mm or less in diameter). They produce galactorrhea and amenorrhea. Prolactinomas also occur as larger masses in an older population of men and women. In this setting, there is typically little or no functional activity, though serum prolactin levels may be high. Prolactinomas are sensitive to dopamine antagonists and shrink significantly under treatment with bromocriptine. This facilitates subsequent surgical removal.
Growth hormone secreting or somatotrophic adenomas are second in incidence, comprising 20-30% of pituitary adenomas. There is a 2:1 male preponderance. The endocrine effects were described above, as were those for corticotrophin (ACTH) secreting adenomas which comprise about 10% of pituitary adenomas. Here again, there is a 4:1 female preponderance. Thyrotrophic and gonadotrophic (LH-FSH) adenomas are relatively rare (@ 1% each).
During embryogenesis, epithelial and mesodermal tissues can be trapped inside the skull and spinal canal. These tissues, along with residual embryonal tissues that have failed to differentiate, may all give rise to tumors. As representatives of this group we will discuss craniopharyngiomas and chordomas. However, lipomas, dermoid cysts, epidermoid cysts, disgerminomas, teratomas and others are also encountered.
CRANIOPHARYNGIOMAS account for about 3% of all intraranial tumors. They are most frequently seen in children and adolescents. These tumors are suprasellar in location and it is thought that they arise from epithelial remnants of Rathke's pouch that are trapped in the stalk of the pituitary gland. Grossly, the tumor appears partially solid and partially cystic and compresses the optic chiasm in front, the pituitary below, and the third ventricle above (image below, arrows).
Craniopharyngiomas are composed of sheets of squamous epithelial cells and keratin set in a loose connective tissue stroma. Note in the image below that sheets of epithelial cells (E) surround cysts, some of which contain keratin (K).
CHORDOMAS--These tumor may arise at the base of the skull or along the vertebra, especially in the lumbar area.. They are thought to derive from notochordal remnants and consist of mucoid-like tissue and sheets or nest of vacuolated cells often in grape-like clusters [physaliforous cells-arrow below]