Day 20 - Micro vs. Macro

PITUITARY ADENOMA - MICRO VS MACRO DIFFERENTIATION

Pituitary adenomas constitute 10% of all intracranial neoplasms. They are classified anatomically based on size:

1. micro-adenoma if less than 10mm and
2. macro-adenoma if more than 10mm in diameter.

They are also classified functionally based on whether there are

1. secretory or
2. non-secretory.

The most common secretory macro-adenoma is the prolactinoma. Although pituitary micro-adenomas are much more common than macro-adenomas on pathologic examinations, macro-adenomas are twice as frequent on imaging studies. Macro-adenomas are the single most common suprasellar mass (1/3 to 1/2 of lesions).Prolactin-producing hypophyseal adenoma (prolactinoma) is the most common functional pituitary adenoma. Its prevalence peaks in women between 20 and 30 years of age. Hyperprolactinemia can be a cause of infertility and is associated with diminished gonadotropin secretion, secondary amenorrhea, and galactorrhea.
When a patient is suspected to have hyperprolactinemia not associated with drugs, MR imaging is the foremost and only imaging technique that can depict a pituitary adenoma.

• Most micro-adenomas have lower signal intensity than the normal pituitary gland on T1-weighted images. A convex outline of the pituitary gland or deviation of the pituitary stalk can also be detected.
• Dynamic study with intravenous bolus injection of contrast medium is the preferred technique for assessing micro-adenomas, as it allows excellent delineation between the tumour and the normal pituitary gland. In the dynamic study, the normal pituitary gland and stalk show strong enhancement in the early phase of dynamic imaging, whereas micro-adenomas show relatively weak enhancement.

Patients with macro-adenomas typically present due to mass effect symptoms rather than hormonal excess. The adenoma may extend superiorly and stretch or compress the optic chiasm, compress the infundibulum, or extend laterally into the cavernous sinus. An expanding macro-adenoma may also erode the sella turcica. The extent of the tumour can be determined by means of contrast-enhanced MR imaging. A central constriction or "waist" where the mass narrows to pass through the diaphragma sella produces a “figure 8” sign. "Invasive adenomas" may grow into the sphenoid sinus and invade the cavernous sinus. Necrosis, haemorrhage and cyst formation are common.