Pituitary Adenoma

            Pituitary adenomas arise within the anterior lobe (adenohypophysis) of the gland . Tumors less than 1 cm are called micro adenomas. Clinical symptoms depend on whether the tumor is secreting or nonsecreting. Nonsecreting adenomas grow and compress adjacent structures, most commonly causing a bitemporal hemianopsia due to chiasmatic compression. Lateral extension into the cavernous sinuses can result in cranial nerve palsies.

            Secreting adenomas manifest with specific endocrine syndromes. Secreting tumors present earlier and are often smaller and confined to the gland. Prolactinomas are the most common functioning adenoma, presenting with amenorrhea, galactorrhea, and infertility. Serum prolactin levels are often greater than 100 ng/ml (normal < 20 ng/ml). Endnote Growth hormone-secreting tumors present with acromegaly in adults. Cushing's disease is heralded by symptoms and signs of hypercortisolism. These tumors are invariably microadenomas and are the most challenging for imaging. Ectopic sources of ACTH must also be considered. Endnote

            On plain T1-weighted images, microadenomas are usually hypointense compared with the normal gland. They may be slightly hyperintense on T2-weighted images, but more often the contrast is better on T1-weighted scans. Secondary signs of microadenoma include asymmetric upward convexity of the gland surface, deviation of the infundibulum, and focal erosion of the sellar floor. Endnote Coronal and sagittal are the preferred imaging planes for pituitary adenomas.

            The macroadenomas are more or less isointense to the normal gland and brain parenchyma, unless cystic or hemorrhagic components are present. Hemorrhage is depicted better by MR than CT, and the signal characteristics suggest the age of the hemorrhage. Central necrosis is not common until the tumors become quite large. Pituitary apoplexy refers to acute hemorrhagic necrosis of a macroadenoma, and patients may present with symptoms related to mass effect of the hematoma or associated subarachnoid hemorrhage. Endnote

            The enhancement characteristics of pituitary adenomas followed those observed with CT. With microadenomas, the normal gland enhances more than the tumor to increase the conspicuity of the small lesions. Gadolinium significantly increases the sensitivity of MR for detecting microadenomas. Endnote If scanning is delayed more than 30 minutes after injection of the gadolinium, the microadenoma may become isointense to the normal gland. The macroadenomas exhibit homogeneous enhancement and are clearly demarcated from normal suprasellar structures. Cavernous sinus extension is seen better on plain scans because the tumor tissue is contrasted against the flow void within the sinuses. On Gd-enhanced scans, the flow void is maintained within the carotid arteries, but both the cavernous sinuses and the macroadenoma enhance, so they cannot be separated easily. Endnote  

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