State of art imaging of the pituitary tumors

Abstract

Over the years imaging of the pituitary has passed through different stages of evolution. Currently magnetic resonance imaging (MRI) is the modality most often used in studies of the pituitary gland. The role of computed tomography (CT) scanning is only appropriate to address specific questions in evaluating abnormalities within the pituitary gland. CT is used more often in lesions located in the parasellar and suprasellar regions or in the bones at the base of the skull. There are various issues that need to be addressed when dealing with the possibility of pituitary dysfunction. Biochemical tests and the clinical symptomatology are indispensable in the diagnosis of a hormonal abnormality that may have its origin in the pituitary but such tests are not always conclusive. Adenomas in the pituitary are benign lesions that cause hyper secretion of a hormone. In order of descending frequency we encounter prolactin (PRL), growth hormone (GH), adrenocorticotrophic (ACTH), gonadotrophic (FSH, LH) and thyrotrophic (TSH) secreting adenomas. The role of imaging is not to distinguish one type of adenoma from the other but to document the presence of the lesion, to show its exact location within the gland and to estimate its size. Adenomas can be small lesions imbedded within the pituitary gland or large enough to increase its size. The terms of microadenomas and macroadenomas have been adopted to describe lesions smaller or larger than 10 mm. The diagnostic issues in these two types of adenomas are different. The main problem in the case of small tumors which may measure 2–3 mm in diameter is that of detection. Small adenomas can easily escape detection because they often enhance in a fashion similar to normal pituitary. Also, the appearance of such small tumors is subjected to the effects of averaging in tomographic sections of 3 mm thickness. Depending on the histologic type, adenomas can be treated medically or by surgical excision. Prolactinomas, at least initially, may be treated medically, whereas the treatment for most of the other adenomas is surgical excision. Targeted radiation therapy is also used in cases that cannot be surgically resected or in patients with incomplete resection of the tumor. Pituitary adenomas selected to be treated by surgery are often small when first diagnosed and yet produce profound clinical symptoms. Therefore detection of small adenomas by imaging is critical for surgical planning. It has been shown that when imaging has accurately identified the adenoma within the pituitary the rate of cure is 80–90 %. When the diagnosis of microadenoma is ambivalent or incomplete, the rate of cure after surgery drops to 50–70 % [1–3]. Adenomas are usually round space occupying lesions commonly separated by a pseudo capsule from the normal pituitary. Even when a pseudo capsule is formed, the border of the adenoma and its interface with the pituitary parenchyma may not be well defined by imaging. This leads to underestimation of its size, which in turn, can result in an incomplete surgical excision and recurrent symptoms. Another diagnostic problem is encountered when the adenomatous mass occupies the entire sella without increasing the overall size of the pituitary. Then the adenoma is not clearly separated from the normal pituitary and may be mistaken as a normal gland. In other instances there is no discreet mass within the pituitary but instead there a diffuse infiltration of its parenchyma by hypersecreting cells [4]. Both the above abnormalities are not detected preoperatively by imaging and may not be appreciated even during surgery. The correct diagnosis is N. J. Patronas (&) C.-Y. Liu Bethesda, MD, USA e-mail: npatronas@nih.gov