Prolactinoma in Women

Abstract

Hyperprolactinemia is by far the most frequent indication of an MRI of the pituitary region. The typical situation is that of a young woman with infertility and hyperprolactinemia. Irregular or absent menses can be masked by a contraceptive pill. Galactorrhea is inconstant and not specific. Nonadenomatous hyperprolactinemias have to be ruled out, such as those related to some drugs or secondary to hypothyroidism. In our experience, a prolactinoma has very little chance of being demonstrated on MRI if the prolactin level is below 35 ng/ml. Most prolactinomas in women are microadenomas (less than 10 mm in diameter), and many are less than 5–6 mm or even less than 3 mm. We call these “picoadenomas” (Fig. 5.1). There is usually a good correlation between the adenoma size and the prolactin level: microprolactinomas are found when the prolactin level is roughly between 35 and 80 ng/ml; macroprolactinomas correspond to higher prolactin levels. A very high prolactin level, such as more than 200 ng/ml, is not consistent with a microprolactinoma and must raise the question of a macroprolactinemia, particularly if the clinical situation is unclear. Conversely, a mild elevation of the prolactin level is not consistent with a macroprolactinoma: in this case, a nonfunctioning pituitary adenoma has to be suspected. Nevertheless, the rule of a good correlation between prolactin level and adenoma size has exceptions : hemorrhagic adenomas (Fig. 5.2) proportionally display lower prolactin levels, and cystic or necrotic adenomas present areas of marked T2 hyperintensity (Fig. 5.3); just as the extremely rare calcified macroprolactinomas, at least in young women, may shrink moderately after dopamine agonist treatment (Fig. 6.6). Unusual T2-hypointense noncalcified microprolactinomas proportionally display higher prolactin levels than the common T2-hyperintense ones (Fig. 5.4). Microprolactinomas are round or oval, sometimes triangular in shape, or can present with irregular contours (Fig. 5.5) and are usually located off-midline. Mostly they are hypointense on T1 if compared with the normal anterior pituitary gland, and more or less hyperintense on T2WI. In other terms, most microprolactinomas present with a T1 signal very similar to that of the cerebral gray matter, while the normal anterior pituitary gland has the same T1 signal as the cerebral white matter. Microprolactinoma T1 hyperintensity resulting from subclinical hemorrhage is not uncommon and nearly always totally asymptomatic. Ancillary signs such as localized bulging of the upper surface of the pituitary gland or mild depression of the sellar floor are rarely absent if the microadenoma is more than 3 or 4 mm in diameter. Displacement and distortion of the posterior lobe are well appreciated on axial T1 sequence (Fig. 5.6) and can constitute a useful additional feature, for instance in cases of rare isointense microadenoma. Conversely, tilting of the pituitary stalk is not always a faithful sign. Gadolinium injection shows a lesser degree of enhancement of the adenoma compared with the normal pituitary gland (Fig. 5.7). Contrast injection is not mandatory if the diagnosis is, as is usual, obvious on high-quality T2WI (Fig. 5.8). Late enhancement of the prolactinoma itself can be observed if an additional T1W sequence is obtained 30 or 40 min after contrast injection (Fig. 5.9), especially with prolactinomas displaying a marked T2 hyperintensity. Dynamic MRI (Fig. 5.10) is usually unnecessary and can lead to false-positive diagnoses (Fig. 15.6). Differential diagnosis includes unrecognized artifacts, mainly partial volume effect artifacts, intrasellar cysts, and particularly Rathke cleft cysts, as well as the normal posterior lobe or a deep fossula hypophyseos thinning the dorsum sellae.