Prolactinoma volume and serum prolactin level: Evidence for the 'silent lactotroph' tumour
MJ Levy1, P Thompson2, M Powell1 & JA Ahlquist1,3
Hyperprolactinaemia in the presence of pituitary tumour can occur from tumour secretion or from stalk compression causing loss of dopaminergic inhibition. It is generally accepted that, in the presence of a large pituitary mass, a serum prolactin level up to 3000mU/l indicates stalk compression rather than a prolactinoma; the clinical diagnosis of prolactinoma depends on the degree of hyperprolactinaemia in the context of pituitary tumour size. Our aim was to examine more formally the relationship between pituitary volume and serum prolactin in patients with prolactinoma. We have undertaken a retrospective study of 14 prolactinoma patients diagnosed by positive prolactin immuno-histochemistry following surgery, assessing the relationship between the degree of hyperprolactinaemia and pituitary volume. Serum prolactin was measured before dopamine agonist treatment. Pituitary volume was determined using the Cavalieri principle, taking tumour diameter measurements in three orthogonal planes on MRI. There was a clear correlation between pituitary volume and prolactin level (r = 0.76, P=0.002), serum prolactin ranging from 1723 to 205100 mU/l. However, there was also a small group of patients (4/14, 29%) in whom there was marked discrepancy between serum prolactin and tumour volume. All 4 patients had a large prolactinoma, with only modest elevation in serum prolactin (1831-2136, mean 1953 mU/l), suggesting that prolactin secretion is disproportionately low in these individuals. We conclude that there is a group of patients with modest hyperprolactinaemia who have a large 'silent lactotroph' tumour. Initial interpretation of the serum prolactin level may be misleading, suggesting that the tumour is not a prolactinoma. These patients should be identified, as serum prolactin does not reflect tumour size in this group, and should not be used as a tumour marker during follow-up. It is not yet known whether silent lactotroph adenomas exhibit a different long term behaviour and response to therapy from that of other prolactinomas, as is the case with silent corticotroph adenomas. Prolonged follow-up is required to determine the natural history of these tumours.