A multicenter study of clinical outcomes and volumetric trends in suspected microprolactinomas

The diagnosis of pituitary microprolactinomas is often obscured by relatively low levels of elevated prolactin compared to macroprolactinomas. This may lead to varying patterns of medical therapy versus observation. We sought to correlate prolactin levels in suspected microprolactinomas with tumor volumes and clinical outcomes. This was a multicenter retrospective study of patients with pituitary microadenomas with baseline prolactin levels > 18ng/ml for males and > 30ng/ml for females. A linear-mixed model was used to depict changes in tumor volume over time. There were 65 patients with a mean tumor volume of 95.9mm³ and mean prolactin level of 59.4ng/ml. There were significantly higher prolactin levels in patients with tumors above the mean volume versus below (74.0 versus 53.4ng/ml, p = 0.027). 26 patients were observed, 31 were treated with anti-dopaminergic therapy, and 8 had surgery. There were significantly greater baseline prolactin levels for patients who were treated surgically (mean 86.4ng/ml) than those treated medically (mean 61.7 g/ml) or observed (mean 48.5ng/ml) (p = 0.02). Among the 26 patients who were surveilled, 13 patients demonstrated spontaneous tumor shrinkage, 12 remained stable, and 1 patient's tumor grew but was lost to follow-up. Linear mixed modeling demonstrated a statistically significant rate of tumor shrinkage over time of 3.67mm³/year (p = 0.03). When analyzing patients who were observed versus those requiring surgery after initially being surveilled, there were significantly greater baseline PRL/volume ratios in surgical patients versus those observed (8.1 ng/ml/mm³ versus 2.4 ng/ml/mm³, p = 0.025). Suspected microprolactinomas may demonstrate more convincingly elevated prolactin levels when measuring over 95.9mm³. Tumors with baseline prolactin levels over 50ng/ml may be more inclined to undergo medical treatment. In tumors with levels below 50ng/ml, it may be reasonable to undergo surveillance as these tumors tend to spontaneously shrink over time. In tumors that are surveilled, an elevated baseline PRL/volume ratio of > 8 ng/ml/mm³ may be indicate serial tumor growth that may necessitate medical and/or surgical intervention.