Is a stable or decreasing prolactin level in a patient with prolactinoma a surrogate marker for lack of tumor growth?

The optimal interval for follow-up imaging of patients with prolactinomas is unclear. We wish to determine the likelihood of tumor enlargement in patients with prolactinomas who have a stable or reduced prolactin (PRL) level over time, whether or not they are treated with a dopamine agonist (DA). We identified 80 patients with prolactinomas (34 men, 46 women) who had at least two paired sets of serum PRL levels and pituitary MRIs, 3 or more months apart. Patients with hyperprolactinemia due to drug or stalk effects were excluded. The median (range) age was 45 (25–77) years. Sixty-three patients (78.8 %) were treated with DA. PRL levels (ng/mL) at the initial and latest sets were 114 (0.3–15,732) and 16 (0.3–1,204), respectively. In patients with identifiable tumors, the maximum tumor diameters (mm) at the initial and latest MRI studies were 12.5 (2–60) and 12.5 (2–39) respectively, with an interval of 2.9 (0.3–9.7) years. Sixty percent of patients (n = 48) had a macroadenoma. Forty-two (52.5 %) patients had either disappearance of the tumor (n = 22) or reduction (n = 20) in tumor size. In the remainder, tumor size was stable in 35 but increased in 3 patients. One of these patients, observed off therapy had a concomitant rise in PRL level. The other 2 had evidence of pituitary hemorrhage with no PRL increase. Tumor growth in prolactinoma patients with a stable or decreasing PRL level, regardless of size, is a rare event. Repetitive pituitary imaging in these patients may not be warranted.