D2-receptor imaging has limited use in patients with nonfunctioning pituitary adenoma

de Herder WW et al. (2006) Dopamine agonist therapy of clinically non-functioning pituitary macroadenomas. Is there a role for 123I-epidepride dopamine D2 receptor imaging? Eur J Endocrinol 155: 717–723

Surgery is the preferred treatment for clinically nonfunctioning pituitary macroadenoma, but these tumors often recur. Since some of these tumors express dopamine D2 receptors, dopamine-agonist therapy has been explored as an alternative to surgery and as a postoperative treatment, with favorable albeit heterogeneous results; de Herder and colleagues, therefore, investigated whether single-photon-emission CT (SPECT) imaging with ¹²³I-epidepride, a radiolabeled dopamine D2 receptor antagonist, could predict patients' responses to dopamine-agonist therapy.

Of 54 patients with clinically nonfunctioning pituitary adenoma who underwent SPECT, 18 patients (age range 32–86 years; 12 men) received dopamine-agonist therapy with quinagolide (150–300 µg daily) or cabergoline (1–2 mg weekly, titrated as required), for 34–187 months (mean ∼7.5 years). Five patients had previously undergone surgery and one required surgery after dopamine-agonist therapy was initiated. The other 36 patients required surgery or refused dopamine-agonist therapy. Patients underwent pituitary MRI at baseline, after 3–6 and 12 months of treatment, and annually thereafter.

Mean tumor shrinkage was 30% in cabergolide-treated and quinagolide-treated patients, which suggested that both agents reduced the need for postoperative radiotherapy. Pituitary uptake of ¹²³I-epidepride did not predict patients' responses to dopamine-agonist therapy, although when tumor shrinkage was defined as a decrease of >20% on MRI, there was a trend for shrinkage to correlate with uptake of ¹²³I-epidepride, especially for tumors with moderate uptake.

The authors conclude that ¹²³I-epidepride SPECT is most useful to differentiate scar tissue from recurrent or residual tumor.