Imaging of dopamine D2 and somatostatin receptors in vivo using single-photon emission tomography in a patient with a TSH/PRL-producing pituitary macroadenoma

A 28-year-old man with a thyroid stimulating hormone/prolactin (TSH/PRL)-secreting pituitary macroadenoma is discussed in relation to dopamine D₂ and somatostatin receptor single-photon emission tomography (SPET). The patient presented with decreased vision in the left eye as a result of a temporal visual field defect and with mild hyperthyroidism. Medical therapy was tried. A test dose of both octreotide and bromocriptine resulted in an acute reduction in serum levels of TSH, -subunits and PRL, whereas there was no response to TRIAC. Somatostatin and dopamine D₂ receptors were present on the tumour as visualised by SPET with the ligands indium-111 diethylene triamine penta-acetic acid (DTPA)-octreotide (¹¹¹In-SMS) and iodine-123 iodobenzamide (¹²³I-IBZM), respectively. Therefore, treatment with octreotide 150 g t.i.d. subcutaneously and bromocriptine 10 mg b.i.d. orally was given for > 12 and > 6 weeks, respectively. Following this treatment the visual defects disappeared, although tumour size, as measured by CT scanning, and serum TSH levels did not decrease. SPET with ¹¹¹In-SMS and ¹²³I-IBZM after therapy revealed no change or a possible increase in somatostatin receptor binding potential and a possible decrease in dopamine D₂ receptor binding potential. The lack of long-term effects of the medical treatment is discussed. It is concluded that a high somatostatin and dopamine D₂ receptor binding potential in vivo in a TSH/PRL-producing adenoma does not necessarily predict a successful outcome of medical treatment.Correspondence to: N.P.L.G. Verhoeff