111In-octreotide scintigraphy in metastatic medullary thyroid carcinoma before and after octreotide therapy: in vivo evidence of the possible down-regulation of somatostatin receptors.

We have investigated the presence of somatostatin receptors on the cell surface of metastatic medullary thyroid carcinoma in vivo using 111In-Octreotide scintigraphy. Five patients were studied before and three months after therapy with octreotide (300-600 micrograms/day). After each 111In-Octreotide scintigraphy the target/background (T/B) radioactivity ratio was calculated for each detectable metastases. A total of 14/18 metastases showed a reduction in the T/B ratio after therapy, suggesting saturation or down-regulation of the somatostatin receptors on metastases induced by octreotide therapy. Patients also showed a reduction in serum calcitonin levels after therapy. We conclude that 111In-Octreotide scintigraphy may be useful in medullary thyroid carcinoma to evaluate the rationale for somatostatin therapy and to monitor the effect of treatment.