Somatostatin receptor scintigraphy in pituitary adenomas: A somatostatin receptor density index can predict hormonal and tumoral efficacy of octreotide in vivo

Previous studies have failed to predict somatostatin analog response with s omatostatin receptor scintigraphy in pituitary adenomas. In vitro studies h ave shown that the density of somatostatin receptors in pituitary tumors mi ght be critical for octreotide response. Methods: The density of somatostat in receptors was calculated in vivo combining the uptake index obtained fro m somatostatin receptor scintigraphy and the tumor volume obtained by MRI. The ratio of these two values, called density index (DI), was established i n 32 of 37 consecutive patients with pituitary adenomas (11 had growth horm one-secreting adenomas, 4 thyroid-stimulating hormone-secreting and 17 nonf unctioning). It was compared with hormonal response, assessed in 15 secreti ng adenomas on growth hormone or thyroid stimulating hormone suppression (w hich was considered significant when it reached at least 50% of basal level ), and with tumor shrinkage (which was considered significant when greater than or equal to 20% of pretherapeutic value) in 12 secreting and 14 nonfun ctioning adenomas. Results: In agreement with previous reports, uptake inde x is not predictive of octreotide response. In contrast, DI predicts both h ormonal suppression and tumor shrinkage (P = 0.009 and P = 0.0002, respecti vely) obtained with octreotide therapy. DI sensitivity specificity and accu racy were 92% each, and a positive correlation was found between DI and the percentage of tumor shrinkage (r = 0.54, P = 0.004). Conclusion: The combi nation of scintigraphic and MRI data allows the computation of a DI for som atostatin receptors that points out patients who can profit from somatostat in analog treatment.