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

UNLABELLED
Previous studies have failed to predict somatostatin analog response with somatostatin receptor scintigraphy in pituitary adenomas. In vitro studies have shown that the density of somatostatin receptors in pituitary tumors might be critical for octreotide response.


METHODS
The density of somatostatin receptors was calculated in vivo combining the uptake index obtained from somatostatin receptor scintigraphy and the tumor volume obtained by MRI. The ratio of these two values, called density index (DI), was established in 32 of 37 consecutive patients with pituitary adenomas (11 had growth hormone-secreting adenomas, 4 thyroid-stimulating hormone-secreting and 17 nonfunctioning). It was compared with hormonal response, assessed in 15 secreting adenomas on growth hormone or thyroid stimulating hormone suppression (which was considered significant when it reached at least 50% of basal level), and with tumor shrinkage (which was considered significant when > or =20% of pretherapeutic value) in 12 secreting and 14 nonfunctioning adenomas.


RESULTS
In agreement with previous reports, uptake index is not predictive of octreotide response. In contrast, DI predicts both hormonal suppression and tumor shrinkage (P = 0.009 and P = 0.0002, respectively) obtained with octreotide therapy. DI sensitivity, specificity and accuracy 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 combination of scintigraphic and MRI data allows the computation of a DI for somatostatin receptors that points out patients who can profit from somatostatin analog treatment.