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Results of somatostatin receptor scintigraphy do not predict pituitary tumor volume- and hormone-response to octreotide therapy and do not correlate with tumor histology

The value of somatostatin receptor scintigraphy (SRS) to predict the effect of somatostatin analog therapy on pituitary adenomas is not clear, due to the use of different radiopharmaceuticals (¹²³I-Tyr³-octreotide and ¹¹¹In-pentetreotide) and the small number of patients in previous studies. We used ¹¹¹In-pentetreotide scintigraphy in 49 patients in order to (i) correlate SRS results with basal tumor volume as well as volume- and hormone-response to 3 months of octreotide therapy (Oct-Tx), (ii) identify tumor remnants after incomplete surgery and (iii) evaluate any correlation with immunohistology. Twenty-five patients had a GH-secreting adenoma (GH-A, 15 prior to intended surgery, 10 with persistent/recurrent disease after previous therapy). Twenty-four patients had a clinically nonfunctioning adenoma (NF-A). For SRS, planar and single photon emission computer tomographic images (SPECT) were recorded 4 h and 24 h post injection. SRS grading was as follows: GO, no uptake; Gl, uptake comparable to normal pituitary; G2, increased uptake; G3, very intense uptake. G2/3 was seen in 8/25 GH-A and in 12/24 NF-A. Pretreatment tumor volume (magnetic resonance imaging (MRI)) tended to be related to ¹¹¹In-pentetreotide uptake in GH-A with a tumor visible on MRI (G0/1 (n=10) vs G2/3 (n=8): 3·6±1·9 vs 10·±6·5 cm³ (mean±S.E.), P=0·051), but not in NF-A (g0/1 (n=12) vs G2/3 (n=12): 17·0±10·1 vs 14·3±3·6 cm³). SRS did not identify a tumor remnant in the 7 MRI-negative patients with persistent post-operative acromegaly. Basal GH (6-h profile) and IGF-I in GH-A did not correlate with SRS results (G0/1 (n=17) vs G2/3 (n=8), GH: 32·3±18·2 vs 29·3±7·4 µg/l, IGF-I: 851±80 vs 1038±153 µg/l). During Oct-Tx of GH-A neither tumor shrinkage nor GH suppression was related to SRS results. In 6 NF-A classified as gonadotropinomas (by their plasma glycoprotein hormone or -subunit concentrations, basally and/or in response to TRH) ¹¹¹In-pentetreotide uptake was not different from that of the non-gonadotropin/non-secreting adenomas. SRS results were not related to the immunohistological subtype in 22 GH-A (monohormonal, mixed somatotrope/lactotrope, plurihormonal) or in 22 NF-A (null-cell adenomas, gonadotropinomas, silent hormonal adenomas). We conclude that ¹¹¹In-pentetreotide SRS reflects tumor volume poorly in GH-A and not at all in NF-A. It does not predict the effect of Oct-Tx on the volume of both GH-A and NF-A, nor on the GH concentration in GH-A. ¹¹¹In-pentetreotide SRS is unable to identify postoperative tumor remnants not visible on MRI.

European Journal of Endocrinology 136 369–376