68Ga-DOTATOC PET/CT and somatostatin receptor (sst1-sst5) expression in normal human tissue: correlation of sst2 mRNA and SUVmax

Purpose By targeting somatostatin receptors (sst) radiopeptides have been established for both diagnosis and therapy. For physiologically normal human tissues the study provides a normative database of maximum standardized uptake value (SUVmax) and sst mRNA. Methods A total of 120 patients were subjected to diagnostic Ga-68-DOTATOC positron emission tomography (PET)/CT (age range 19-83 years). SUVmax values were measured in physiologically normal tissues defined by normal morphology, absence of surgical intervention and absence of metastatic spread during clinical follow-up. Expression of sst subtypes (sst1-sst5) was measured independently in pooled adult normal human tissue by real-time reverse transcriptase polymerase chain reaction (RT-PCR). Results SUVmax revealed a region-specific pattern (e.g., mean +/- SD, spleen 31.1 +/- 10.9, kidney 16.9 +/- 5.3, liver 12.8 +/- 3.6, stomach 7.0 +/- 3.1, head of pancreas 6.2 +/- 2.3, small bowel 4.8 +/- 1.8, thyroid 4.7 +/- 2.2, bone 3.9 +/- 1.3, large bowel 2.9 +/- 0.8, muscle 2.1 +/- 0.5, parotid gland 1.9 +/- 0.6, axillary lymph node 0.8 +/- 0.3 and lung 0.7 +/- 0.3). SUVmax was age independent. Gender differences were evident within the thyroid (female/male: 3.7 +/- 1.6/5.5 +/- 2.4, p<0.001; Mann-Whitney U test) and the pancreatic head (5.5 +/- 1.9/6.9 +/- 2.2, p<0.001). The sst mRNA was widely expressed and heterogeneous, showing sst1 to be most abundant. SUVmax values exclusively correlated with sst2 expression (r=0.846, p<0.001; Spearman rank correlation analysis), whereas there was no correlation of SUVmax with the expression of the other four subtypes. Conclusion In normal human tissues Ga-68-DOTATOC imaging has been related to the expression of sst2 at the level of mRNA. The novel normative database may improve diagnostics, monitoring and therapy of sst-expressing tumours or inflammation on a molecular basis.