99mTc-EDDA/HYNIC-TOC and 18F-FDG in thyroid cancer patients with negative 131I whole-body scans

Several studies have reported on the expression of somatostatin receptors in patients with differentiated thyroid cancer (DTC). The aim of this study was to evaluate the imaging abilities of a recently developed technetium-99m labelled somatostatin analogue, 99mTc-EDDA/HYNIC-TOC (99mTc-TOC), in terms of precise localisation of disease. The study population comprised 54 patients (24 men, 30 women; age range 22–90 years) with histologically confirmed DTC who presented with recurrent or persistent disease as indicated by elevated Tg levels after initial treatment. All patients were negative on the iodine-131 post-therapy whole-body scans. Fluorine-18 fluorodeoxyglucose positron emission tomography (18F-FDG PET) was performed in a subgroup of 36 patients. The study population consisted of two groups: Group A (n=22) comprised patients with disease recurrence as shown by elevated Tg levels but without detectable pathology. In group B (n=32), pre-existing lesions were known. Among the 54 cases, SSTR scintigraphy was true positive in 33 (61.1%), true negative in 4 (7.4%) and false negative in 17 (31.5%) cases, which resulted in a sensitivity of 66%. A total of 138 tumour foci were localised in 33 patients. The fraction of true positive 99mTc-TOC findings was positively correlated (P<0.01) with elevated Tg levels (higher than 30 ng/ml). Despite two false positive findings, analysis on a lesion basis demonstrated better diagnostic efficacy with 18F-FDG PET (P<0.001); however, it also revealed substantial agreement between the imaging techniques [Cohen’s kappa of 0.62 (0.47–0.78)]. In conclusion, scintigraphy with 99mTc-TOC might be a promising tool for treatment planning; it is easy to perform and showed sufficient accuracy for localisation diagnostics in thyroid cancer patients with recurrent or metastatic disease.