Objective: Conventional radiological techniques such as CT and MRI have limitations for the differentiation of adrenal lesions.Some adrenal tumouts express somatostatin receptors, and can be visualized by radiolabelled somatostatin analogue scintigraphy.The aim of this study was to assess the differential diagnostic value of somatostatin receptor imaging with 99mTc-HYNIC-TOC (99mTc-hydrazinon-icotinyl-Tyr3-octreotide) for adrenal tumors, including adrenal pheochromocytoma, adrenal adenoma, adrenocortical carcinoma and so on.Methods: 91 patients (47 men, 44 women; age range, 26-67 years) with documented adrenal lesions were enrolled in this study, including 38 adrenal pheochromocytoma, 30 adrenal adenoma,7 adrenal struma, 5 adrenal cyst, 3 adrenocortical carcinoma, 2 adrenal metastases, 2 adrenal lymphoma and so on.The diagnoses were confirmed by pathohistology.All patients had one or more results of conventional imaging techniques, such as CT, MRI or ultrasonography (US).All patients gave written informed consent.Regional, whole body and SPECT or SPECT/CT (in positive cases) imagings were obtained at 1 and 4 hours after an intravenous injection of 350-400MBq 99mTc-HYNIC-TOC.Results: Positive findings were obtained in 25 (13 left, 8 right, 4 bilateral) of 38 patients with adrenal pheochromocytoma, and 13 (5 left, 8 right) cases were negative.The detection rate of adrenal pheochromocytoma with 99mTc-HYNIC-TOC imaging was 65.8%.Among the 25 positive cases, mean adrenal lesion-background (T/B) activity ratio was 4.8(range 3.7-7.6).11 cases (44%)were localized by 99mTc-HYNIC-TOC SPECT or SPECT/CT imaging, but whole body and regional imaging failed to detect them.The range of lesion size was 0.8-3.5cm.So the diagnostic efficacy of 99mTc-HYNIC-TOC imaging in adrenal pheochromocytoma can be raised by using SPECT or SPECT/CT imaging.The lesion was too small and the near organs including liver,spleen and kidneys had high uptake of 99mTc-HYNIC-TOC, which disturbed the finding of regional imaging.Of 13 negative lesions (range 1.5-4.0cm), 2 had cystic degeneration.The reason of negative findings included:(1) the lesion was too small; (2) the lesion didnt express somatostatin receptors or expressed too low; (3)the lesion had cystic degeneration.There were only 2 (1 left, 1 right) positive findings in 30 patients with adrenal adenoma,and 28 (9 left, 17 right, 2 bilateral) negative results.The detection rate of adrenal adenoma with 99mTc-HYNIC-TOC imaging was 6.7%.T/B ratio of the 2 positive lesions (1.3 and 1.5) were not high, which were localized by SPECT imaging.Morever, 1 metastatic adrenal gland lesion with breast cancer, 1 adrenal struma, 1 adrenal cyst, 1 adrenocortical carcinoma, 1 adrenal lymphadenosis and l adrenal synoviosarcoma were positive too, but the T/B ratio was less than 2.0.Conclusions: 99mTc-HYNIC-TOC imaging is a highly accurate and helpful method for differentiating adrenal pheochromocytoma from other adrenal tumours and benign lesions.It is also powerful for differentiating benign from malignant adrenal masses, and finding metastatic lesions particularly.Occasional adrenal tumours show mild increased 99rmTc-HYNIC-TOC uptake compared with that of the backgroung, but the T/B ratio of the lesions is lower than that of adrenal pheochromocytoma.