[Somatostatin receptor scintigraphy. When would a SPECT study be performed?]

Somatostatin receptor scintigraphy has become an essential tool in the treatment planning of patients with carcinoid tumor (CaT) and pancreatic neuroendocrine tumors (PNET). However, the contribution of SPECT images in the diagnosis and staging of these tumors has not been well established.

1) To find out if SPECT improves the sensitivity of planar imaging and adds any value to tumor diagnosis, and 2) To determine under which circumstances SPECT images must be obtained.

We have evaluated 49 patients, 26 females and 23 males, using both planar and SPECT 111In-DTPAOC images. The patients were classified into three groups according to their clinical indication: Group A: Indolent symptoms of CaT or PNET (n = 9); Group B: Known or functioning CaT or PNET tumors (n = 22); Group C: Post-surgical follow-up (n = 18). The total number of SPECT studies were 51 (37 abdomen, 13 thorax and 1 brain). In all cases SPECT was performed 24 hours after 111In-DTPAOC injection. Scintigraphic images were compared with surgical findings in 17 patients and with a synthesis of biochemical, clinical and CT findings in the other patients.

16 primary lesions were detected with planar images whereas SPECT detected 18. The two tumors detected only by SPECT were a bronchial CaT and an insulinoma located in the pancreatic tail. Planar and SPECT image sensitivities were 66.6% and 75% respectively. Twenty one patients developed metastatic disease. Sensitivities of planar and SPECT images for metastases were 76.1% and 90.4% respectively. Furthermore, SPECT helps planar images in the evaluation of six patients with uncertain images (lesions with low receptor density or with an uncertain scintigraphic localization). Therefore, in 15.6% of patients with primary or metastatic tumors, planar images were normal whereas SPECT was abnormal and SPECT provided an added value in 21.8% of the studies with abnormal planar images.

We recommend the use of SPECT images in: 1) Patients with specific functional clinical syndrome or with a known CaT or PNET and normal planar images. 2) When planar images are abnormal only in primary tumor. 3) In order to confirm the absence of other metastases in patients with known liver metastases. 4) In order to enhance the detection of lesions with low receptor density and uncertain planar images and 5) In order to determine exactly the anatomic location of some lesions