Frequency of normal/abnormal focal uptake of small intestine on ¹⁸F-FDG PET-CT

Objectives: ¹⁸F-Fluorodeoxyglucose (FDG) PET reflecting whole body glucose metabolism and is widely used for assessment of malignant tumors. However, physiological uptake often complicates interpretation. Especially, gastrointestinal tracts occasionally show focal physiological uptake, making it difficult to differentiate physiological from abnormal uptakes such as malignant tumor and inflammatory disease. In contrast to stomach and large intestine, endoscopic investigation of small intestine is not available in many institutes. Therefore, nuclear medicine physicians are highly expected to determine whether to recommend endoscopic test for small intestine. In this retrospective study, we aimed to reveal frequencies of normal (physiological) vs. abnormal (malignant and inflammatory) uptakes in small intestine. [Methods]We reviewed radiology reporting system in our institute. Between December 2008 and September 2017, a total of 23,216 FDG PET-CT examinations were performed, among which nuclear medicine physician(s) diagnosed incidental uptake in small intestine for 120 locations in 115 examinations. Any diffuse uptake findings were excluded. Mean patient age was 66.5±12.1 years old. Two PET-CT scanners were used; Biograph 64 PET-CT scanner (Asahi-Siemens Medical Technologies Ltd., Tokyo, Japan) and Gemini TF64 TOF-PET/CT scanner (Hitachi Medical Corporation Ltd, Tokyo, Japan). The patients were required to fast longer than 6 hours before the exam. One hour after intravenous FDG (4.5MBq/kg) injection, the 3-min/bed emission acquisition was followed by CT scanning for attenuation correction. Attenuation-corrected radioactivity images were reconstructed using OSEM-based

Methods: The images and any clinical records were reviewed by 2 physicians in this study to classify focal uptakes in small intestine into 3 groups: a) abnormal uptake, b) normal uptake, and c) unknown. Any discrepancy was solved by discussion. The uptake was considered as ‘abnormal’ when any lesions were confirmed by small intestine endoscope, capsule endoscope, laparotomy, and/or pathological investigation. The uptake was considered as ‘normal’ when follow-up FDG PET-CT revealed disappearance of the uptake, follow-up CT (>1 year) detected no related diseases, or endoscopic examination detected no abnormality. Otherwise, the uptake was classified as ‘unknown’. SUVmax was measured for all the locations. Contrast/non-contrast CT finding was considered abnormal when nodule, wall thickening, or intussusception was observed. [Results]Among 120 uptakes, 51 (43%), 40 (33%), and 29 (24%) were classified into abnormal, normal, and unknown group. Fifty-one abnormal uptakes consisted of 17 (33%) metastatic disease, 13 (25%) malignant lymphoma, 3 (6%) GIST, and 7 (14%) others. Importantly, all the 26 lesions accompanied with CT abnormality were ‘abnormal’ (100%). In contrast, of 65 uptakes without CT abnormality, 51 (78%) were normal whereas 14 (22%) were abnormal (table). Based on CT findings, sensitivity, specificity, and accuracy for existence of lesion were 65%, 100%, and 84%, respectively. Overall, SUVmax (mean±SD) was higher in abnormal uptake (11.1±6.5) than in normal uptake (7.9±3.3, P=0.0030). However, if only the uptakes without CT abnormality were considered (N=65), SUVmax was not different between groups. [Conclusion]When focal FDG uptake and corresponding CT abnormality were observed in small intestine, the positive predictive value (PPV) was 100% and thus small intestine endoscopy should be recommended. In contrast, in case of focal uptake without CT abnormality, PPV was only 22% and thus short-term follow-up or less-invasive capsule endoscope may be better than small intestine endoscopy. Note that SUVmax was not useful for diagnosis.

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