PET/CT detection of incidental colorectal foci of F.D.G. uptake: Correlation with colonoscopy results

Purpose: F.D.G.-PET is an established tool for the diagnosis of recurrent or metastatic colorectal carcinoma. Several case series suggest that F.D.G.-PET often detects incidental adenomatous polyps or colorectal adenocarcinomas. The aim of this study was to correlate unexpected colorectal foci of F.D.G. uptake to pathology findings after systematic colonoscopy. Patients and methods: We reviewed the records of 3541 patients who underwent F.D.G. PET/CT in our institution over a 30-month period for the assessment of a known or suspected malignancy. In 85 of them, incidental, nodular shaped and well-circumscribed foci of abnormal uptake were identified in the area of the colon or rectum. Patients with segmental or diffuse abnormal colorectal uptake were excluded, as well as patients with known benign or malignant colorectal disease. Colonoscopy and complete pathology report was available in 29 patients. Maximal standardized uptake value (SUVmax) was measured in all lesions. Results: Unexpected colorectal foci of F.D.G. uptake were associated with colonoscopic abnormalities in 23 patients (true positive rate: 79 %). Adenocarcinomas were found in six patients (SUVmax = 7.3 ± 2.6), tubular adenomas in four patients (SUVmax = 7.3 ± 4.9) and tubulovillous adenomas in 12 patients (SUVmax = 4.2 ± 1.1). Hyperplasic polyps with no sign of dysplasia were found in the last patient (SUVmax 3.3). Concomitant CT abnormalities were found on PET/CT fusion in eight patients and consisted of wall thickening (n = 5) or nodular mass (n = 3). Conversely, PET was falsely positive in six patients (21 %), with no concomitant CT abnormalities and no abnormal findings at endoscopy (SUVmax 6.2 ± 2.8, no significant difference with true positive lesions). Conclusion: Our findings emphasize the need to perform a colonoscopy in front of incidental nodular colorectal foci of F.D.G. uptake because malignant or pre-malignant neoplasms, which are not clinically apparent, are found in more than three-quarter of cases. (authors)