Objective To investigate the predictive values of ¹⁸F-fluorodeoxyglucose (FDG) PET/CT in patients with incidental focal colorectal FDG uptake in diagnosing early colorectal adenocarcinoma and advanced adenomas by comparing such values with colonoscopic and histopathologic findings.

Methods Fifty-six patients (31 males and 25 females, aged (66±11) years, range 38–84 years) were retrospectively enrolled for the study. These patients underwent ¹⁸F-FDG PET/CT for noncolorectal lesions with incidental focal colorectal FDG uptake in Beijing Hospital between January 2016 and June 2020. Taking the results of colonoscopy and histopathology within 3 months after ¹⁸F-FDG PET/CT examination as the final diagnostic criteria, the predictive values of ¹⁸F-FDG PET/CT for early colorectal adenocarcinoma and advanced adenoma were analyzed. The normally distributed measurement data were compared between two groups using independent sample t-test (with equal variance) and among multiple groups using analysis of variance. Skewed distribution measurement data were compared between these two groups using Mann-Whitney test, whereas those for multiple groups were compared using the Kruskal-Wallis test. Rate enumeration data were also compared using the Chi-square test. Furthermore, the receiver operating characteristic (ROC) curve was delineated to analyze the diagnostic efficacy and optimal diagnostic threshold of maximum standardized uptake value (SUV_max) for diagnosing early colorectal adenocarcinoma and advanced adenomas.

Results Fifty-six patients were enrolled in this study. A total of 74 incidental colorectal ¹⁸F-FDG uptake foci were found, while colonoscopy revealed 140 lesions. Among 74 incidental colorectal ¹⁸F-FDG uptake foci, 59 and 15 foci were found to be positive and negative, respectively, during colonoscopy. Among 59 positive colonoscopic findings, 69.5%(41/59) were early colorectal adenocarcinoma (n=7) and advanced adenomas (n=34). Among 81 lesions that were considered missed diagnoses via ¹⁸F-FDG PET/CT, 55.6% were nonadvanced adenomas (n=45) and 27.2% were nonadenomatous polyps (n=22). The sensitivities of ¹⁸F-FDG PET/CT in nonadenomatous polyps, nonadvanced adenomas, advanced adenomas, and early colorectal adenocarcinoma were 26.7% (8/30), 10.0% (5/50), 72.3% (34/47), and 100% (7/7), respectively (χ²=35.09, P<0.001). The positive predictive value of ¹⁸F-FDG PET/CT in early colorectal adenocarcinoma and advanced adenomas was 55.4% (41/74). In terms of the ROC analysis, the optimal cut-off value of SUV_max for diagnosing early colorectal adenocarcinoma and advanced adenomas was 11.6.

Conclusions In this study, ¹⁸F-FDG PET/CT showed high positive predictive value and sensitivity in diagnosing early colorectal adenocarcinoma and advanced adenomas. According to the findings, follow-up colonoscopy is needed, especially in incidental focal colorectal ¹⁸F-FDG uptake lesions with high SUV_max.