Endoscopic and histopathological correlation of incidental colorectal uptake of 18F-FDG in oncological patients

Introduction: Incidentalomas can be defined as an unexpected but clinically significant area of uptake unrelated to the primary neoplasm. It is common to find them in organs such as the thyroid, adrenal, parathyroid, prostate, and gastrointestinal system. More findings are identified in the colon and rectum because they are glandular tissues with a high rate of cellular metabolism. Three different types of uptakes have been described at this level: diffuse, segmental, and focal.

The aim of the study is to identify incidental colon uptakes that can be correlated with malignant or premalignant findings and require further study.

Methods: We retrospectively analyzed oncological patients PET CT in which incidental ¹⁸F-FDG colorectal uptake was observed, and who underwent endoscopy within 40 days, and biopsy if required. Patients with known colon cancer were excluded.

Results: Forty-one patients were included, of whom 63.4% (26/41) were women and 36.6% (15/41) were men, with a mean age of 60 years (SD ± 15.8). 34.1% (14/41) had a haematological malignancy diagnosis, 24.4% (10/41) breast cancer, 9.8% (4/10) lung cancer, and 31.7% (13/41) another neoplasm (hepatocarcinoma, lung cancer, gallbladder cancer, melanoma, GIST, cervical cancer, esophageal cancer, ovarian and endometrial cancer). 61% (25/41) were focal uptake, 29% (12/41) segmental uptake, and 10% (4/41) diffuse uptake. The most frequent location was the ascending colon 26.8% (11/41), 24.4% (10/41) in rectum, 14.6% (6/41) in the sigmoid, 12.2% (5/41) in descending colon, 9.8% (4/41) in the cecum, 7.3% (3/41) in the transverse colon and 4.9% (2/41) in the anal canal.

Within focal uptake, the endoscopic findings were polyps in 48%, tumors in 28%, inflammation in 12% and normal findings in 12%, wich did not require biopsy. In the pathology report, 58% of the polyps corresponded to premalignant lesions, 25% to malignant lesions, and the rest 17% to benign lesions. Regarding the tumors, the pathology report was 86% for malignant lesions, and 14% for benign findings. The correlation of focal uptake with endoscopy (p<0.05) and biopsy (p<0.01) findings were statistically significant, but the correlation with SUVmax was not.

Conclusions: The focal uptake pattern has greater clinical relevance since it correlates with the presence of premalignant or malignant lesions in subsequent colonoscopies. The diffuse and segmental pattern has been seen mostly related to physiological or inflammatory uptake, making them less clinically relevant.

A correlation greater than 85% was demonstrated between the location of the anatomical lesion by endoscopy and the focal uptake of [¹⁸F]FDG, however it does not provide adequate differentiation between a neoplastic lesion from a non-neoplastic or inflammatory one. In addition, optimal discrimination between benign lesions such as hyperplastic or inflammatory polyps, and premalignant and malignant lesions such as adenomas and adenocarcinomas based on the semiquantitative values of the PET/CT has not been shown, so that in the case of a colorectal incidentaloma, subsequent examination is essential.

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