Experience in interpreting the anorectal region in computed tomography colonography

Abstract

Computed tomography (CT) colonography is a highly useful diagnostic examination method for colorectal cancer screening. However, one of its disadvantages is that it cannot be used to easily perform diagnoses in the anorectal region. To clarify the optimal considerations for accurately diagnosing anorectal lesions, we retrospectively compared CT colonography images with colonoscopic images. Fifty-five cases in which colonoscopic images could be obtained for comparisons included 1, 3, 3, and 48 cases with false-negative, false-positive, true-positive, and true-negative findings in the anorectal region, respectively. The false-negative case involved a type 4 advanced signet ring cell carcinoma, while the false-positive cases involved hypertrophied anal papillae, benign polyps, and suspected residues. The findings that could result in false-positive assessments included ridges caused by longitudinal folds that converged at the anal canal and pseudo-elevation of lesions as a result of the use of a rectal catheter balloon. Candidates for true-positive lesions were those with lesions at some distance from the anus and lesions > 5 mm in size. Notably, lesions very close to the anus may be deformed or obscured by an inflated balloon. Contrastingly, some lesions may be concealed in the anal canal and obscured due to the deflation of the balloon. The findings of this report indicate the importance of recognizing the greater likelihood of false-negative and false-positive findings in the anorectal region. Physicians needs to interpret anorectal region carefully.