How to accurately measure the distance from the anal verge to rectal cancer on MRI: a prospective study using anal verge markers

To determine an accurate method for localizing rectal cancer using the distance from the anal verge on preoperative MRI. This prospective study included 50 patients scheduled for MRI evaluation of rectal cancer. After rectal filling with gel, MRI was performed with two markers attached at the anal verge. The distance between the tumor and the anal verge on a sagittal T2-weighted image (T2WI) was measured independently by two radiologists using six methods divided into three groups of similar measurement approaches, and compared to those obtained on rigid sigmoidoscopy. The anal verge location relative to the external anal sphincter was assessed on oblique coronal T2WI in reference to the markers. Correlation analysis was performed using the intraclass correlation coefficient (ICC) for verification, and a paired t test was used to evaluate the mean differences. The highest correlation (ICC 0.797–0.815) and the least mean difference (0.74–0.85 cm) with rigid sigmoidoscopy, and the least standard deviation (3.12–3.17 cm) were obtained in the direct methods group using a straight line from the anal verge to the tumor. The anal verge was localized within a range of − 1.4 to 1.5 cm (mean − 0.31 cm and − 0.22 cm) from the lower end of the external anal sphincter. The direct methods group provided the most accurate tumor distance among the groups. Among the direct methods, we recommend the direct mass method for its simplicity. Despite minor differences in location, the lower end of the external anal sphincter was a reliable anatomical landmark for the anal verge.