Defining the distal margin of rectal cancer for surgical planning.

Accurate measurement of the distal rectal tumor margin is essential in selecting the appropriate surgical procedure. However, there is no standard measurement method. The National Cancer Institute consensus group recommends use of the anal verge (AV) as a landmark, and the European Society of Gastrointestinal and Abdominal Radiology recommends use of the anorectal ring (ARR). In addition, whether measurements should be made on double contrast barium enema (BE) radiographs or magnetic resonance (MR) images remains controversial. We measured the distal tumor margin on both BE and MR images obtained preoperatively from 52 patients who underwent sphincter-saving resection for rectal cancer. The distances from the distal end of the tumor to the AV and the ARR were measured on both types of images, and the variability was investigated by Bland-Altman analysis. The mean distance from the tumor to the AV was 8.9 cm on the BE radiographs and 7.7 cm on the MR images (P=0.013). The mean distances to the ARR were 6.8 and 5.6 cm, respectively (P=0.070). Significant proportional bias was shown as the measured distances increased, the difference between the BE- and magnetic resonance imaging (MRI)-based measurements increased. Use of one or the other landmark did not affect selection of the appropriate surgical procedure. We conclude that an approximate 1-cm underestimation should be taken into account when MRI-based measurement of the distal rectal tumor margin is used to choose between sphincter-saving resection and abdominoperineal resection.