Magnetic resonance imaging predicts sphincter invasion of low rectal cancer and influences selection of operation

Background. With the development of numerous sphincter-saving surgical techniques in the last 2 decades, the indication for abdominoperineal resection in radical-elective operations has been markedly reduced. The preoperative assessment of the extent of local tumor growth is essential for the planning of the optimal surgical procedure. Magnetic resonance imaging (MRI) proved to be a reliable method for local staging of low rectal carcinoma. The objective of this study was to determine the frequency of sphincter invasion in an unselected population with low rectal cancer. Methods. From 1997 to 1999, 40 patients with histologically verified adenocarcinoma of the lower rectum ( 5 cm above the linea dentata) without evidence of metastases underwent a MRI with a body coil (no anal endocoil). The MRI results were compared with the operative situs and with pathohistologic findings. Results. An infiltration of the sphincter ani internus was observed in 11 cases (28%), and a combined infiltration of the sphincter ani internus and externus was found in 2 patients (5%). The median distance of the lower tumor edge to the upper border of the anal canal was 2.0 cm (range, 0-4.5 cm). No infiltration of the external sphincter was observed in-patients with cancers above the anal canal. Nine patients (22%) were treated with intersphincteric resection and coloanal anastomosis, 12 (30%) with ultralow resection, and 11 (28%) with low anterior resection of the rectum in conjunction with coloanal anastomosis or a stapled anastomosis. Eight (17%) of, the patients were treated with abdominoperineal resection. Conclusion. An infiltration of the internal sphincter occurs only in 28% of low rectal cancers; an infiltration of the external anal sphincter is extremely rare and occurred only in patients with cancers located in the anal canal. Pelvic MRI offers a precise preoperative visualization of sphincter infiltration in patients with low rectal cancers and is therefore a valuable tool for planning of rectal surgery.