Histopathologic tumor spread in very low rectal cancer treated with abdominoperineal resection

We have pathologically evaluated the tumor spread in low rectal cancer treated with abdominoperineal resection to clarify the potential indication of intersphincteric resection and other anus-preserving operations with external sphincter muscle resection.

A total of 197 patients received abdominoperineal resection between 1982 and 2001. We determined histopathologically any invasion or metastasis into the anal canal structures.

When the lowest edge of a tumor was located above the dentate line, the invasion was rarely beyond the internal sphincter muscle, in particular, where the distance between the tumor and the dentate line was longer than 2 cm. When the lowest edge was located at or below the dentate line (Pb-cancer), invasion tended to extend into the external sphincter muscle and into the intermuscular groove. A logistic regression analysis showed that the Pb-cancer, any distant metastasis, and the tumor histology of mucinous carcinoma were each an independent significant risk factor to invasion beyond the internal sphincter muscle, whereas the Pb-cancer, the poorly differentiated adenocarcinoma, and the mucinous carcinoma were each an independent significant risk factor to invasion into the intermuscular groove.

The anus-preserving operation with sphincter muscle resection was theoretically possible for low rectal cancer in patients who underwent abdominoperineal resection. However, the procedure cannot be indicated for a tumor where the lowest edge is below the dentate line and where a preoperative biopsy shows a poorly differentiated adenocarcinoma or mucinous carcinoma, even if the intermuscular groove is macroscopically unaffected by the tumor.