Suspected recurrent rectosigmoid carcinoma after abdominoperineal resection: MR imaging and histopathologic findings.

Eleven patients who underwent abdominoperineal resection for rectosigmoid carcinoma were examined with magnetic resonance (MR) imaging for suspected recurrent tumor in the presacral space. There were 12 suspected masses in the 11 patients. Ten lesions were malignant, and in two lesions only nonneoplastic inflammatory tissue was found. The signal intensity (SI) of each lesion relative to that of muscle was determined visually and correlated with the histologic findings at surgery or biopsy. The SIs of malignant and benign lesions were indistinguishable. All lesions showed areas of high SI that correlated with carcinoma and tumor necrosis or with non-neoplastic inflammation and edema. Areas of low SI corresponded to reactive fibrous tissue (desmoplastic reaction) with small islands of tumor or to nonneoplastic fibrosis. MR imaging can be useful in determining the extent of suspected tumors, but SIs on T2-weighted images do not permit histologic diagnosis. In particular, distinction of benign from malignant tissue is difficult in tumors with a desmoplastic reaction.