Rectal carcinoma: MRI with histologic correlation before and after chemoradiation therapy

OBJECTIVE. The purpose of this study was to use MRI to compare the morphologic features of rectal cancer before and 6 weeks after chemotherapy and radiation treatment to correlate the posttreatment MRI appearances with the histologic findings in resected tumors. MATERIALS AND METHODS. High-resolution T2-weighted MRI was performed before and immediately after a standardized 5-week course of chemoradiation therapy in the care of 30 patients with locally advanced adenocarcinoma of the rectum. Changes in morphologic features were evaluated with respect to primary tumor and nodal downstaging. The MRI findings after chemoradiation therapy were compared with the histologic findings in the resected specimens with respect to prediction of tumor stage and showing the relation between the tumor and the circumferential margin of resection. RESULTS. Tumor shrinkage > 30% was found in 19 (63%; 95% CI, 46 - 81%) of 30 patients, but changes in MRI T stage occurred in only five (17%; 95% CI, 3 - 30%) of 30 patients. Tumor regression from the circumferential resection margin was found in five patients, all findings confirmed at histologic examination. Nodal downstaging was observed in 13 (68%; 95% CI, 48 - 89%) of 19 patients; 11 patients were node free on the basis of both MRI findings and subsequent histologic results. Overall prediction of distance between tumor and circumferential resection margin was good, with a mean difference of - 0.2 mm and an interclass correlation coefficient of 0.74. MRI was not useful for gauging disease activity of persistent abnormalities in mucinous tumors that often represented inactive mucin lakes. CONCLUSION. Decreases in tumor size and nodal downstaging can be seen on MRI after chemoradiation therapy in approximately two thirds of patients. The surgically more relevant parameter - distance between tumor and circumferential resection margin - can be accurately predicted. Errors were caused by the presence of considerable tumor, rectal wall fibrosis, and mucinous tumors.