Rectal carcinoma treated by preoperative irradiation: MR imaging and histopathologic correlation

We evaluated the role of MR imaging in assessing the effect of preoperative irradiation in 11 patients with primary rectal carcinoma. Findings on MR images obtained before radiotherapy and 5-6 weeks afterward were analyzed and correlated with the histopathological findings (nine patients) or the findings at laparotomy (two patients). Before irradiation, the tumor volumes on MR images were between 3.3 and 51.7 cm3 (mean 19.7 cm3). After irradiation, the volumes were from 0.8 to 33.2 cm3 (mean, 10.4 cm3), representing a decrease in volume of 11% to 88% (mean, 55%). On the MR images obtained before irradiation, the tumors were confined to the bowel wall in four cases (stage A-B1), penetrated the perirectal fat in six cases (stage B2), and involved an adjacent organ in one case (stage B3). After irradiation, no apparent changes were seen in the MR appearance of the local tumor stage in nine of the 11 patients. In one patient, progression of stage was suspected on the postirradiation MR images, but this was not confirmed at histologic examination. In one patient, possible downstaging occurred after irradiation, although this could not be proved. Our findings suggest that MR imaging may be useful for determining the effect of preoperative radiotherapy on rectal carcinomas.