POSTIRRADIATION CHANGES IN THE PELVIC WALL - FINDINGS ON MR

MR images of 45 patients who had received radiation therapy for carcin oma of the anus or recurrent carcinoma of the rectum were reviewed wit h regard to postirradiation changes of the pelvic wall. High signal in tensity in bone marrow on T1-weighted images due to fatty replacement was almost always observed. Presacral edema occurred in 7 of 36 patien ts who were examined 4 to 6 weeks after the end of irradiation and was more frequent at later studies. The pelvic wall muscles showed high s ignal intensity on T2-weighted images compatible with edema. This find ing was most frequent on studies performed more than 6 weeks after the end of irradiation. The changes subsided more than a year after radia tion therapy. To avoid an erroneous diagnosis of tumor infiltration in to the pelvic wall, it is important to be familiar with the normal pos tirradiation changes of the presacral space and the muscles.