Colorectal adenomas: inflammatory changes that simulate malignancy after laser coagulation--evaluation with transrectal US.

To evaluate the assumption that inflammatory changes in the rectal wall after treatment of colorectal adenoma with photocoagulation may simulate malignant infiltration, the findings were reviewed in 23 follow-up examinations performed with transrectal ultrasound (TRUS) in 17 patients with benign adenoma. Special attention was given to the appearance of the wall layers underneath the adenoma. After partial coagulation, changes in the colorectal wall were usually seen on TRUS scans obtained early in follow-up; sometimes the thickened muscle layer had an irregular outer margin. In three of the 13 early examinations, lesions mimicked malignancy. To correlate these findings with histologic proof, an experimental study was performed: Colorectal specimens from pigs were examined with ultrasound (US) and microscopy. The abnormal sonographic appearance of specimens on US scans was caused by an inflammatory reaction in the deep wall layers; concomitant serositis appeared to be responsible for the malignancy-mimicking irregular outer margin. In screening for malignancy in colorectal adenoma, TRUS may cause overstaging during the first 6 weeks after polypectomy or after a session of laser treatment and should be avoided.