Smooth muscle pseudotumours: a potentially confusing artefact of rectal biopsy.

An artefactual smooth muscle lesion was found in seven of 500 consecutive rectal biopsy specimens. The lesions had the deceptive appearance of a genuine tumour although none of the patients with the lesion had presented with a rectal mucosal swelling. The morphology of the lesion and its poor reproducibility under experimental conditions suggested that it was an artefact of the biopsy procedure: it was easily reproduced in resected specimens of large bowel using punch or basket forceps but not when using flat forceps. The presence of the lesion seems to depend on the type of forceps used rather than on differences in deployment and seems to be caused by avulsion of the superficial part of the muscularis propria and its incorporation into the tissues included in rectal biopsy specimens.