Colonic Endometriosis with Extensive Smooth Muscle Metaplasia Simulating Leiomyoma

It is difficult to distinguish colonic endometriosis with smooth muscle metaplasia from a colonic \nneoplasm. Especially, in a small biopsy specimen, marked smooth muscle proliferation can be \nmisdiagnosed as a spindle cell lesion such as leiomyoma or as a gastrointestinal stromal tumor. \nWe present the case of a woman aged 48 years who had a colonic polyp, raising suspicion of a \nsubmucosal tumor. In a endoscopic polypectomy specimen, the lesion showed marked smooth \nmuscle proliferation but in the resection specimen, a nodular mass, composed of endometrial \nglands, stroma, and a smooth muscle component, was identified. By immunohistochemical \nstaining, smooth muscle cells were positive for estrogen receptor, progesterone receptor, smooth \nmuscle actin, and smooth muscle myosin heavy chain. Endometriosis in the colon can be \nmisinterpreted as a benign or malignant neoplasm. When examining small biopsy specimens, \nclinicians and pathologists should be aware of the potential of this condition to mimic other \nintestinal diseases.