An 80-year-old man was diagnosed with rectal cancer and underwent Hartmann's procedure. Although no tumors were identified during the preoperative examination, gross examination of the resected specimen incidentally revealed a submucosal tumor that was 9 mm in diameter at the oral side and located in the proximal stump of the specimen from the sigmoid colon. We suspected a concurrent gastrointestinal stromal tumor (GIST) and performed a histopathological examination. An L-shaped nodular lesion measuring 9 × 6 mm was histologically composed of a patternless proliferation of spindle cells intermingled with eosinophilic globules. Cellular atypia, prominent mitotic figures and necrotic foci were not observed in the nodule. The spindle cells were positive for CD34, CD117 and vimentin, but negative for CD56, smooth muscle actin and S-100 protein. MIB-1 positivity was estimated to be as low as approximately 1–2%. Electron microscopy showed a bundle of wool-like fibers with a periodicity of approximately 40 nm. We therefore considered the lesion to be a low-risk GIST with skeinoid fibers in the large intestine. Although numerous previous reports have reported skeinoid fibers in the stomach and small intestines, there have been only 9 cases (including the present case) of skeinoid fibers in the large intestine.
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