Primary osteosarcoma arising in the colon: report of a case.

A 53-year-old female presented with abdominal pain, and computed tomography scan revealed a small, calcified lesion in the wall of the transverse colon. The symptoms later disappeared spontaneously, and she remained in good health. However, four years later, she developed lancinating abdominal pain and was admitted to our hospital. A large tumor with calcification was found in the left upper abdominal cavity. Curative resection of the tumor was performed, and the histology was compatible with extraskeletal osteosarcoma. We speculated that the tumor originated from the colonic wall, because a small calcification was observed in the wall of the transverse colon four years before surgery. To the best of our knowledge, this is the first report of primary colonic osteosarcoma.