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A high-grade primary osteosarcoma arising in the splenic flexure of the colon: an extremely rare entity.

Extraskeletal osteosarcoma (ESOS) is an uncommon malignant mesenchymal tumour deriving from soft tissues or visceral organs without any evidence of a primary bone osteosarcoma. Although ESOS has been mainly reported in the extremities, it can also occur in unusual locations such as the colorectal region. A 58-year-old man with a 3-month history of dyspepsia, constipation and soreness in the left upper abdominal quadrant was admitted to hospital. Computed tomography and magnetic resonance imaging demonstrated a huge, irregular, mixed cystic and solid, partially calcified mass measuring 116mm×140mm×200mm. Because of the patient's obstructive symptoms, surgery was performed. A tumour involving the splenic flexure of the colon was excised in line with oncological principles by extended left hemicolectomy with end-to-side colo-colonic anastomosis. The specimen was interpreted as a primary osteosarcoma of the colon after immunohistochemical straining. Considering the limited data regarding ESOS arising in colon, we present the second case to date that involves the transverse colon, to further elucidate its clinical aspects, prognosis and treatment options.