Radiation-induced sarcoma of the transverse colon: a case report and its clinical implications

• Radiation-induced sarcoma can present as a late complication of abdominal radiotherapy. • Colonic involvement outside the rectum is an exceptionally rare presentation. • Transverse colon leiomyosarcoma developed 12 years after gastric cancer treatment. • Early diagnosis enabled curative surgical resection with favorable clinical course after 2 years. • Long-term surveillance is essential in patients previously treated with radiotherapy. Radiation-induced sarcomas are rare late complications of radiotherapy and represent aggressive mesenchymal tumors arising in previously irradiated tissues. They most commonly occur in the pelvis and thorax, while involvement of the colon is exceptional, particularly outside the rectum. A 48-year-old male with a history of gastric adenocarcinoma treated twelve years earlier with subtotal gastrectomy followed by adjuvant chemoradiotherapy according to the INT-0116 protocol presented with a five-month history of progressive abdominal pain, weight loss, hematemesis, melena, and recurrent vomiting. Abdominal computed tomography revealed an endoluminal mass in the transverse colon without evidence of distant metastasis. Colonoscopy demonstrated a large pedunculated tumor with necrotic borders occupying the entire colonic lumen. The patient underwent an extended right hemicolectomy with intracorporeal ileocolic anastomosis and was discharged on postoperative day five without complications. Histopathological examination revealed a high-grade inflammatory leiomyosarcoma with myogenic differentiation, extensive necrosis, and a high proliferative index. Surgical margins and lymph nodes were negative, and genetic testing showed no evidence of a hereditary cancer syndrome. The patient remains under close oncologic and colorectal surgical follow-up with a favorable clinical course after 2 years. This case highlights the importance of long-term surveillance in patients undergoing radiotherapy, as early recognition of radiation-induced sarcomas may allow curative surgical treatment despite their traditionally poor prognosis.