Gastroenterology clinical image challenge: A 71-year-old, asymptomatic man underwent incomplete colonoscopy for screening. He had a history of surgically resected retroperitoneal well-differentiated liposarcoma 10 years prior, with local recurrence as dedifferentiated liposarcoma six years later. Physical examination and laboratory tests were unremarkable. He underwent computed tomography colonography (CTC) to evaluate the colon segments not assessed by conventional colonoscopy. The CTC demonstrated a 3-cm pedunculated polyp in the ascending colon on the supine acquisition and on three-dimensional endoluminal view (figure). The polyp was homogeneous and showed soft tissue attenuation, with no fat attenuation.
What is the diagnosis?