Lipoma of the sigmoid colon, gross specimen. A lipoma is a benign tumour (bulbous shape, left) that forms from fat cells. Lipomas are most common in the skin, and are rare in the intestinal tract. Here, the tumour is found in the sigmoid colon, in the large intestine. In this case the tumour is typically solitary but large, hence obstructing the passage of food, with an ulcer capping its dome. In rare cases a lipoma may turn cancerous, when it is termed a liposarcoma.