When a colonic mesenchymal mass is seen on colonoscopy, it is important to consider other, nonbenign lesions that present this way. The most common malignant mesenchymal tumor of the GI tract is a gastrointestinal stromal tumor (GIST), a soft-tissue sarcoma that occurs predominantly in the stomach and small intestine.6 These tumors arise from the interstitial cells of Cajal (ICC) and are associated with mutations of KIT and PDGFR-α genes.7 The incidence in the United States is approximately 0.70 per 100,000 people per year, predominantly found in adults in their fifth or sixth decade of life.8 While this tumor typically occurs in the upper GI tract, very rarely, GISTs can be found in the colon.6 Common constitutional symptoms of colon GIST are similar to those of colon lipomas and include abdominal pain, changes in bowel habits, nausea, vomiting, and in some cases, weight loss.
CT imaging is often enough to differentiate a colon lipoma from a colon GIST. On CT, large GIST tumors tend to show irregular, lobulated margins, mucosal ulceration, central necrosis, cavitation, hemorrhage, and hypervascularity—vastly different from the CT findings of colon lipomas. If imaging is equivocal, an ultrasound-guided fine needle aspiration biopsy may be performed, differentiating GIST through the presence of ICC tumor cells as well as KIT and PDGFR-α proteins.
In our patient, colonoscopy showed a positive Cushion sign (tumor indented on depression with biopsy forceps), pathognomonic for a colon lipoma, and CT imaging showed a radiolucent, well-circumscribed lesion.9 This was more consistent with a colon lipoma than a GIST. Because the patient was symptomatic with a near obstructing lesion, the appropriate next step was removal of the lesion. Had this instead been a GIST tumor, a more extensive oncologic surgical resection would have been warranted, with adequate mesentery and lymph nodes collected.
This case is notable because colon lipomas exceeding 2 cm are rare and are usually an incidental finding on CT. However, larger lipomas can lead to symptoms, including obstruction if not removed in a timely manner.