Differential diagnosis of stromal tumors in colon
• Gastrointestinal stromal tumors (GISTs) in the GI tract most commonly occur in the stomach (50%), followed by the small bowel (25%), colon and rectum (10%), and, least commonly, esophagus (5%). Histologically, these can be spindled or epitheloid and show strong reactivity with CD117 (95%), and 60% to 70% show positive staining with CD34. These are also stain positive with DOG 1 antibody (including some of KIT negative tumors). Approximately one third can also show reactivity with smooth muscle markers (smooth muscle actin).These arise from interstitial cells of Cajal, and KIT mutations are seen in 85% to 90% of GISTs. Approximately 5% show mutation within the PDGFRA gene and these are usually seen in gastric GISTs. These have epitheloid morphologic characteristics and a less aggressive clinical course. All the GISTs are potentially aggressive. The clinical behavior can be predicted on the basis of size, mitotic figures, and site. Gastric GISTs have a better prognosis than the small bowel GISTs. The GISTs with exon 11 mutation have a low risk for progressive disease (as opposed to exon 9 mutation) and respond better to imatinib mesylate in the metastatic disease setting.
• Schwannomas are well-circumscribed, nonencapsulated spindle cell tumors with strong immunoreactivity with S100 protein. Dense lymphoid cuffing is seen around schwannomas.
• Leiomyoma is another spindle cell tumor arising from the smooth muscle in muscularis mucosae that shows strong positive immunostaining with smooth muscle actin.
• Lipoma is a sporadic, benign, well-circumscribed submucosal lesion of adipose tissue.
• Kaposi sarcoma shows proliferation of slitlike vascular channels, spindle cells, and inflammatory infiltrate. It is seen in some patients with human immunodeficiency virus infection and is associated with human herpesvirus 8.
• Other lesions include hemangiomas, lymphangiomas, vascular malformations, and, rarely, angiosarcomas.