S1726 A “Low-Risk” Colonic Gastrointestinal Stromal Tumor With Metastatic Features

Abstract

INTRODUCTION: Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumor of the GI tract, yet only make up about 0.1% of colon and rectal tumors. Studies on colonic GISTs are currently limited to case reports and small case series. As a result, risk assessments for malignant potential primarily focus on gastric and small bowel lesions. We present a case of a transverse colon GIST that developed metastatic disease despite having low risk features. CASE DESCRIPTION/METHODS: A 69-year-old male with iron deficiency anemia was referred for endoscopic evaluation. He was asymptomatic with a normal colonoscopy 4 years prior. His EGD found H. Pylori-associated chronic active gastritis and his colonoscopy revealed a 3-to 4cm mass in the transverse colon with central ulceration (Figure 1). Histopathology showed marked spindle cell proliferation, positive for CD-117 and CD-34 stain, confirming gastrointestinal stromal tumor. He underwent right hemicolectomy demonstrating the 3.5 cm GIST, negative margins and 33 negative regional lymph nodes. Surgical pathology revealed 4 mitoses/50 HPF, positive CD117 and DOG1 stains. Follow-up PET CT scan found ascites, bilateral pleural effusions, and focal activity in a cardiophrenic lymph node consistent with metastatic disease. Oncology is planning to initiate imatinib therapy. DISCUSSION: As colorectal GISTs tends to primarily occur in the rectum, this finding in the transverse colon was rare. Their presentation is often due to vague abdominal pain, anemia, or incidental finding on screening colonoscopy. Surgical resection is definitive therapy with imatinib used as adjuvant therapy for those with high risk of recurrence. The NCCN estimates risk for metastatic rate between 1.9%-8-5% for tumor sizes between 2-5cm and < 5 mitoses/50 HPFs. Given the uncommon presentation, specific risk assessments for malignant progression or reoccurrence for colonic GISTs are limited. While gastric and small bowel GISTS are labeled low-risk for metastasis based on a small tumor size and low mitotic rate of < 5/50 HPF, our case displays the aggressive nature of colonic GISTs despite meeting these criteria. Colorectal GISTs should be treated as high risk lesions despite size and mitotic rate.Figure 1.: Colonoscopic view of GIST.Figure 2.: H&E stain demonstrating spindle cell architecture.Figure 3.: PET with cardiophrenic lymph node involvement.