S4048 Uncommon Location, Rare Tumor: Granular Cell Tumor of the Transverse Colon

Introduction: Granular cell tumors (GCTs) are rare, usually benign neoplasms derived from Schwann cells. While most have an indolent course, 1%–2% may exhibit malignant behavior. GCTs commonly arise in the skin, tongue, and esophagus. Involvement of the lower gastrointestinal tract is uncommon, with a predilection for the right colon. We report a rare case of a GCT located in the transverse colon, identified incidentally during an index screening colonoscopy. Case Description/Methods: A 48-year-old white woman with a history of IDA, hypertension, and gastroesophageal reflux disease presented for evaluation of chronic abdominal pain and alternating bowel habits. She reported menorrhagia, but denied GI bleeding, weight loss, or systemic symptoms. There was no family history of colorectal cancer. Imaging was unremarkable. Colonoscopy revealed a normal colon except for a single 8 mm smooth, submucosal-appearing polyp in the transverse colon, which was removed by cold snare polypectomy. Histopathology showed polygonal cells with abundant eosinophilic granular cytoplasm, consistent with GCT. Immunohistochemistry was positive for SOX10, S100, and CD68. No malignant features were identified. Due to superficial resection and absence of submucosal tissue in the specimen, margin status was uncertain. The patient was scheduled for a repeat colonoscopy with endoscopic mucosal resection (EMR) and tattooing to ensure complete excision. Discussion: Colonic GCTs are rare and usually incidental findings found in the right colon. Transverse colon involvement, however, is unusual. GCTs appear as small, firm, yellow-white submucosal nodules, often mimicking other submucosal tumors such as GISTs, carcinoid tumors, lipomas, or leiomyomas. Histologically GCTs consist of nests/sheets of polygonal cells with abundant granular eosinophilic cytoplasm due to intracytoplasmic lysosomes, with small and centrally located nuclei. Immunohistochemistry shows positivity for S100, SOX10, and CD68, confirming Schwann cell origin. Cold snare polypectomy, while effective for small mucosal polyps, may be inadequate for submucosal lesions like GCTs. Incomplete resection can lead to uncertain margin status with potential for recurrence or even malignant transformation. Therefore, EMR with tattooing of the site is recommended for complete removal and accurate assessment. Management and surveillance decisions of GCTs rely on accurate histologic diagnosis and completeness of resection, especially in the setting of an index screening colonoscopy.