Rectal gastrointestinal stromal tumors (GISTs) are rare mesenchymal neoplasms, accounting for only a small proportion of gastrointestinal stromal tumors. Their nonspecific clinical presentation and predominantly submucosal and exophytic growth pattern may make diagnosis challenging. We report the case of a 61-year-old man presenting with anal pain, pruritus, and chronic constipation. Pelvic magnetic resonance imaging revealed a large rectal mass with predominantly exoluminal growth, diffusion restriction, and enhancement after gadolinium administration. Thoracoabdominopelvic computed tomography demonstrated hepatic and pulmonary metastases associated with peritoneal carcinomatosis. CT-guided biopsy of the rectal mass confirmed the diagnosis of a gastrointestinal stromal tumor, with positive immunohistochemical staining for CD117 and DOG1. Given the metastatic stage at diagnosis, systemic treatment with imatinib was initiated. This case highlights the rare rectal location of GISTs and emphasizes the complementary role of pelvic MRI and CT in local tumor assessment and metastatic staging.

Keywords: Gastrointestinal Stromal Tumor; GIST; Rectum; Magnetic Resonance Imaging; Computed Tomography; Metastasis; Imatinib

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