S6058 Metastatic Adenocarcinoma Masquerading as Primary Duodenal Cancer: A Case of Obstructive GI Lesion Without a Primary Source

Introduction: Metastatic disease to the small intestine is uncommon and often mimics primary gastrointestinal tumors, particularly when the overlying mucosa is involved. The duodenum is a rare site of metastasis, and lesions here may present with gastric outlet obstruction. This case describes an obstructing duodenal mass initially presumed to be a primary adenocarcinoma but later confirmed as metastatic disease from an unidentified primary tumor. Case Description/Methods: A 69-year-old woman with hypertension and chronic kidney disease presented with progressive weakness, postprandial vomiting, and 2 days of poor oral intake. Labs revealed hypokalemia, metabolic alkalosis, and mild azotemia. Computed tomography imaging showed a 6.1 × 5.0 × 4.7 cm mass in the third portion of the duodenum, encasing the superior mesenteric artery and abutting the ligament of Treitz, with adjacent mesenteric lymphadenopathy. Esophagogastroduodenoscopy demonstrated an ulcerated, partially obstructing mass. Biopsies revealed moderately differentiated adenocarcinoma beneath intact duodenal mucosa without dysplasia. Immunohistochemistry showed CK7-negative, CK20 patchy-positive, CDX2 positive, SATB2 weakly positive, and TTF-1 negative—suggesting a lower gastrointestinal (GI) tract origin. Mismatch repair proteins (MLH1, MSH2, MSH6, PMS2) exhibited intact nuclear expression, indicating microsatellite stability. Extensive imaging, including PET-computed tomography, failed to identify a primary tumor. The patient remained hospitalized for 37 days due to nutritional compromise and electrolyte derangements. Surgical resection was not feasible due to comorbidities and vascular encasement. Palliative chemotherapy was planned, but she was readmitted shortly after discharge with persistent vomiting. With declining functional status, she opted for hospice care. Discussion: Metastatic adenocarcinoma to the GI tract may closely resemble primary malignancy, particularly when mucosal colonization is present. In this case, the absence of epithelial dysplasia raised suspicion for a metastatic process. IHC findings were consistent with colorectal origin, while negative TTF-1 excluded pulmonary or gastric sources. Intact mismatch repair status ruled out Lynch syndrome. This case underscores the importance of integrating histology, IHC, and imaging to distinguish primary from metastatic GI lesions. Diagnostic accuracy is essential, as treatment strategies differ substantially—surgical resection for resectable primary disease versus systemic therapy or palliative interventions for metastases.