Duodenal mucinous adenocarcinoma in a dog: Correlation between cytology, histopathology and proliferation index

Abstract An 8‐year‐old female Schnauzer presented with chronic postprandial vomiting. Abdominal ultrasonography revealed a thickened duodenal segment with loss of wall layering and decreased mesenteric reactivity. Exploratory laparotomy and segmental duodenectomy were performed on the patient. Intraoperative cytology revealed cohesive clusters of atypical epithelial cells with a mucinous background. Histopathology confirmed infiltrative mucinous adenocarcinoma, characterised by disorganised neoplastic glands, extracellular mucin lakes and occasional signet‐ring cells. Immunohistochemistry revealed a high Ki67 proliferation index (∼80%) and focal cyclooxygenase‐2 (COX‐2) expression. No postoperative complications were noted; however, no adjuvant treatment was performed, and long‐term follow‐up was unavailable. This case highlights the diagnostic value of intraoperative cytology and histological features of mucinous adenocarcinoma and potential role of Ki67 and COX‐2 as prognostic markers.