Malignant transformation of a duodenal duplication cyst in a cat

CASE SUMMARY: A 15-year-old domestic shorthair cat presented for lethargy, vomiting and anorexia. Abdominal ultrasound showed a bi-lobed cystic duodenal mass. Based on ultrasonographic features, malignant transformation of a duodenal duplication cyst was suspected. A resection and anastomosis was performed. Histology of the mass was consistent with carcinoma and an intestinal mucosa was present along the inner surface of the mass, suggestive of a duplication cyst. The patient returned 3 months postsurgery with recurrence of clinical signs. Abdominal ultrasound showed a recurrent duodenal mass at the surgery site and body wall nodules. Fine-needle aspirates of these lesions showed epithelial neoplasia. Owing to the poor prognosis, the owners elected euthanasia. RELEVANCE AND NOVEL INFORMATION: Malignant transformation of duplication cysts in cats is rare but can be detected on ultrasound. The described ultrasonographic features can aid in prioritization of malignant transformation of duplication cysts as a differential diagnosis.