Bronchogenic cysts are congenital anomalies usually seen in the mediastinum but rarely in the abdomen. Abdominal bronchogenic cysts with malignant regeneration are extremely rare, and only 2 cases have been reported. We report a case of retroperitoneal bronchogenic cysts, one of which was diagnosed as a malignant cyst using preoperative fine-needle aspiration cytology. A 57-year-old man admitted for back pain was found in computed tomography to have a well-defined, circumscribed 8 cm multilobular mass in the retroperitoneal space, attached to the left crus of the diaphragm, and was widely attached to the pancreatic body, pressing it anteriorly. Cytological examination of fluid obtained by ultrasonography-guided fine-needle aspiration revealed adenocarcinoma cells, indicating a malingnant retroperitoneal cystic tumor. Because the tumor was widely attached to the pancreatic body, it was resected with distal pancreatectomy and splenectomy. Histologically, cysts were lined with pseudostratified ciliated columnar epithelium resting on fibrous connective tissue. Some atypical cells were growing papillary, diagnosed as papillary adenocarcinoma, resulting in a definitive diagnosis of malignant retroperitoneal bronchogenic cyst with malignant regeneration.
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