An 80-year-old male presenting with abdominal distension was referred to our institution after being detected to have a positive test for fecal occult blood. Computed tomography revealed a retroperitoneal tumor. Endoscopic ultrasound-guided fine needle aspiration (EUS-FNA) showed an invasive mass inside the inferior duodenal angle, and cytological examination confirmed the diagnosis of squamous cell carcinoma (SCC) . Despite careful examination, the origin of the tumor remained unclear. Analysis of specimens obtained during right hemicolectomy, partial hepatectomy, and mesenteric tumor resection confirmed the diagnosis of SCC.
Typically, retroperitoneal carcinoma is a metastatic cancer, and primary retroperitoneal carcinoma is extremely rare. This paper describes a case of retroperitoneal SCC diagnosed by EUS-FNA and considered as a case of carcinoma of unknown primary (CUP) . According to a widely accepted hypothesis about the onset of primary cancer in the retroperitoneum, these cancers arise from serous or mucinous metaplasia of pre-existing retroperitoneal coelomic mesothelium. In our case, the tumor specimens revealed no serous or mucinous metaplasia; therefore, the diagnosis of retroperitoneal SCC with CUP was considered more reasonable.
If a carcinomatous lesion does not contain an epithelial component, it is unlikely to be a primary tumor. In this case, EUS-FNA strongly suggested the diagnosis of SCC before surgery; therefore, the treatment course was changed. Thus, EUS-FNA was useful both for diagnosis and selection of the therapeutic strategy.
