A 69-year-old man presented with a 2-month history of intermittent epigastric pain. At the time of presentation, the patient had hypertension that was controlled with medication, and he had no other remarkable past medical history. The physical examination and laboratory tests were unremarkable.

After admission, the patient underwent an upper gastroduodenal endoscopy examination and the result was normal. We then performed ultrasonography (US) and computed tomography (CT) examinations to further evaluate the abdominal pain. Based on the US findings, a large well-defined anechoic mass was identified and it was located in the posterior aspect of the splenic vein. The lesion was seen with posterior acoustic enhancement that suggested the presence of a cystic or necrotic mass (Fig. 1A). Helical CT was performed using a multidetector-row CT (MDCT) scanner (LightSpeed VCT; GE Healthcare; Milwaukee, WI) with 64 detectors. The abdominal CT axial and coronal images (Fig. 1B, C) showed an approximately 5 cm sized, well-defined, necrotic mass with an even peripheral wall in the retroperitoneal area. The mass was located between the left renal vein and the splenic vein and the mass compressed the adjacent pancreas without evidence of direct invasion. There was no calcification detected in the mass. Several abnormal enlarged lymph nodes in the left gastric area were also noted. Based on these imaging findings, metastatic lymph nodes, tuberculosis lymphadenitis and an unusual form of Castleman's disease were considered in the differential diagnosis. The main necrotic mass and abnormal enlarged lymph nodes were removed by surgical excision. On a gross specimen, the mass showed extensive internal necrosis that correlated to the image findings seen on US (Fig. 1D). Spindle to ovoid cells that made up fascicles were identified on a light microscopic examination of the resected mass and lymph nodes. The tumor cells had slightly eosinophilic plump cytoplasm with an indistinct cell border and small nuclei (Fig. 1E). After immunohistochemical staining, the tumor cells showed focal positivity for CD 21 and CD 35. These findings were consistent with a follicular dendritic cell sarcoma.

A 52-year-old woman presented with hematochezia and dyspepsia. The symptoms had developed two weeks prior to the patient's presentation to a local clinic and then colonoscopy was performed at that clinic. According to the colonoscopic findings, an approximate 12 cm intraluminal fungating mass was detected in the descending colon. A gastrointestinal stromal tumor (GIST) in the colon was initially suspected. The patient was transferred to our hospital for further evaluation and management. All of the laboratory findings, including the levels of tumor markers (CA 19-9 and carcinoembryonic antigen), were unremarkable except for a low hemoglobin level (10.5 g/dL). The hemoglobin level was slightly decreased due to hematochezia.

A CT scan (Sensation 64; Siemens Healthcare, Erlangen, Germany) was performed for surgical planning and the multiplanar reconstruction images were obtained. On a pre-enhanced CT scan, the mass showed multiple foci of irregular and dense calcifications in the central portion (Fig. 2A). The axial and coronal post-enhanced CT images (Fig. 2B, C) showed that the mass was well-defined, with a size of 12 cm at the longest diameter. The mass was seen with two homogenously enhancing components and a lobulated margin. The mass was located in the mid-descending colon and the mass was composed of intraluminal and extraluminal components. The intraluminal component showed less enhancement than did the extraluminal component. There was no evidence of adjacent organ invasion as seen the coronal images, but multiple enlarged lymph nodes were noted around the mesocolon and inferior mesenteric vessels. Based on these imaging findings, we suspected that the mass was a malignant GIST or an unusual sarcoma that arose from the descending colon with metastatic lymph nodes for the differential diagnosis. The patient then underwent a left hemicolectomy. During surgery, a well-defined mass that measured 12×10 cm was identified in the mid-descending colon. There was direct focal invasion of the left lateral parietal peritoneum by the mass, but the overall margin of the tumor was well-defined. On a gross specimen, the mass showed a relatively dull yellow color with some portions of necrosis in the intraluminal component, and the non-necrotic extra-luminal component had a pink-tan, solid appearance (Fig. 2D). On a light microscopic examination, the cellular component of the mass was composed of plump spindle cells with open chromatin and small nuclei. These cells had a rather bland appearance and any mitotic figures were not identified. Sclerotic bundles with multifocal calcifications, several lymphocytes, plasma cells and eosinophils were admixed with foamy macrophages. The tumor cells showed focal positivity for CD35 (Fig. 2E). These findings were compatible with a diagnosis of follicular dendritic cell sarcoma.