Castleman's disease is a rare benign lymphoproliferative disorder of unknown etiology. It is characterized by giant lymph node hyperplasia and a nonmalignant course. Castleman's disease usually involves the mediastinum, with about 70% of cases occurring in the thorax. However, the disease can also occur in an extrathoracic site where lymphoid tissue is present such as the neck, axilla, shoulder area, mesentery, pelvis, pancreas, and retroperitoneum (1).

The presence of Castleman's disease in the abdomen or pelvis is rare, and extremely rare at the omentum. To the best of our knowledge, only one case was reported in 1990 (2). We report one case of omental Castleman's disease in a 69-year-old female patient, imaged by three cross-sectional modalities: US, CT, and MRI with gadolinium.

A 69-year-old woman with no previous history of medical illness visited the emergency room with right upper quadrant pain and mild sweating. She had no fever or weight loss. Initial laboratory testing showed an elevated white blood cell number of 12,000/uL and C-reactive protein level of 17.80 mg/dL. Other laboratory tests, such as electrolytes and urine analysis, were normal. A subsequent constrast-enhanced abdominal CT revealed several small stones within the mild dilated intrahepatic biliary trees. The findings were consistent with acute cholangitis. In addition, a 3.3 × 2.7 cm well enhancing mass was incidentally shown in the greater omentum (Fig. 1A). The mass showed a well-defined margin without adjacent abdominal solid organ invasion and heterogeneous hypoechogenicity to muscle with internal vascularity on ultrasonography (Fig. 1B). To gather more information, the patient underwent MRI with gadolinium enhancement. The results indicated a homogenous mass that was hypointense with T1 weighting (Fig. 1C) and hyperintense with T2 weighting (Fig. 1D) compared to the signal intensity of muscles. Homogeneous enhancement was present on a dynamic study with gadolinium. With these findings, this mass was suspected to be a lymphoma, desmoid tumor, or Castleman's disease.

The omental mass was surgically resected (laparoscopic mass excision) to get a definite diagnosis. On histopathologic examination, diffusely scattered large follicles with markedly vascular proliferation and small hyalinized germinal center were compatible with the hyaline vascular type of Castleman's disease (Fig. 1E, F). The mass was completely excised and no other lymph node involvement was seen.

Castleman's disease was first described by Dr. Benjamin Castleman in 1954 in a group of patients with large mediastinal lymphadenopathy (3). For the successive cases reported afterward, the names were relatively confusing, including giant lymph node hyperplasia, hamartoma of the lymphatics, giant benign lymphoma, angiofollicular lymphadenosis, tumor-like hyperplasia of lymphoid tissue, and so on (4). The etiology of Castleman's disease is unknown, but there are several hypotheses involving chronic low-grade inflammation, a hamartomatous process, and abnormal immunological functions that have been proposed as likely pathogenetic mechanisms (156).

Histologically, Castleman's disease is divided into two clinical types: localized-type and diffuse-type according to the extent of disease spread (78). The hyaline vascular type is the most common variant (encountered in 80-90% of the cases), which is considered to undergo a benign clinical course. Most patients with hyaline vascular type are asymptomatic and a surgical resection is the primary choice of treatment and has the most favorable prognosis. In contrast, the plasma cell type accounts for 9% of cases and is more aggressive. Patients with the plasma cell type tend to experience systemic symptoms, characterized by fever, sweating, weight loss, anemia, an elevated sedimentation rate, and hypergammaglobulinemia. Unlike the hyaline vascular type, masses in the plasma cell type are characterized by multiple discrete enlarge lymph nodes, which are mainly treated with radiotherapy, chemotherapy, and immunosuppression, with a poor prognosis (8).

The localized hyaline vascular Castleman's disease was characterized by homogeneous high enhancement in the early phase of dynamic enhancements, and a persistent enhancement mode similar to that of large arteries in CT and MR images. The imaging feature of the diffuse type was multiple lymphadenitis demonstrated by mild to moderate enhancement (9). MRI shows a low intensity mass on T1-weighted images and higher signal intensity on T2-weighted images. The differential diagnosis to distinguish it from lymphoma, leiomyoma, and leiomyosarcoma must be performed (10). However, the radiologic findings of Castleman's disease are non-specific, and in most cases, the diagnosis of Castleman's disease is confirmed after the resection and histopathological examination of the specimen.