Radiographic, gross and histopathological findings of resected unicentric Castleman's disease. T1-weighed magnet resonance imaging revealed a sharply demarcated hyperintense mass lesion of 8.5 × 6 × 3.5 cm, located between the adductor and quadriceps femoris muscle surrounding the femoral vessels (A, arrow). Grossly, the resected tumor specimen displayed a single well-circumscribed round nodule covered by hemorrhagic soft tissue (B). On microscopic view, the tumor consisted of lymphoid tissue with numerous germinal centers with central fibrosis and marked vascularization. Some lymph follicles showed germinal centers with concentric onion skin-like layering of surrounding lymphoid cells (C). Immunohistochemical staining with CD5 revealed CD5-positive lymphocyte at the periphery of the follicles (D), whereas the follicular dendritic cells in the germinal centers expressed KiM4P (E). The spaces between the lymph follicles (stars) were filled out with CD138-positive plasma cells (F) (× 100).