Osteosarcoma Manifesting Systemic Inflammation and Histological Features Mimicking Plasma Cell-type Castleman Disease

A 73-year-old man was referred to our hospital with a persistent fever, anemia, and a mass in the left pubic region. The findings of biopsy evaluations of the mass and a left inguinal lymph node were consistent with Castleman disease (CD) of plasma cell type. His serum interleukin 6 (IL-6) level was remarkably elevated, supporting the diagnosis of CD. However, imaging analyses revealed destruction of the pubic bone by the mass, which was atypical for CD. Therefore, another deeper biopsy was performed, which finally led to the diagnosis of IL-6-producing osteosarcoma. We conclude that clinicians should carefully exclude malignancies prior to making a CD diagnosis.