Rosai-Dorfman Disease Arising in Patella

The patient was a 12-year-old girl with localized pain in the right suprapatellar region for 4 months. Laboratory workup showed normal results (white blood cells=6470/µL, hemoglobin=13.4 g/dL, platelet= 210 000/µL, erythrocyte sedimentation rate=4 mm/h, calcium= 9.4 mg/dL, phosphorus =4.8 mg/dL, parathyroid hormone = 36 pg/mL, alkaline phosphatase = 828 IU/L, lactate dehydrogenase = 283 IU/L). Plain X-ray of the right knee showed a well-circumscribed lytic lesion with a sclerotic margin (Figure 1). Right knee magnetic resonance imaging (MRI) with and without contrast exhibited a 14 × 12 mm well-defined round lesion on the superior aspect of the patella which was hyperintense on T1W/SPIR images (fat suppressed sequences), isointense on T2W images and showed homogenous enhancement after contrast administration (Figure 2). A thin sclerotic margin and perilesional edema were observed. The previous findings along with internal enhancement suggested radiologic differential diagnoses of benign lesions such as chondroblastoma. Subsequently, surgical resection of the patellar mass was undertaken and histologic evaluation revealed bony trabecula surrounded by large histiocytes, lymphocytes, and plasma cells (Figure 3A). The large histiocytes showed emperipolesis (engulfed lymphocytes within vacuoles) (Figure 3B).