We report a case of IgG4-positive inflammatory pseudotumor mimicking malignant bone tumor. Biopsy revealed no tumor cells. Surgical excision was performed and an abscess developing beneath the periosteum was observed with Streptococcus constellatus. Preoperative serum IgG4 value of 120 mg/dl normalized postoperatively to 80.6 mg/dl. It was difficult to distinguish inflammatory pseudotumor from sarcoma because it developed under the periosteum. In such cases, it is important to measure blood IgG4 values and perform tissue staining and culturing.
Keywords: IgG4-related disease; Malignant bone tumor; Serum IgG4 value; Streptococcus constellatus; Subperiosteal inflammatory pseudotumor.