• Kunihiro Ikuta, Tomohisa Sakai, Hiroshi Koike, Kan Ito, Shiro Imagama, and Yoshihiro Nishida.
• Department of Orthopedic Surgery, Nagoya University Graduate School and School of Medicine, 65 Tsurumai, Showa, Nagoya, Japan.
• Medicine (Baltimore). 2021 Dec 10; 100 (49): e28138e28138.

RationaleFibrous dysplasia is a rare disorder that results in fractures, pain, and disability and can affect any bone in the body. The treatment of symptomatic fibrous dysplasia is determined based on the affected bones. Although some lesions are often too extensive for surgical procedures, there are currently no effective or recommended medical treatments available for them.Patient ConcernsA 27-year-old woman developed right buttock pain and was diagnosed with a bone tumor in the right ilium. Clinical images revealed an expansive osteolytic lesion with thinning of the cortex and cystic change from the acetabulum to the sacroiliac joint.DiagnosisAn incisional biopsy was performed, and the lesion was diagnosed as cystic fibrous dysplasia. Occasional osteoclast-like giant cells and woven bone were observed. The patient had no evidence of polyostotic lesions or findings of McCune-Albright syndrome. Biochemical blood test results showed no obvious abnormal values, except for an increase in serum tartrate-resistant acid phosphatase 5b to 459 mU/dL.InterventionsSince surgical treatment appeared to be challenging, she was treated with denosumab with decreased dose-intensity schedules.OutcomesThe administration of denosumab caused osteosclerosis within the lesion, resulting in the elimination of bone pain. The patient received denosumab treatment for 18 months. Pain relief and lesion radiodensity were maintained for 9 months after denosumab discontinuation. The serum level of tartrate-resistant acid phosphatase 5b was measured to monitor the response to denosumab, which was suppressed during denosumab treatment.LessonsWe described successful denosumab treatment in a patient with cystic fibrous dysplasia (FD) who maintained efficacy for 9 months after treatment. Although the use of denosumab in fibrous dysplasia is currently off-label, our experience with this patient supports the potential of denosumab therapy for patients for whom surgical treatment is challenging.Copyright © 2021 the Author(s). Published by Wolters Kluwer Health, Inc.

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