Fibrodysplasia Ossificans Progressiva (FOP) is a disease that begins in infancy and is characterized by progressive ossification of skeletal muscles throughout the body. Stimuli such as injury and infection can trigger flare-ups involving subcutaneous mass formation, resulting in subsequent ossification at the site. Imaging diagnosis is often difficult in the initial phase where only a mass due to flare-up is present. In most cases, the diagnosis is made only after ossification has occurred, typically several years after the onset. Herein, we report a case of FOP diagnosed 4 months after onset. Careful imaging evaluation of the mass showed characteristics of FOP distinct from those of malignant tumors and other mass-forming lesions. Since early intervention can improve the outcome for patients with FOP, early diagnosis through careful imaging evaluation and awareness of the disease is important.
