Infantile myofibroma of deltoid muscle with atypical immunostaining and novel PDGFRB mutation

Myofibroma/myofibromatosis is a rare benign pericytic tumor, sometimes difficult to diagnose based on histomorphological characteristics alone. Recent studies have revealed the occurrence of platelet-derived growth factor receptor beta recurrent mutations in myofibromatosis. A 1-month-old female infant presented to our hospital with a right shoulder nodule that had been diagnosed at birth. Magnetic resonance imaging revealed a well-demarcated mass measuring 20 mm in the left deltoid muscle. Incisional biopsy indicated myofibroma and infantile fibrosarcoma, but the immunohistochemical findings were incompatible with these tumors. Wide tumor resection with a 1-cm margin was performed, and the histological diagnosis was the same as that of the incisional biopsy. RNA-Seq to confirm the pathological diagnosis showed a platelet-derived growth factor receptor beta (PDGFRB) p. N666S mutation, a rare alteration in myofibromatosis. No recurrence or metastasis was observed 3 years after treatment. Genetic testing can aid the diagnosis of infantile myofibroma with unusual histomorphological and immunohistochemical features.