Diagnosis of Pediatric Myositis Ossificans Based on Cytomorphology and Molecular Analysis From <scp>FNAB</scp> Sample: A Case Report

Myositis ossificans (MO) is a benign soft tissue lesion, characterized by ectopic ossification due to inappropriate fibroblast differentiation, most commonly affecting skeletal muscles. It often occurs in young adults after muscle trauma, predominantly in male patients and very rarely in children. We describe the case of a previously healthy 3-year-old boy who developed a lesion in his deltoid muscle after vaccination against tick-borne encephalitis. During an MRI scan, performed under general anesthesia, fine needle aspiration biopsy (FNAB) and core needle biopsy (CNB) were performed. While the CNB sample resulted in a non-diagnostic finding, the FNAB sample showed cytomorphology consistent with the diagnosis of MO. A molecular analysis performed on the FNAB sample confirmed the presence of COL1A1::USP6 fusion, which is considered diagnostic for MO in the appropriate clinical context. The boy was then referred to the orthopedic surgeon. Extracorporeal shock-wave therapy was chosen as the first-line treatment, but as it was too painful, the lesion was surgically removed, and histopathologic evaluation confirmed the diagnosis. Although rare in children, the diagnosis of MO should be considered in soft tissue lesions after trauma. We present the first pediatric case of MO diagnosed by FNAB and propose that FNAB, as a minimally invasive diagnostic procedure, is a suitable diagnostic approach, especially when molecular testing is available to confirm the diagnosis.