Myositis Ossificans Mimicking Osteosarcoma: Case Report

Abstract

Introduction: Myositis ossificans is a benign, self-limiting condition characterized by heterotopic bone formation in skeletal muscles or other extra-skeletal soft tissue sites. It is most common in physically active adolescents and young adults. Its incidence is related to trauma, but it can be triggered by systemic diseases or be idiopathic. Case report: A 48-year-old female patient was investigated due to a 2.5-cm soft tissue lesion on the right thigh after tomography, with ill-defined edges, described as suspicious for neoplasia, submitted to biopsy and sent to the pathology laboratory. The sample was included for microscopic analysis, consisting of a spindle cell component showing atypia, without mitoses, and arranged in bundles. The presence of smaller quantities of osteoclasts surrounding the osteoid-like material was described. The material was also subjected to immunohistochemistry. The findings favored the diagnosis of osteosarcoma. In a later anatomopathological review, including a molecular study of the USP6 gene, it was concluded that it was myositis ossificans mimicking osteosarcoma. Conclusion: The differential diagnosis between osteosarcoma and myositis ossificans can be challenging due to the radiological and clinical similarities between these conditions. In the case reported, a 48-year-old female patient was diagnosed with osteosarcoma, but after re-evaluation, the diagnosis was myositis ossificans. This finding highlights the importance of correlating clinical and morphological findings with molecular biology analyses for accurate characterization of the lesion, ensuring appropriate management and avoiding unnecessary treatments for the patients.