Proliferative Myositis of the Erector Spinae Mistaken for Malignancy

We report the case of a 56-year-old woman who presented with enlarging, painful upper back mass that had developed 4 days ago. The patient was referred to Emergency Department from primary care facility after ultrasonography showed an ill-defined mass-like lesion in muscle potential for malignancy. Magnetic resonance imaging and at Eunpyeong St.Mary’s Hospital revealed hypervascular longitudinally oriented left erector spinae. initial differential diagnosis included inflammatory such as proliferative myositis, sarcoidosis, low-grade lymphoma infection. underwent ultrasound-guided biopsy pathology confirming myositis. Proliferative myositis is rare benign proliferation skeletal muscles can be mistaken malignancy due its rapid progression.