Proliferative Myositis of the Erector Spinae Mistaken for Malignancy

Abstract

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