Proliferative myositis of the biceps brachii: A sarcoma mimicker.

Proliferative myositis is a rare benign intramuscular inflammatory pseudotumor that may clinically mimic a soft tissue sarcoma due to its rapid growth and painful presentation. We report the case of a 50-year-old man presenting with a rapidly enlarging, firm, and painful mass of the left arm, which had appeared 3 weeks earlier, without any history of trauma or systemic symptoms. Ultrasound demonstrated an oblong intramuscular pseudomass within the biceps brachii muscle, with preserved fibrillar architecture and mild Doppler vascularity. Magnetic resonance imaging revealed a lesion involving the biceps brachii and coracobrachialis muscles, appearing hypointense on T1-weighted images and hyperintense on STIR sequences, with homogeneous enhancement after gadolinium administration and associated subcutaneous edema. The preservation of muscle fascicles within the lesion resulted in a characteristic fibrillar pattern. A follow-up ultrasound performed 1 month later showed complete resolution. In this clinical and imaging context, proliferative myositis should be considered as a leading diagnostic hypothesis. Although the absence of histopathologic confirmation represents a limitation in our case, the characteristic imaging features combined with complete spontaneous resolution strongly support this diagnosis and make a malignant process such as sarcoma highly unlikely. Recognition of this presentation is essential to guide appropriate management and potentially avoid unnecessary biopsy in selected cases.