Occurrence of proliferative myositis after COVID-19 vaccination: A case report.

Proliferative myositis (PM) is a benign intramuscular tumor that might mimic a malignant one due to its unusual pseudosarcomatous inflammatory nature. In this report, we describe a patient who developed PM after vaccination with Sinopharm coronavirus disease (COVID-19) vaccine. A 73 years old man was admitted due to rapidly-growing painful mass in his left thigh from a few days ago, curtailing his walking. He received a recent COVID-19 vaccination (Sinopharm COVID-19 vaccine) about 5 days before the beginning of symptoms. No history of trauma was present. On physical exam, a round firm mass was found in lateral side of mid portion of left thigh within the muscle with tenderness on palpation. An oval-shaped well-defined intramuscular mass measured 15 × 41 mm was noted in vastus lateralis muscle in ultrasonography. Left thigh magnetic resonance imaging (MRI) showed a well-defined intramuscular mass with a definite margin of 19 × 39 mm. Finally, ultrasound (US)-guided core needle biopsy showed muscular tissue with a loose mass composed of plump fibroblasts and myofibroblasts and large ganglion-like cell with abundant amphophilic to basophilic cytoplasm, vesicular nuclei and prominent nucleoli. Pathology report showed a very rare case identified as proliferative myositis. It should be noted that we cannot make a direct link between these 2 events. PM is an extremely rare entity; however, its relation with COVID-19 vaccination might be a coincidence.