Muscular Involvement of Extranodal Natural Killer/t Cell Lymphoma Misdiagnosed As Polymyositis: A Case Report and Review of Literature.

BACKGROUND Natural killer (NK)/T cell lymphoma is a rare and highly aggressive malignant tumor, and is a special form of non-Hodgkin's lymphoma. Although extranodal involvement is frequently found in tissues such as the skin, testicular and gastrointestinal tract etc, its presence in skeletal muscle has scarcely been reported in the literature. CASE SUMMARY We report a case of extranodal NK/T cell lymphoma with muscle swelling as the first clinical manifestation. A 42-year-old man, who initially presented with localized swelling in the double lower extremities, demonstrated gradual facial and eyelid swelling, and his imaging results showed multiple sites of muscle damage throughout the body. The final pathological results suggested NK/T cell lymphoma, and immunohistochemistry showed CD20 (-), CD3Ɛ (+), CD30 (+), CD56 (-), EBER (+), Ki67 (60%), TIA-1 (+) and CD68 (±) staining. The muscle swelling significantly improved after treatment with chemotherapy regimens. CONCLUSION This disease is difficult to diagnose and highly invasive, and should be included in the differential diagnosis of unexplained muscle swelling.