Unmasking peripheral T-cell lymphoma in dermatomyositis with anti-PM/Scl-100 and anti-Mi-2 positivity: a case-based review.

Elderly-onset dermatomyositis (DM) is associated with an increased risk of malignancy. Serology supports clinical phenotyping of DM and risk assessment for malignancy, but malignancy cannot be completely excluded by serology alone. T-cell lymphomas can rarely mimic DM or coexist with established DM, posing an important diagnostic pitfall. An 82-year-old woman presented with fever, polyarthralgia, rash, and interstitial lung disease. Myositis autoantibodies assessed by immunoblot were positive for anti-PM/Scl-100 and anti-Mi-2. Aldolase was elevated, and skin biopsy findings were compatible with DM. Computed tomography showed interstitial lung disease and mild bilateral inguinal lymphadenopathy, while the inguinal lymph node biopsy suggested reactive lymphadenitis. She was diagnosed with DM and treated with oral prednisolone. During tapering, she rapidly developed systemic deterioration with severe thrombocytopenia and marked elevations in lactate dehydrogenase, ferritin, and soluble interleukin-2 receptor, raising suspicion for lymphoma-associated hyperinflammation. Bone marrow aspirate ultimately revealed peripheral T-cell lymphoma, not otherwise specified, with bone marrow involvement. A literature review identified 16 reported cases, including ours, of T-cell lymphoma either "mimic" DM or "coexisting" with established DM. 7 cases were classified as mimic and 9 as coexisting. These cases suggest that repeat tissue evaluation, particularly of skin lesions, may be critical in atypical or treatment-refractory cases. Our case highlights that even a DM phenotype with autoantibodies generally considered less strongly associated with malignancy does not exclude occult lymphoma in elderly patients.