Malignancy-associated Multicentric Reticulohistocytosis Mimicking Dermatomyositis-like Features, with a Significant Ultrasound Finding

Multicentric reticulohistocytosis (MRH) is a rare systemic non-Langerhans cell histiocytosis characterized by destructive polyarthritis and nodular skin lesions, often associated with malignancy. We present the case of a 74-year-old Japanese man initially suspected of having dermatomyositis due to polyarthritis, typical skin manifestations, and laboratory findings. Malignancy screening revealed invasive bladder cancer, which necessitated surgery and chemotherapy. Although the skin symptoms improved, aggressive joint destruction persisted, and histopathological analyses confirmed MRH. Remarkably, ultrasonography revealed significant inflammation and tendon involvement, highlighting its diagnostic utility. This case emphasizes the challenge of distinguishing MRH from autoimmune diseases and the critical role of ultrasound in capturing destructive joint damage.