Erdheim-Chester Disease with Emperipolesis: A Unique Case Involving the Heart.

Histiocytosis is an uncommon disease characterized by excessive accumulation of histiocytes. Here, we report a rare case of non-Langerhans-cell histiocytosis in a 51-year-old woman who presented with severe symptoms of pericardial effusion. Radiologic investigation also detected multiple bone (lower limbs, vertebrae, ribs, and ilium) lesions. Resected pericardium showed abundant mono- or multi-nucleated non-foamy histiocytes (CD68+/CD163+/S-100+/CD1α-/langerin-) in a fibroinflammatory background. The histiocytes demonstrated emperipolesis of lymphocytes, a hallmark feature of Rosai-Dorfman disease (RDD). However, molecular analysis revealed a BRAF V600E mutation of the proliferating histiocytes, highlighting the neoplastic features frequently observed in another non-Langerhans-cell histiocytosis known as Erdheim-Chester Disease (ECD). We consider this case to be a unique presentation of ECD harboring some RDD-like cells with emperipolesis, but not a case of RDD with a BRAF mutation concerning its clinical manifestation (involvement of the heart and bones) and neoplastic features.