How I diagnose Erdheim-Chester disease.

Abstract

Erdheim-Chester disease (ECD) is one of 3 major types of histiocytic neoplasms, along with Rosai-Dorfman disease and Langerhans cell histiocytosis. It was first recognized as a neoplasm in the 2016 World Health Organization Classification of Haematolymphoid Tumours. About 80% to 90% of cases harbor MAPK pathway mutations. We discuss the diagnosis of ECD using 3 illustrative cases, consider interpretative challenges, and present a diagnostic algorithm. Erdheim-Chester disease is a systemic disease that shows a characteristic pattern of organ involvement including bone, perinephric soft tissue, skin, heart, and central nervous system. Histologically, ECD shows a spectrum of xanthogranulomatous, lymphohistocytic, or fibrohistiocytic infiltrates. By immunohistochemistry, the neoplastic histiocytes have a macrophage phenotype with expression of CD163, negative to focal S100, and negative CD1a and langerin. These features alone are not specific and thus insufficient for diagnosis. Immunohistochemistry for cyclin D1 and BRAF V600E, correlation with clinical and imaging findings, and mutational analysis for MAPK pathway mutations all play important roles in establishing an ECD diagnosis. The diagnosis of ECD is often challenging due to its clinical and histologic overlap with reactive inflammatory processes. Correlation of histology features with clinical findings, immunohistochemistry, and genetic studies is essential for an accurate and timely diagnosis.