From a Long‐Standing Yellowish Plaque to a Diagnosis of a Rare Disorder: A Case of Erdheim–Chester Disease Treated With Vemurafenib

Erdheim-Chester disease (ECD) is a rare, systemic, and potentially malignant non-Langerhans cell histiocytosis characterized by the infiltration of foamy histiocytes into multiple organ systems. The diagnosis of ECD is often complicated and time-consuming due to its rarity and heterogeneous presentation. A 39-year-old female presented with a progressively enlarging yellowish plaque measuring 5 × 2 cm on left periorbital skin over the past 2 years. Besides, the patient had bilateral lower extremity pain persisting for 1.5 years, significantly affecting her mobility and quality of life. Histological examination of skin biopsies revealed frequent Touton giant cells, bland-appearing histiocytes characterized by abundant foamy (xanthomatous) cytoplasm in the dermis, which were positive for CD68, Factor XIIIa, and BRAF; negative for CD1a and S100. In bone marrow biopsy, histiocytic cells were positively stained with CD68, Factor XIIIa, and CD14, and negatively stained for CD1a, Langerin, and S100. Positron emission tomography/computed tomography imaging revealed pathological 18-fluoro-2-deoxyglucose uptake in the bone marrow and widespread sclerotic bone lesions on the bilateral lower extremity. BRAF sequencing determined a BRAFV600E mutation. The patient was diagnosed with ECD, and vemurafenib was initiated. After 6 months of treatment, significant improvements were observed in both bone pain and the size and discoloration of the yellowish plaque. In conclusion, our case highlights that the primary diagnostic clue for ECD may be a single yellowish plaque, which requires further investigation in relation to other systemic symptoms. Vemurafenib treatment may lead to regression of systemic symptoms and cutaneous yellowish plaques associated with ECD carrying a BRAF mutation.