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Erdheim-Chester disease is a rare form of non-Langerhans histiocytosis. A 40-year-old woman was diagnosed as Erdheim-Chester disease based on typical bone scintigraphy, symmetric osteosclerosis and findings of foamy, non-Langerhans histiocytes in bone marrow. BRAF V600E mutation was detected in a bone biopsy. Treatment with IFN-α showed significant improvement. The BRAF V600E mutant was detected in plasma cell-free DNA (cfDNA) by a droplet-digital PCR assay. Longitudinal analysis of BRAF V600E in plasma cfDNA showed a decreasing trend during treatment. We could not detect the mutant in urinary cfDNA. While, similar studies have detected the BRAF V600E mutant in urine, but not in plasma. A combination of allele burden assessments in plasma and urine may be helpful for detecting the residual mutant burden and monitoring therapeutic response.

Objective: To explore the clinicopathologic features, immunophenotype, differential diagnosis and gene mutation status of the Erdheim-Chester disease (ECD)., Methods: Clinical and pathologic findings of 3 ECD cases were examined by gross, microscopic, immunohistochemical methods and BRAF V600E mutation. Related literatures were reviewed., Results: Two male patients and one female patient presented clinically with multiple skin nodules, bone pain and bony lesions by imaging study. Microscopically, the lesions were composed of spindle-shaped fibroblasts, foamy histiocytes and scattered Touton-type giant cells embedded in reactive fibrous tissue. Lymphocytes, plasma cells, and multinucleated giant cells were also found. Immunohistochemically, all histiocytes were positive for CD68, none of which expressed CD1a, although 2 cases focally expressed weak S-100 stain. In 2 cases,BRAF V600E mutation was detected., Conclusions: ECD is a rare disease of xanthogranulomatous histiocytosis.Its diagnosis relies on pathological and immunohistochemical findings, but correlation with clinical information, especially radiographic findings should be performed.No effective treatment of the disease is currently available.