Erdheim-Chester disease: a case treated with IFN-α monitored using plasma and urine cell-free DNA.

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. BRAFV600E mutation was detected in a bone biopsy. Treatment with IFN-α showed significant improvement. The BRAFV600E mutant was detected in plasma cell-free DNA (cfDNA) by a droplet-digital PCR assay. Longitudinal analysis of BRAFV600E in plasma cfDNA showed a decreasing trend during treatment. We could not detect the mutant in urinary cfDNA. While, similar studies have detected the BRAFV600E 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.