A Case of Primary Cutaneous BRAFV600E+ Langerhans Cell Histiocytosis, With Underlying Chronic Myelomonocytic Leukaemia, Responsive to Dabrafenib With Subsequent Undetectable Circulating Cell Free DNA.

We present a case of BRAFV600E+ primary cutaneous Langerhan cell histiocytosis (LCH), with corresponding circulating cell free DNA (cfDNA), which responded to treatment with dabrafenib, and simultaneously saw corresponding BRAFV600E cfDNA become undetectable. A concomitant diagnosis of chronic myelomonocytic leukaemia (CMML) was suspected with identification of TP53, TET2, RAS and CBL variants identified through molecular testing on the initial skin biopsy, and CMML later confirmed on bone marrow biopsy. Whilst the link between LCH and several haematological malignancies is documented, the mechanism is still unclear. In this case, the LCH most likely developed from an already abnormal myeloid compartment and highlights the need for clinical vigilance and appropriate investigation, where effective treatments for these conditions exist.