Leptomeningeal dissemination as a first sign of progression in metastatic melanoma: a diagnostic lesson.

One of the most serious complications of advanced melanoma is the diffusion of cancer cells to the central nervous system. The diagnosis of leptomeningeal metastasis (LMM) is notoriously challenging and requires a combination of consistent MRI and cerebrospinal fluid (CSF) cytology. In ambiguous cases, mutations like BRAF V600E in CSF-cell-free (cf)DNA may help to clarify diagnosis of LMM. Here we present the case of a young woman who developed isolated LMM after the diagnosis of a node-positive primary melanoma with normal LDH. The CSF was negative for tumour cells by cytology but positive for cfDNA BRAF V600E mutation, thus allowing us to diagnose LMM. To our knowledge, this is the first case where CSF sampling for the detection of BRAF mutation was used to identify leptomeningeal disease in the presence of negative MRI and without involvement of any other distant sites.