The page below is a sample from the LabCE course Body Fluid Differential Tutorial. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Metastatic Melanoma in the Cerebrospinal Fluid (CSF)

Melanoma is the most deadly form of skin cancer and is the third most common form of cancer to spread to the brain. Approximately 30% of patients with metastatic melanoma will experience involvement of the brain and CSF. Once melanoma has spread to the brain, it is no longer curable.

The top image shows a CSF cytospin from a teenage patient who presented with headaches, mental status changes, and new onset of seizures. There had been no prior recognition of a melanoma and there was no evidence of such on the physical exam. 

The CSF showed a few cells with the "classic" look of melanoma - a "shaggy" mesothelial cell with blue-black melanin inclusions. This is very typical of metastatic melanoma.
Sometimes the melanin granules can be so sparse that they are difficult to find, as shown in the bottom image.