Cytology coupled with immunocytochemistry identifies Merkel cell carcinoma: A rare intruder in the cerebrospinal fluid.

Merkel cell carcinoma is an uncommon aggressive skin tumour which is well known for its recurrence and metastasis. Leptomeningeal metastasis involving the cerebrospinal fluid is extremely rare. The diagnosis may easily be missed as it simulates other much more common small round blue cell tumours. A patient history with a primary diagnosis may not always be available. Awareness and cytomorphological vigilance with judicious employment of appropriate immunomarkers on limited cerebrospinal fluid samples is indispensable for reaching the correct diagnosis. The tumour cells are usually monotonous small round cells, singly scattered or arranged in loosely cohesive clusters. Cells have round to elongated nuclei with coarsely granular chromatin, nuclear moulding in places and scanty basophilic cytoplasm. A limited immunocytochemistry panel employing CK20 and CD56, for which the tumour is positive, is confirmatory. We herein present a case of infiltration of cerebrospinal fluid by Merkel cell carcinoma.