Primary leptomeningeal melanoma. Diagnosis by ultrastructural cytology of cerebrospinal fluid and cranial computed tomography.

A case of primary leptomeningeal melanoma is presented in which the diagnosis was made by ultrastructural demonstration of melanoma cells from the cerebrospinal fluid (CSF) at a time when cranial computed tomography (CT) still gave negative results. Later CT examinations documented the emergence of a tumor mass of the left temporoparietal lobe. This case clearly illustrates the complementary role of these investigational procedures for the diagnosis of cerebrospinal melanoma: leptomeningeal involvement, characterized by two-dimensional diffuse spread of melanoma tissue ("leptomeningeal melanomatosis"), is invisible with CT, but easily recognisable by CSF cytology; in contrast, nodular parenchymal tumor deposits can be readily detected by CT. Identification of pigmented cells recovered from the CSF requires ultrastructural confirmation.