The brain is surrounded by several layers of protective coverings collectively called meninges. The semi-translucent innermost layers, called the leptomeninges, form a “shrink-wrap” around the brain that allows for easy flow of cerebrospinal fluid (CSF) along the outer surface of central nervous system structures. Unfortunately, it also allows for easy spread of neoplastic cells. The image shows the inferior aspect of a cerebellum with edematous cortices and thickened opaque leptomeninges secondary to infiltration of tumor cells in a process sometimes referred to a malignant meningitis. The nomenclature can be further classified based on neoplastic etiology, such as leptomeningeal carcinomatosis (in cases due to carcinoma) or lymphomatosis (in cases due to lymphoma).
For further study on this topic and others related to neuropathology and neuroanatomy, the following books come highly recommended, and a small portion of the proceeds (or any Amazon purchase through these links) help with expenses of hosting this site:
Adventures in Neuropathology 
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