The JPA is a WHO Grade I lesion. The
mass is usually sharply circumscribed, with a very narrow zone of microscopic
infiltration. The tumor nodule is supplied by capillaries that lack a complete
blood-brain-barrier (BBB). The absence of the BBB is the cause of both the
contrast enhancement, as well as the source of production of the proteinaceous
fluid that accumulates in the "cyst". Because there is no true "lining" the
lesions are not "true" cysts. The wall of tissue surrounding the fluid is normal
or compressed brain, or may have a mild reactive (not neoplastic)
gliosis.
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