These tumors are composed of well-differentiated fibrillary astrocytes that typically form mixed solid and cystic masses in one of the cerebellar hemispheres (Chapter 2, Fig. 18). The tumor-associated cyst can often be very large and produce significant mass effect in the cerebellum and brainstem. These tumors are histologi-cally described as juvenile pilocytic astrocytomas (JPA), are benign neoplasms (WHO grade I), and as such display an indolent pattern of growth. Although JPAs can be found throughout the CNS, 80% are located in the cerebellum.
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