Fig. 4

Histologic spectrum of epigenetically defined pilocytic astrocytomas harboring FGFR1 alterations. These tumors are characterized by a glial component with predominantly oligodendroglial morphology often in a mucin-rich stroma with variable presence of microcalcifications. Occasional tumors demonstrate neurocytic differentiation and neurocytic rosettes resembling RGNT (case PA #1). In contrast to pilocytic astrocytomas with BRAF mutation or fusion that typically have piloid morphology, frequent Rosenthal fibers, and biphasic pattern of alternating loose and compact growth, these pilocytic astrocytomas with FGFR1 alterations typically have an oligodendroglial morphology in a prominent myxoid stroma without Rosenthal fibers