Pilocytic Astrocytoma and Pilomyxoid Astrocytoma

Abstract

Pilocytic astrocytoma (PA)

• These are circumscribed, slow-growing, and often cystic astrocytic neoplasm, typically with a biphasic compact and loose architecture by histology and corresponding to WHO grade I.

• Though most are sporadic, there is a well-established association of pilocytic astrocytoma and neurofibromatosis Type 1 with germline mutations of the NF1 (neurofibromin) gene. Optic nerve involvement, particularly bilateral, is the most common central nervous system (CNS) finding.

• It has been referred to as “optic nerve glioma,” “cerebellar astrocytoma,” or “dorsal exophytic brain stem glioma.”

Pilomyxoid astrocytoma (PMA)

• A recent addition to the 2007 WHO classification of tumors of the CNS, this lesion shares some histologic features of PA, though tends to occur in younger children and exhibits a more aggressive clinical behavior. It is correspondingly given a WHO grade II designation.

• It is sporadic in nature, and only a single example was reported arising in the context of NF1.

• Though its histogenesis remains uncertain, theories include the following:

  • The term “tanycytoma” has been proposed by some authors in reference to the fact that PMA have been shown to exhibit ultrastructural characteristics similar to those of “tanycytes,” which are precursor cells found within circumventricular organs and floor of the third ventricle.

  • Others have proposed that PMA represents some variant of glioneuronal neoplasm in reference to immunohistochemical and ultrastructural features outlined elsewhere.

  • PMAs may represent an “immature pilocytic astrocytoma,” as there have been several documented cases in which recurrent lesions have shown features of classic PA.