Clinicopathological features of pilocytic astrocytoma-unusual clinical features and new findings from molecular analysis

Abstract

Pilocytic astrocytoma (PA) is a WHO grade I tumor that is histologically characterized by a mixture of compact bipolar cells and loose textured astrocytes. PA has been classified into the other astrocytic tumors which is a different category from diffuse astrocytic and oligodendroglial tumors in the recent revised WHO 2016 classification.PA frequently occurs in cerebellum, as well as the supratentorial hemispheric and optico-hypothalamic regions. Recent molecular analysis demonstrated that gene expression profiles of the tumor differ among these three regions. BRAF tandem duplication resulting in the K1AA1549-BRAF fusion gene is the most frequent genetic alteration in PA. The BRAF V600E mutation is less frequently detected than the K1AA1549-BRAF fusion. More recently, it has been recommended that pilocytic astrocytoma with anaplasia should be designated as anaplastic astrocytoma with piloid features according to the results of methylation-based classifications. Clinical trials involving molecular targeted therapies using MAPK pathway inhibitors are ongoing and several promising results have been reported. Clinically, some PAs show spontaneous regression, while other PAs demonstrate spinal dissemination without the evidence of histological malignant transformation. The biological mechanism of these unusual clinical features will be dissected using molecular techniques in the near future.