These tumors form the pediatric counterpart to the adult-type diffuse gliomas and are universally aggressive. Outcome data for all but the infant-type hemispheric glioma suggest a median survival of less than 2 years from diagnosis. They occur in the breadth of childhood classically, but a subset affects young adults as well (10).
Diffuse midline glioma, H3 K27-altered. Diffuse midline gliomas, H3 K27-altered, occur in the midline and most frequently in mid-childhood (5 to 8 years of age) but can occur in adolescence and early adulthood. They occur most frequently in the brainstem (commonly the pons, hence the terminology of “diffuse infiltrating pontine glioma”), bithalamic, unilaterally thalamic, or unilaterally in the spinal cord, with unilateral presentations being more common in adolescence and young adulthood (12; 14). The tumors show a diffusely infiltrative histology, with tumor cells being generally small and showing varied differentiation patterns, including astrocytic and epithelioid (15). Irrespective of their histologic appearance, these tumors are WHO grade 4 by definition, in keeping with their aggressive behavior (11; 06). From a molecular perspective, they show four major subtypes: H3.3 p.K28M (K27M)-mutant, H3.1 or 3.2 p.K28M (K27M)-mutant, H3-wildtype with EZHIP overexpression, and EGFR-mutant (02; 01; 13).
Diffuse hemispheric glioma, H3 G34-mutant. Diffuse hemispheric gliomas, H3 G34-mutant, are cerebral hemispheric tumors arising in adolescents and young adults and are assigned a WHO grade 4 designation due to their aggressive behavior (11). They show an infiltrating growth pattern, with astrocytic tumor cells and frequent necrosis, mitotic activity, and perivascular pseudorosettes akin to those seen in the higher-grade adult-type diffuse gliomas. The key molecular finding is a mutation in the H3-3A gene (07).
Diffuse pediatric-type high-grade glioma, H3-wildtype and IDH-wildtype. Diffuse pediatric-type high-grade gliomas, H3-wildtype and IDH-wildtype, are aggressive infiltrating gliomas affecting children, adolescents, and young adults. They arise anywhere but the spinal cord and have several molecular subtypes. The pHGG RTK1 subtype is notably the most frequent type among those associated with germline tumor predisposition syndromes and post-radiation gliomas. The other two subtypes are pHGG RTK2 and pHGG MYCN. Histologically, they show an infiltrating astrocytic tumor with high-grade morphologic features (microvascular proliferation and necrosis) (08; 11; 09).
Infant-type hemispheric glioma. Infant-type hemispheric glioma is a pediatric-type high-grade glioma notable for its occurrence chiefly in the first year of life. They are supratentorial and typically hemispheric. They frequently harbor receptor tyrosine kinase alterations or mutations in ROS1, ALK, or MET. They have an astrocytic histology and frequent high-grade morphologic features (microvascular proliferation, necrosis, and elevated mitotic activity) (05). Comprehensive outcome data are lacking due to the scarcity of this tumor, but targeted therapies may be available if an RTK fusion is present (04; 16; 03).