PAs represent the most frequent glioma subtype in children. They are characterized by elongated, bipolar astrocytes usually with bland nuclear features. The classic architecture is a biphasic pattern, with alternating Rosenthal fiber-rich compact areas and loose, microcyst rich regions. Eosinophilic granular bodies may be present in these regions and even abundant. Additional variable features include hyalinized or glomeruloid vessels, hemosiderin deposition, and degenerative nuclear pleomorphism or multinucleated cells. Monotonous oligodendroglial-like cells may predominate in some examples. Occasional mitoses and non-pseudopallisading necrosis may be present, but they have an inconsistent relation with clinical outcome [6, 7]. As other astrocytomas, these tumors express GFAP and OLIG2. P53 and Ki-67 show low labeling indices. Elevated Ki-67 labeling indices have been associated with worse outcome in some studies [8] but not in others [9, 10]. Unlike the category of diffuse gliomas, the development of anaplasia/histologic malignancy in PA is a very rare event (<2 %), and has been defined as the presence of brisk mitotic activity with or without necrosis [11].

Pilomyxoid astrocytoma (PMA) is considered a variant of PA, characterized by monophasic morphology within a myxoid background and conspicuous aggregates around blood vessels [12]. Unlike conventional PA, PMA lacks a biphasic pattern and Rosenthal fibers. Eosinophilic granular bodies are rare to absent. The classic presentation of PMA is the hypothalamic region of young children. Because of its higher likelihood for aggressive behavior and leptomeningeal dissemination [13], the WHO assigns a grade II to this variant. Morphologic and molecular overlap occurs with conventional PA, which is supported by the recognition of tumors with intermediate features between PMA and PA or PMAs that mature into PA over time [14].

SEGA is a unique astrocytoma subtype that arises almost always within the lateral ventricles near the foramen of Monro in tuberous sclerosis patients. It is composed of large cells with voluminous eosinophilic cytoplasm, round nuclei and prominent nucleoli. The mitotic activity is very low. At an immunohistochemical and ultrastructural level, these tumors demonstrate evidence of neuronal differentiation in addition to a glial phenotype, which is more accurately consistent with a glioneuronal neoplasm [15]. This includes immunoreactivity for synaptophysin and neurofilament protein in addition to GFAP.

PXA is a rare neoplasm with distinctive histology. It is composed of spindle neoplastic cells with often conspicuous pleomorphism, sometimes including giant cell forms. Mitotic activity is very low in most cases, and this disconnect between the cellularity and pleomorphism of the tumor and its relative lack of mitoses is one of the first clues to the diagnosis. Eosinophilic granular bodies, particularly large pale forms, are also frequent.

PXA may also demonstrate immunoreactivity for neuronal markers in addition to S100 and GFAP. P53 immunolabeling is typically weak or negative, while CD34 expression is relatively frequent [16]. An additional characteristic finding is the presence of pericellular staining with reticulin special stains, particularly in areas juxtaposed to leptomeninges.

Although PXA is low grade (WHO grade II) by definition, among the circumscribed gliomas it has the highest propensity for recurrence and biologic aggressiveness. A subset develops anaplastic features in the form of brisk mitotic activity and/or necrosis and microvascular proliferation [17]. However, these changes do not consistently predict an adverse outcome, and pending more definite data, a grade III is not yet allowed under the WHO classification.

Angiocentric glioma is a distinctive neoplasm that was added to the WHO 2007 classification [18, 19]. It is slow growing, frequently associated with epilepsy and therefore shares many clinical properties with dysembryoplastic neuroepithelial tumor. Despite gross circumscription, it is histologically infiltrative. The most characteristic feature of angiocentric glioma is the presence of thin elongated nuclei with little pleomorphism and a perivascular aggregation in a parallel and perpendicular pattern (Fig. 4.3). GFAP expression is a constant feature by immunohistochemistry, but in addition EMA frequently stains in a dot-like fashion, and microlumina may be present on electron microscopy, which suggests a dual astrocytic/ependymal phenotype.

Diffuse astrocytoma (DA) is a specific subtype characterized by neoplastic astrocytes with nuclear hyperchromasia, atypia, usually low cellularity and rare to absent mitotic figures (Fig. 4.3). Unlike the circumscribed glioma group, DA demonstrates an exquisite infiltration of underlying brain parenchyma, making them surgical challenges. They also have a strong tendency for progression to higher grade neoplasms (anaplastic astrocytoma, glioblastoma), particularly in adults.

Oligodendroglial tumors include oligodendrogliomas and mixed oligoastrocytomas. They may be low (grade II) or high grade (grade III) (Fig. 4.4). Oligodendroglial morphology is defined by round nuclei with perinuclear halos and little internuclear variability. A delicate “chicken-wire” type microvasculature may be present. Limited mitotic activity may be present, but brisk mitotic activity, endothelial hypertrophy and necrosis define higher grade tumors. Adult oligodendroglioma in particular has classic molecular alterations involving chromosomal arms 1p and 19q, and has almost become a combined pathologic/molecular diagnosis.

Prior observations have hinted that morphologically similar neoplasms in the adult and pediatric populations may have different clinical behavior. DA, for example, may not always have the same predictable progression to anaplasia in children as in adults. This has been confirmed by molecular studies [20]. Diffuse intrinsic pontine gliomas represent a distinct subset of diffuse gliomas in children, and are essentially defined by their anatomic development within the brainstem. Although in theory, a subset of these may be identified as grade II early in their course, high-grade histologic features are almost always present postmortem [21]. Oligodendrogliomas in children for example lack 1p19q co-deletion and IDH1/2 mutations in contrast to adult tumors [22–24]. Furthermore, the mutational landscape uncovered by recent, large scale sequencing efforts is different, and simpler, in pediatric diffuse gliomas.

The earliest insights into the molecular alterations contributing to glioma formation have evolved from the study of inherited tumor syndromes. Neurofibromatosis type 1 (NF1) is associated with germline mutations in the NF1 gene that encodes for the protein neurofibromin, a negative regulator of RAS signaling. These patients are predisposed to gliomas of various grades, particularly PA of the optic pathways. Furthermore, biallelic NF1 inactivation is a feature of these patients tumors [36]. Tuberous sclerosis complex is associated with germline mutations in TSC1 or TSC2, which leads to increased mTOR activity and a predisposition to SEGA. Conversely, Li–Fraumeni syndrome is associated with TP53 mutations, which predisposes to a variety of tumors, including infiltrating astrocytomas [37]. These early observations suggested that the pathways deregulated by these germline alterations were important for gliomagenesis.

More recent studies have also highlighted a role for germline polymorphisms that predispose to diffuse glioma development. For instance, genotyping efforts have uncovered single nucleotide polymorphisms (SNP) variants at 8q24.21 (near the MYC gene) to be associated with an increased risk for gliomas with IDH1/2 mutations, both astrocytic and oligodendroglial [38].

One of the earliest molecular alterations described in diffuse gliomas, with morphologic and prognostic relevance was the identification of 1p19q co-deletion, particularly in tumors with oligodendroglial morphology (Fig. 4.7). Subsequent studies highlighted a strong association between 1p19q co-deletion and therapeutic response, particularly in anaplastic oligodendroglioma [74]. This alteration may be identified by a variety of methods that work in formalin-fixed paraffin-embedded tissue, most commonly FISH [75], but also array based platforms, such as SNP arrays [76]. Although partial deletions involving chromosome arm 1p and/or 19q are not uncommon in gliomas with various histologies, it is whole arm 1p and 19q co-deletion that is most closely associated with oligodendroglial histology, which is mediated by an unbalanced (t1;19) translocation [77, 78]. Subsequent whole exome sequencing studies have identified recurrent mutations in FUBP1 (Ch 1p) and CIC (Ch 19q) as likely tumor suppressor genes inactivated at these locations [79, 80].

Another remarkable subsequent discovery in the biology of diffuse gliomas was the identification of recurrent point mutations in genes encoding for the cytosolic metabolic enzyme IDH1 (and less frequently IDH2) through exome sequencing efforts. Although initially identified in a subset of glioblastomas, it was subsequently noted that these mutations were highly prevalent (>80 %) in diffuse gliomas grade II and III, both astrocytomas and oligodendrogliomas as well as in secondary glioblastomas [81–83]. These IDH mutations occur almost always at the same site (Arg132 of IDH1 and analogous Arg172 site in IDH2) and result in a neoenzymatic function leading to increased 2-hydroxyglutarate (2HG) [84], which has numerous cellular effects including inhibition of histone and DNA demethylation and global epigenetic alterations (reviewed in [85, 86]). IDH1/2 mutations appear to be early events in tumorigenesis, since they show similar prevalence in grade II astrocytomas and oligodendrogliomas, and may occur earlier than 1p19q loss and TP53 mutations. Diagnostically, an antibody directed against the most frequent IDH1 mutant protein in gliomas (IDH1 R132H), is in clinical use and valuable in differentiating infiltrating gliomas from other tumors and non-neoplastic conditions (i.e., gliosis) [87, 88].

Another molecular property of a subset of infiltrating astrocytomas is the presence of a telomerase independent mechanism of telomere maintenance known as the alternative lengthening of telomeres (ALT) (Fig. 4.8). Although present in a small subset of cancers of various types, this phenotype is enriched in DA (WHO grade II), anaplastic astrocytoma (WHO grade III), as well as secondary and pediatric glioblastoma (WHO grade IV). Subsequent studies found this phenotype to be strongly associated with mutations in the gene encoding the chromatin remodeling protein ATRX [89]. These mutations lead to ATRX protein loss and are strongly associated with IDH mutations [90–92], but are mutually exclusive with 1p19q co-deletion/CIC/FUBP1 mutations [93].

All these recent studies highlighted an important role for telomere maintenance in the biology of diffuse gliomas, findings subsequently reinforced by the finding of mutations in the TERT promoter which leads to increased transcriptional activity [94]. Interestingly, among diffuse gliomas, TERT promoter mutations are more frequent in oligodendrogliomas and primary glioblastomas and mutually exclusive with ATRX mutations and the ALT phenotype. An expanding picture is now emerging with distinctive molecular signatures separating various low-grade glioma subtypes (Fig. 4.9).