Nondiffuse Astrocytoma Variants

Definitions and Synonyms

The pilocytic astrocytoma (PA) is a circumscribed, well-differentiated neoplasm of childhood and young adults that can occur throughout the central nervous system (CNS), but is most common in the cerebellum, where it is often cystic. It is composed of cytologically bland fibrillar, rounded, or long thin tumor cells arranged in either a solid, microcystic, or biphasic pattern, typically with associated Rosenthal fibers and eosinophilic granular bodies. Its circumscription often lends itself to a complete neurosurgical resection that can result in a cure. It corresponds to a World Health Organization (WHO) grade I neoplasm.

Numerous nonspecific clinical terms have been used to refer to pilocytic astrocytomas, including cerebellar astrocytoma, optic pathway glioma, tectal glioma, and dorsal exophytic brainstem or medullary glioma. These terms should be avoided or at least clarified whenever possible, because diffuse astrocytomas may also involve those sites, albeit considerably less often.

Incidence and Demographics

When considering all age groups, PA accounts for only 2% of CNS neoplasms and 6% of all gliomas, with an annual incidence rate of 0.84 per 100,000 person-years. However, in children, PA is the most common form of low-grade brain tumor and represents 33% of all gliomas in this age group. While more than 75% of PAs occur in children, with a peak incidence between 8 and 13 years, they can occur at any age, even into the 60s and 70s, although they account for less than 1% of adult brain tumors. In these older patients, it is generally assumed that tumors have been present in an asymptomatic form for decades. No gender or racial predilection has been appreciated.

Clinical Manifestations and Localization

Pilocytic astrocytomas can be found throughout the neuroaxis, but are predisposed to a smaller number of stereotypic sites, including the cerebellum, the optic pathway (including nerves, chiasm, and tracts), hypothalamus, dorsal brainstem, and spinal cord. PA accounts for approximately 10% of cerebral and 85% of cerebellar astrocytomas. These tumors are typically well circumscribed, slowly growing intra-axial masses that grossly and radiologically appear to displace adjacent brain rather than diffusely infiltrate it. Symptoms depend largely on the site. In the case of cerebellar tumors, the most frequent symptoms are headache, nausea, vomiting, ataxia, and cranial nerve deficits. Tumors that include the optic pathways present with slowly deteriorating vision or occasionally with proptosis if the orbit is involved. Tumors localized to the hypothalamus can cause endocrine dysfunction, including but not limited to obesity and diabetes insipidus, due to impingement on either the hypothalamus or pituitary gland. Those that grow in the periventricular or periaqueductal regions, such as the thalamus or brainstem, will often present with signs of hydrocephalus due to cerebrospinal fluid (CSF) obstruction. The majority of PAs are sporadic; however, NF1 patients are predisposed to this tumor type, particularly in the optic pathway and, to a lesser extent, the brainstem and other sites (see Chapter 22 ). Note that 15% of all PAs arise in this setting.

Radiologic Features and Gross Pathology

On neuroimaging, the majority of PAs are well circumscribed, with either a dominant cystic or solid pattern. The classic radiologic finding of a cerebellar pilocytic astrocytoma is a cyst with an enhancing mural nodule ( Fig. 7.1A and B ). When PAs involve the optic nerves or chiasm, they are usually solid, expansile, and variably enhancing ( Fig. 7.1C ). PAs of the optic tracts typically have a wedge-like growth pattern that expands posteriorly. In regions of the hypothalamus and brainstem, tumors are circumscribed, lobulated or exophytic, and intensely contrast enhancing. In the spinal cord, PAs are intra-axial, fusiform, enhancing masses and may be associated with a syrinx that extends over multiple spinal levels.

On gross inspection of surgical resections or autopsy material, pilocytic astrocytomas typically appear well demarcated, though there may be some regions where the tumor blends in with adjacent brain. Cystic degeneration is common.

Histopathology

The classic pilocytic astrocytoma has a biphasic appearance with alternating densely fibrillar and loose/microcystic components ( Fig. 7.2A and C ). In some cases, only one of the two patterns is encountered. The dense regions often resemble fibrillary astrocytoma, except that the cytoplasmic processes of PA are particularly long and hair-like (i.e., “piloid” as indicated by the tumor's name). The latter is best appreciated on cytologic specimens, such as intraoperative smears, in which the hair-like processes extend long distances, often across an entire low magnification microscopic field ( Fig. 7.2B ). PAs also differ from fibrillary astrocytoma by their mostly solid growth pattern, such that the dense fibrillarity noted in PAs is due almost entirely to the cellular processes of neoplastic cells. There are generally no entrapped axons or neuronal cell bodies within the central portions of the tumor, and there is a general paucity of entrapped axons upon neurofilament immunohistochemistry. However, nearly all cases show at least limited infiltration at the periphery and rare examples show extensive invasion ( Fig. 7.3C ).

The presence of corkscrew-shaped brightly eosinophilic Rosenthal fibers (RFs) strongly favors the diagnosis of PA over diffuse astrocytoma, although this is not absolute ( Fig. 7.2E ). PAs can also contain cellular arrangements that are less densely fibrillated and are composed of cells that are more loosely disposed and look remarkably similar to oligodendrogliomas, with round nuclei and perinuclear cytoplasmic clearing ( Fig. 7.2D ). This pattern can be noted in myxoid regions, as well as in regions dominated by microcysts. Even in these areas, however, the long, thin cellular processes typical of PAs can be highlighted with a glial fibrillary acidic protein (GFAP) immunostain ( Fig. 7.3B ). Additionally, this component often harbors mulberry-shaped eosinophilic granular bodies (EGBs), which may be numerous ( Fig. 7.2F ) or rare. In the latter scenario, a PAS with diastase stain may help to draw attention to these structures ( Fig. 7.3A ).

Most often, the nuclei of pilocytic astrocytomas are oval with delicate chromatin, but significant atypia (perhaps degenerative in nature) may occasionally be seen and is generally unassociated with increased mitotic/proliferative activity ( Fig. 7.2G ). True multinucleated forms are often appreciated with tight clustering of nuclei in a horseshoe-like configuration, often described as having a “pennies on a plate” arrangement ( Fig. 7.2G ), wherein the nuclei resemble a stack of pennies splayed out peripherally on a plate (i.e., cytoplasm). Glomeruloid single-layered vessels with multiple lumens are also typical, especially around the cyst lining, and should not be mistaken for the multilayered endothelial hyperplasia of diffuse gliomas ( Fig. 7.2I ). Nevertheless, the latter may also be encountered in pilocytic astrocytomas and does not have the ominous prognostic significance it does in the diffuse gliomas. Others show degenerative vascular changes resembling cavernous angioma or other vascular malformations ( Fig. 7.2J ). The presence of occasional mitotic figures is acceptable ( Fig. 7.3D ), although a high mitotic index is uncommon. Bland infarct-like necrosis is encountered in roughly 5% of PAs and, similarly, has no clinical significance ( Fig. 7.3G ). Likewise, extension into the subarachnoid space is quite common and does not alter the prognosis ( Fig. 7.2H ). In fact, some examples create “tissue bridges” with bundles of tumor cells resembling bales of hay; this results from spread of tumor across the subarachnoid space from one cerebellar folium to the next ( Fig. 7.2K and L ). In contrast, palisading necrosis and foci of hypercellularity with increased proliferative activity should prompt consideration of an alternate diagnosis or malignant transformation, an exceptionally rare complication in pilocytic astrocytomas, with most examples being encountered after radiation therapy.

Histologic Variants and Grading

The classic pilocytic astrocytoma corresponds to a WHO grade I neoplasm. The pilomyxoid astrocytoma (PMA) is a variant recognized by the WHO that occurs in early childhood or adolescence and has been reported to have a more aggressive clinical behavior. Because of uncertainties in the diagnostic criteria and insufficient clinical experience, the current WHO classification no longer provides a specific grade for pilomyxoid astrocytoma. PMAs most often arise in the hypothalamic region, where they are well circumscribed, generally solid, and homo­geneously contrast-enhancing midline masses ( Fig. 7.4A ), but can also arise in the thalamus, cerebellum, brainstem, temporal lobe, and spinal cord. PMA consists of a hypercellular, monomorphous population of piloid cells ( Fig. 7.4B and C ) that are typically embedded within a rich myxoid matrix and often display an angiocentric arrangement ( Fig. 7.4C ). Like PA, the PMA has a relatively discrete architecture, with only a slight tendency for peripheral infiltration of adjacent brain. Individual tumor cells have elongate “piloid” processes, are moderate in size, and contain hyperchromatic nuclei with only modest nuclear pleomorphism. Mitotic figures can be noted but are not abundant. The diagnosis of PMA is made only when this tissue pattern is predominant, since focal myxoid or angiocentric cell arrangement may be noted in typical pilocytic astrocytoma ( Fig. 7.3H ) or infiltrating astrocytoma. Unlike ordinary pilocytic astrocytoma, PMAs typically lack a biphasic appearance, do not contain Rosenthal fibers, and only exceptionally contain eosinophilic granular bodies. Those that have concluded that PMAs are associated with a more aggressive clinical course than typical pilocytic astrocytoma have reported higher recurrence rates, occasional CSF dissemination, and increased risk of patient death.

While the vast majority of PAs are histologically low grade and have an indolent clinical behavior, a small subset, representing no more than 1% to 2%, will undergo anaplastic transformation and are referred to as pilocytic astrocytomas with anaplasia. The most prominent feature that characterizes these cases is a brisk mitotic rate (>4 mitoses per 10 HF), but nuclear anaplasia, hypercellularity, microvascular proliferation, and necrosis can be noted as well ( Fig. 7.5A–C ). In the majority, a PA with classic histology coexists with anaplastic foci or was documented in a prior resection. In a recent series of pilocytic astrocytomas with anaplasia, tumors were categorized by the dominant histology as follows: classic pilocytic astrocytoma with brisk mitotic figures (41%); poorly differentiated with small cells (32%); epithelioid or rhabdoid (15%); or resembling diffuse astrocytoma (12%). A subset of PAs with anaplasia have been previously radiated, and the role this may play in inducing anaplastic transformation is not known; many other examples have not been previously treated. Since patients with PA with anaplastic features have a significantly worse prognosis compared to classic pilocytic astrocytoma, these features are important to recognize and document. The WHO has not designated a formal grade for these tumors, although it was suggested in one study that those with and without necrosis have survival times similar to WHO grade III and II diffuse gliomas, respectively.

Differential Diagnosis

The most common differential diagnoses for PA include diffuse astrocytoma, oligodendroglioma (both discussed in Chapter 6 ), and reactive “piloid” gliosis. Given the occasional findings of nuclear atypia, microvascular proliferation, mitotic activity, and necrosis in a classic PA, or in the occasional PA with anaplastic features (see Figs. 7.2 and 7.5 ), one may even find oneself in the unsettling differential diagnosis with a glioblastoma. In most instances, the unique clinical, radiographic, and microscopic features of PA will help distinguish it from these other considerations and immunohistochemical and genetic studies can be performed as needed (see next section). Although neither RFs nor EGBs are entirely specific for pilocytic astrocytoma, they generally suggest a benign or slowly evolving process, such as pilocytic astrocytoma, PXA, and ganglioglioma, all representing tumor types with a favorable prognosis, often with similar radiographic features. PXA has a much greater degree of nuclear pleomorphism, mesenchymal-like spindled elements, and vacuolated or lipidized cells, as well as reticulin-rich foci that are not seen in pilocytic astrocytomas (see Fig. 7.9 ). Gangliogliomas have a well-differentiated neuronal component, characterized by dysmorphic ganglion cells, and will often have a lymphocytic infiltrate. Immunohistochemistry for neurofilament, synaptophysin, or NeuN is helpful for highlighting dysmorphic ganglion cells in ganglioglioma; additionally, stellate CD34 positive cells and BRAF V600E mutant protein positivity are much more common in ganglioglioma. Dysembryoplastic neuroepithelial tumors (DNTs) most often resemble oligodendrogliomas, but may have areas resembling pilocytic astrocytoma as well. Temporal lobe predilection, patterned mucin-rich nodules, and floating neurons serve to distinguish this entity, although the glial nodules of a complex DNT are often indistinguishable from PA. Lastly, RFs are also often encountered in a piloid form of reactive gliosis (similar appearance to Fig. 7.2E ), most often next to craniopharyngiomas, ependymomas, hemangioblastomas, developmental cysts, and syringomyelia. Piloid gliosis is typically less cellular and does not have a microcystic component. Additional sampling and attention to clinical/radiographic features generally allow one to avoid this pitfall.

The striking angiocentric orientation of glial processes in pilomyxoid astrocytoma can resemble the pseudorosettes in ependymoma, causing a diagnostic dilemma in some cases. While both lesions are noninfiltrative and show glial differentiation, PMAs typically have a myxoid matrix and are less densely fibrillar, with nuclei that are small, bland, and slender compared to ependymoma. OLIG2 and SOX10 are highly expressed in PMA (as well as PA) tumor nuclei (see Figs. 7.3I and 7.3J ), but not in ependymoma and may be useful for distinguishing difficult cases.

Ancillary Diagnostic Studies

In the majority of cases, the diagnosis of PA can be established on morphologic grounds alone. PAS with diastase and trichrome stains are occasionally helpful in highlighting rare EGBs and RFs that were not evident on hematoxylin-eosin (H & E) stains (see Fig. 7.3A ). PAs are typically strongly immunoreactive for GFAP ( Fig. 7.3B ), OLIG2 ( Fig. 7.3I ), and SOX10 ( Fig. 7.3J ), and show moderate staining for S-100 protein. In smaller biopsies, the distinction between an infiltrating glioma and pilocytic astrocytoma can be aided using immunohistochemistry for neurofilament, which highlights infiltrated axons in diffuse gliomas, but usually only shows scattered entrapped axons at the edges of PA. In adult patients, isocitrate dehydrogenase (IDH)-mutant oligodendrogliomas and diffuse astrocytomas can be ruled out by a negative IDH1 R132H immunohistochemical stain and follow-up sequencing for less common IDH mutations, if warranted. In the differential of an IDH-mutant diffuse astrocytoma, neoplastic cells that show retention of ATRX and lack of p53 expression by immunohistochemistry would also favor a PA. The MIB-1 labeling index of PA is generally low, ranging from 1% to 5%, yet a higher index does not exclude the diagnosis (see Fig. 7.3E ). Most studies have not shown any prognostic value for MIB-1 indices in this tumor type. However, in the case of subtotal resection, the proliferation index may be taken into account for clinical management in some instances. Although electron microscopy is rarely required, these studies generally show electron-dense cells with prominent cytoplasmic intermediate filaments, corresponding to GFAP.

Genetics

The dominant genetic alterations of pilocytic astrocytoma result in the activation of the mitogen-activated protein kinase (MAPK) pathway and include activating alterations of BRAF and inactivation of neurofibromatosis type 1 (NF1). Array comparative genomic hybridization (CGH) studies were the first to identify a low-level copy number gain of the BRAF gene on 7q34 in a large proportion of cerebellar pilocytic astrocytomas. The BRAF copy gain is due to a tandem duplication producing a KIAA1549:BRAF fusion protein with loss of its Ras-binding domain at the N-terminus and constitutive BRAF activity. BRAF fusions are present in 70% of all pilocytic astrocytomas, but are rare in high-grade pediatric gliomas. These fusions are found in approximately 80% of cerebellar, but only 55% of noncerebellar pilocytic astrocytomas. The frequency of BRAF-KIAA1549 fusions also appears to vary with patient age, decreasing in frequency in older patients. Approximately 50% of pilomyxoid astrocytomas contain BRAF fusions, consistent with the frequencies noted in PAs from the same anatomic locations. More recent studies have uncovered a larger number of gene partners for BRAF in these fusion events, yet the resulting gene products all lack the BRAF N-terminal regulatory domain and show constitutive activity. Due to the multitude of fusion partners and fusion sites with KIAA1549, PCR-based testing is challenging. However, fluorescence in situ hybridization (FISH) testing can reliably detect the tandem duplication of 7q34, as can cytogenetic microarrays (see Fig. 7.3K ).

The constitutively active BRAF V600E point mutation is present in about 10% of pilocytic astrocytomas and is more frequent in supratentorial tumors than in those involving the cerebellum. Thus, over 80% of sporadic PAs have some sort of activating BRAF alteration, most often either an activating gene fusion or point mutation. Other less common MAPK activating genetic alterations include RAF1 gene fusions, NTRK gene fusions, activating FGFR1 point mutations and gene fusions, and KRAS mutations.

Approximately 15% to 20% of NF1 patients develop pilocytic astrocytomas, particularly of the optic nerve, and up to one-third of patients with a pilocytic astrocytoma in this location fulfill the diagnostic criteria of NF1 (see Chapter 22 ). Since the NF1 gene has tumor suppressor function and its gene product, neurofibromin, inhibits the MAPK pathway, NF1 loss represents another mechanism of MAPK pathway activation. NF1-associated PAs therefore rarely contain a BRAF fusion or mutation, since NF1 loss similarly leads to activation of the MAPK/ERK signaling pathway.

Similar to other low-grade, circumscribed pediatric brain tumors, the genetic spectrum of PA is relatively simple and generally includes only a single oncogenic driver. Alterations of genes that are commonly involved in diffuse forms of astrocytoma, such as IDH1, IDH2, ATRX, TP53, EGFR, CDKN2A, and PTEN, are not present in classic PA and their identification usually precludes diagnosis. However, activation of the PI3-kinase/Akt pathway has been associated with aggressive clinical behavior in pilocytic astrocytomas and some of these additional genetic alterations may be involved, especially CDKN2A losses.

Studies of outcome related to the presence of a BRAF gene fusion or mutation have not shown a strong independent association with recurrence. However, gains of whole chromosome 7 have been associated with an increased risk of recurrence after adjusting for surgical status and other genetic alterations. Other whole genome cytogenetic microarray analyses of PA have shown that 45% of adult and 17% of pediatric cases show aneuploidy. The gains that were identified were nonrandom and preferentially involved chromosomes 5, 7, 6, and 11. Interestingly, these cytogenetic alterations were noted most frequently in noncerebellar PAs and those that harbored BRAF V600E mutations rather than BRAF gene fusions.

Treatment and Prognosis

Pilocytic astrocytomas are benign (WHO grade I) neoplasms treated primarily with surgery. The overall prognosis for this tumor type is excellent, with a 10-year survival over 90% and 20-year survival estimated at 80%. As a group, the supratentorial PAs have a less favorable prognosis than those of the cerebellum. In particular, those that involve deep, midline structures, such as the hypothalamus, have higher rates of incomplete resection and recurrence and are associated with shorter survivals. PAs associated with NF1 generally have a better clinical outcome than sporadic PAs, especially those involving the optic tracts. Often the latter grow early in childhood, only to stabilize or even regress spontaneously as the patient gets older.

A small subset of PAs is associated with significant morbidity and mortality, yet predicting which cases will behave in a more aggressive fashion based on histology alone is challenging. Definitive markers associated with anaplastic histology of aggressive clinical behavior have not been identified with certainty, but both chromosome 7 gains and FGFR1 mutations have been suggested. Other cases may be associated with dissemination through the CSF, but this does not necessarily imply that the leptomeningeal deposits are anaplastic or rapidly growing. Some patients with disseminated disease may have stable or slowly progressive disease despite, presumably due to their slow growth. No specific genetic alterations have been identified in disseminated PAs as compared to primary PAs.

Depending on the tumor location, subtotally resected cases may be radiated for enhanced local control. With the identification of MAPK pathway activation and BRAF alterations in PAs, targeted therapeutic approaches have been established and show variable success.