COMMON CHILDHOOD TUMOURS
Pilocytic astrocytomas
Pilocytic astrocytomas most commonly occur in the cerebellum of children. However, they may occur anywhere from the optic nerve to the medulla oblongata. Patients with pilocytic astrocytomas that can be excised have a good prognosis.
There is an increased incidence of pilocytic astrocytomas in NF1 patients, particularly involving the optic nerve, and these tumours in NF1 patients behave in a particularly benign fashion. Pilocytic astrocytomas are generally biologically non-aggressive and are remarkable among astrocytic tumours in maintaining their grade I status over years and even decades (in contrast to the diffuse astrocytic tumours in adults).
However, very occasional cases may prove more sinister and progress to more malignant tumours. Pilocytic astrocytomas show a wide spectrum of morphologies, from the pilocytic, bipolar cellular areas with Rosenthal fibres (fig 1) to less cellular protoplasmic astrocytoma-like areas with eosinophilic granular bodies and clear cells.
The latter are reminiscent of oligodendroglioma and in the posterior fossa can also be confused with clear cell ependymoma. The presence of features typically associated with a malignant biological behaviour (for example, vascular proliferation or mitosis) does not carry the same sinister implications as in the other astrocytic tumours.
This morphological spectrum can make histopathological diagnosis extremely difficult particularly in the absence of the clinical data that must be provided to the pathologist as outlined above.
Figure 1 Pilocytic astrocytoma malignancy grade I (H&E). Note the piloid bipolar cells and Rosenthal fibres (arrows). This shows the classical morphology that is generally found somewhere in a pilocytic astrocytoma; other areas can show very different histological patterns
More than 100 cases of pilocytic astrocytomas have been analysed cytogenetically and many more by comparative genomic hybridisation. No consistent findings have been made. The majority show normal cytogenetic and comparative genomic hybridisation (CGH) findings.
Adult pilocytic astrocytomas have been found to show the most frequent but again variable abnormalities. Molecular genetic studies have been few and have shown allelic losses on both 17p and 17q including the TP53 and NF1 loci. Few TP53 mutations have been reported and no mutations of the NF1 locus have been reported in sporadic tumours.
Recently studies of methylation of the promoter regions of a number of genes reported to be hypermethylated in the adult diffuse astrocytic gliomas have provided somewhat inconsistent data on methylation in pilocytic astrocytomas.
Ependymoma
Ependymomas arise at or close to ependymal surfaces and may occur anywhere in the ventricular system as well as in the spinal cord and very occasionally at extraneural sites. The most common location is in the fourth ventricle, followed by the spinal canal, lateral ventricles, and the third ventricle.
Children have the highest incidence of ependymomas, but they can occur into late middle age. Ependymomas are the most frequent glioma of the spinal cord and this location is common in adults. There are a number of subtypes.
The least biologically aggressive are malignancy graded as grade 1, and consist of the subependymoma (intraventricular and often symptomless) and myxopapillary ependymoma that most commonly occurs at the cauda equina.
The tumour named ependymoma is malignancy graded as grade II and has a number of histopathological variants. Ependymomas show in some area(s) evidence of an ependymal cell phenotype—by the formation of ependymal rosettes and sometimes canals (fig 2). More commonly perivascular pseudo-rosettes are identified but are not specific for ependymomas.
Ependymomas (malignancy grade II) are differentiated from anaplastic ependymomas (malignancy grade III) on the basis of low mitotic rate and a low level of nuclear polymorphism, but the borderline between these remains ill defined. Necrosis and microvascular proliferation do not have the same significance in this tumour type as in the adult astrocytic tumours.
Most ependymomas (malignancy grade II) show immunoreactivity for glial fibrillary acidic protein (GFAP), S-100 protein, and epithelial membrane antigen (EMA).
