Chromosomal copy number abnormalities detected by classical cytogenetics and CGH include chromosomes 1, 6, 7, 9, 10, 13, 17, 19, and 22. Deletions are most common with losses on chromosome 22 a frequent event in adult spinal ependymomas (over 50%) but infrequent in paediatric ependymomas.
Gains have been reported for chromosome 7. These findings have been confirmed by molecular genetic data that have identified losses on 6q, 9p, 10, 11q, 13q, 17p and 19q. The genes targeted by these allelic losses and gains are in most cases unknown, with the exception of the loss of both wild-type copies of the neurofibromatosis type 2 (NF2) gene in sporadic intramedullary spinal ependymomas but not in intracranial ependymomas. Single cases have been reported with loss of other wild type genes such as the MEN1 gene. Germ line mutations of TP53 are uncommon in contrast to the situation in the diffuse astrocytic tumours.
Medulloblastoma
Medulloblastoma has a peak incidence in childhood but also can occur into late middle age. Histologically childhood and adult medulloblastoma are identical, being highly cellular, malignant invasive tumours corresponding to WHO malignancy grade IV.
Medulloblastomas occur in the posterior fossa. They consist of densely packed tumour cells with round to oval or carrot shaped hyperchromatic nuclei with scanty cytoplasm, high mitotic and apoptotic rates, and usually neuroblastic rosettes in some areas (fig 3).
Neuronal differentiation and glial differentiation may be present. Microvascular proliferation is relatively uncommon. Tumours arise with similar frequency in the cerebellar vermis (mainly in children) and the cerebellar hemispheres (older patients), and often invade the fourth ventricle, with occasional brainstem involvement.
There is a high risk of seeding through the subarachnoid space due to the tendency of the tumour to penetrate the ependymal surface. Many antigens have been identified focally in medulloblastomas (nestin, vimentin, neurofilament proteins, GFAP, retinal S-antigen, N-CAMs, Trk-A, -B, -C etc).
However, most are not of any great importance in the day-to-day diagnosis of these tumours. It is most important to differentiate medulloblastomas from atypical teratoid/rhabdoid tumours, as the latter have a very poor prognosis and do not respond to the current relatively successful treatment protocols for medulloblastomas. In adults the possibility of a metastasis of a small cell lung cancer must often be excluded.
