Background
Primitive neuroectodermal tumors (PNET) are neoplasms of which medulloblastoma is the prototype. These are small cell, malignant embryonal tumors showing divergent differentiation of variable degree along neuronal, glial, or rarely mesenchymal lines.
The most recent WHO categorization of embryonal tumors is as follows: [1]
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Medulloblastoma
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CNS primitive neuroectodermal tumor (PNET)
- CNS neuroblastoma
- CNS ganglioneuroblastoma
- Medulloepithelioma
- Ependymoblastoma
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Atypical teratoid/rhabdoid tumor
Only tumors of the CNS are discussed here. Peripheral primitive neuroectodermal tumors are regarded as distinct entities. [2]
Pathophysiology
PNET of the CNS can be divided grossly into infratentorial tumors (medulloblastoma or iPNET) and supratentorial tumors (sPNET).
Considerable controversy exists regarding the histogenesis of these tumors. Initially, these dense, cellular, embryonal tumors were thought to have a common origin from primitive neuroectodermal cells and to differ only in their location, type, and degree of differentiation. In the revised World Health Organization (WHO) classification, however, many of these tumors are given a separate niche on the basis of the assumption that these embryonal tumors also could arise from cells already committed to differentiation. [3]
Regardless of the controversy, these tumors are discussed as infratentorial (medulloblastoma) and supratentorial. The latter occur rarely (25:1) and are more common in young adults than infratentorial tumors.
Spinal dissemination via the cerebrospinal fluid (CSF) is the most common form of metastatic spread of PNETs.
Epidemiology
Frequency
United States
Medulloblastoma represents the most common type of primary solid malignant brain tumor in children (as many as 30% of all solid brain tumors). In contrast, only 1% of brain tumors in adults are medulloblastomas. The overall annual incidence is approximately 0.5 case per 100,000 children. Seventy-five percent arise in the midline (vermis), while 25% occur in the lateral cerebellum.
International
The Swedish Cancer Registry reported, as part of a population-based study, that medulloblastomas represented 21% of all primary brain tumors in children. Similar figures were provided by the British Tumor Registry and from the United States (Surveillance, Epidemiology and End Results Program).
Mortality/Morbidity
Risk of sudden death secondary to obstructive hydrocephalus has been hypothesized; however, it is not often observed clinically.
Race-, sex-, and age-related characteristics
National Cancer Survey suggests a slightly higher incidence in white than in blacks.
A slight male preponderance is observed (male-to-female ratio 1.8:1).
Three fourths of these tumors appear in children younger than 15 years, and 50% are seen in the first decade of life. A second, smaller peak occurs in young adults (aged 21-40 y). [2] Children are 4.6 times as likely to be afflicted by a PNET than adults. [4] Cerebellar medullobastoma occurs rarely in the elderly. [5]
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Cerebellar medulloblastoma. This MRI (axial view, T2-weighted image) demonstrates the heterogeneity of the tumor.
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Cerebellar medulloblastoma. This sagittal view MRI without contrast demonstrates characteristic midline cerebellar location with mild obstructive hydrocephalus.
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Cerebellar medulloblastoma. This axial view CT scan with contrast shows a partially enhancing mass arising in the midline from cerebellum and filling the fourth ventricle.
