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Medulloblastoma

Medulloblastoma is a highly malignant primary brain tumor that originates in the cerebellum or posterior fossa.

Originally considered to be a glioma, medulloblastoma is now known to be of the family of cranial primitive neuroectodermal tumors (PNET).

Tumors that originate in the cerebellum are referred to as infratentorial because they occur below the tentorium, a thick membrane that separates the cerebral hemispheres of the brain from the cerebellum. Another term for medulloblastoma is infratentorial PNET. Medulloblastoma is the most common PNET originating in the brain.

All PNET tumors of the brain are invasive and rapidly growing tumors that, unlike most brain tumors, spread through the cerebrospinal fluid (CSF) and frequently metastasize to different locations in the brain and spine.

Incidence

Brain tumors are the second most common malignancy among children less than 20 years of age. Medulloblastoma is the most common malignant brain tumor, comprising 14.5% of newly diagnosed cases. In adults, medulloblastoma is rare, comprising less than 2% of CNS malignancies.

The incidence of childhood medulloblastoma is higher in males (62%) than females (38%). Medulloblastoma and other PNET tumors are more prevalent in younger children than older children. 40% of medulloblastoma patients are diagnosed before the age of 5, 31% are between the ages of 5 and 9, 18.3% are between the ages of 10 and 14, and 12.7% are between the ages of 15 and 19.

Pathogenesis

Medulloblastomas usually form in the fourth ventricle, between the brainstem and the cerebellum. Tumors with similar appearance and characteristics originate in other parts of the brain, but they are not identical to medulloblastoma.

Although it is thought that medulloblastomas originate from immature or embryonal cells at their earliest stage of development, the exact cell of origin, or "medulloblast" has yet to be identified.

It is currently thought that medulloblastoma arises from cerebellar "stem cells" that have been prevented from dividing and differentiating into their normal cell types. This accounts from the varying histologic variants seen on biopsy. Rosette formation is highly characteristic of medulloblastoma and is seen in up to half of the cases.

Molecular genetics reveal a loss of genetic information on the distal part of chromosome 17, distal to the p53 gene, possibly accounting for the neoplastic transformation of the undifferentiated cerebellar cells. Medulloblastomas are also seen in Gorlin syndrome as well as Turcot syndrome. Another research has strongly implicated the JC virus, the virus that causes multifocal leukoencephalopathy.

Clinical manifestation

Symptoms are mainly due to secondary increased intracranial pressure due to blockage of the fourth ventricle and are usually present for 1 to 5 months before diagnosis is made. The child typically becomes listless, with repeated episodes of vomiting, and a morning headache, which may lead to a misdiagnosis of gastrointestinal disease or migraine. Soon, the child will develop a stumbling gait, frequent falls, diplopia, papilledema, and sixth cranial nerve palsy. Positional dizziness and nystagmus are also frequent and facial sensory loss or motor weakness may be present. Decerebrate attacks appear late in the disease.

Extraneural metastases to the rest of the body is rare, but usually only after craniotomy.

Diagnosis

The tumor is distinctive on T1 and T2-weighted MRI with heterogeneous enhancement and typical location adjacent to and extension into the fourth ventricle.

Histologically, the tumor is solid, pink-gray in color, and is well circumscribed. The tumor is very cellular, many mitoses, little cytoplasm, and has the tendency to form clusters and rosettes.

Correct diagnosis of medulloblastoma may require ruling out atypical teratoid rhabdoid tumor (ATRT) and primitive neuroectodermal tumor (PNET).

Treatment and prognosis

Treatment begins with maximal resection of the tumor. The addition of radiation to the entire neuraxis and chemotherapy may increase the disease-free survival. This combination may permit a 5 year survival in more than 80% of cases. The presence of desmoplastic features such as connective tissue formation offers a better prognosis. Prognosis is worse if the child is less than 3 years old, there is an inadequate degree of resection, or if presence of any CSF, spinal, supratentorial or systemic spread.

Increased intracranial pressure may be controlled with corticosteroids or a ventriculoperitoneal shunt

Model

Using gene transfer of SV40 large T-antigen in neuronal precursor cells of rats, a brain tumor model was established. The PNETs were histologically indistinguashabe from the human counterparts and have been used to identify new genes involved in human brain tumor carcinogenesis. . The model was used to confirm p53 as one of the genes involved in human medulloblastomas, but since only about 10 % of the human tumors showed mutations in that gene, the model can be used to identify the other binding partners of SV40 Large T- antigen, other than p53.