Pathology

Causes and Treatment of Medulloblastoma



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Medulloblastoma is a type of brain tumor that almost always occurs in children below 15 years of age; and appears to affect boys more than girls. It is most commonly seen in kids between 5 and 6 years old and about 20 percent of cases occur in babies below two years. Medulloblastomas comprise an estimated 20 percent of all CNS cancers in children.

Medulloblastoma occurs in the cerebellum, the back portion of the brain responsible for several functions, including balance, walking control and fine motor coordination. The term medulloblastoma is coined from thee smaller words: medulla (Latin for marrow, which means inner substance or core); blastos (Greek for germ, which means young, primitive and not completely developed; and oma (Greek word for tumor). By definition, therefore, a medulloblastoma is a tumor of primitive, undeveloped cells found inside the cerebellum.

As with majority of brain tumors, there are no clear or definitive causes identified for medulloblastoma. Researchers are yet to discover possible causes of this type of brain tumor which does not seem to have a direct hereditary link.

However, an association is somewhat shown between medulloblastoma and certain chromosomal abnormalities that most likely occur during a child’s development. This development does not only cover the postnatal period, but also before the baby is born, during its embryonic or fetal development inside the womb. Although still being validated, exposure of the fetus to certain causative viruses or environmental agents is one probable cause that has been suggested.

Children with medulloblastoma commonly manifest frequent, severe vomiting and other symptoms indicative of cerebellar dysfunction. Proper medical evaluation is necessary to rule out other possible disorders and to establish a diagnosis of medulloblastoma.

Once a medulloblastoma diagnosis has been established, accurate staging of the disease must be done to determine the most effective treatment and to predict outcome. Treatment modalities for medulloblastoma include:

Surgery

Although surgery is not considered as the sole treatment component anymore, it remains the typical first component of therapy for medulloblastoma. It has been shown that surgery alone does not cure this tumor; but it is highly important to perform a surgery that’s as maximal and complete as possible, with the aim is of removing all visible tumor while sparing as much surrounding tissue as can be managed.

After surgery, an MRI is performed to scan the brain for any sign of residual tumor. There are several types of surgery depending on the results of the MRI. A surgery can be called gross total resection if no evidence of any residual tumor is seen either during the operation or on post-surgery MRI. A surgery is considered near-total resection if over 90 percent of the original tumor is removed; while it is categorized as subtotal resection if 51-90 percent of tumor is removed. Partial resection refers to 10-50 percent tumor removal while a 10-percent removal is essentially considered just a biopsy, or sampling of tumor tissue.

Although considered ideal, gross total resection is sometimes not possible because the tumor may be invasive. All the same, the overall goal of surgery is to remove as much of the tumor as possible while avoiding severe brain deficits

Radiation or radiotherapy

External radiation to the entire CNS is recommended after surgery to remove medulloblastoma. Otherwise called craniospinal irradiation (CSI), this radiotherapy helps prevent tumor recurrence (return) in this area. Low-dose radiation to the brain and spine is highly essential to prevent the tumor from returning within the CNS.

The possible long-term complications of radiation therapy to the CNS of a growing child have understandably received considerable attention. The occurrence of such side effects is dependent on certain factors, including extent of pre-radiation surgery, amount and location of brain tissue that is radiated, as well as the dose of radiation administered.

But the potential long-term side effects of radiotherapy can be minimized with proper planning and delivery of optimal radiation dose.

Proton beam radiotherapy

This is a special type of particle radiotherapy which employs a beam of protons to irradiate tumor tissue. The main advantage of proton beam radiotherapy is its ability to confine the radiation dose more precisely compared with other forms of external beam radiotherapy. As a result, this type of radiotherapy can prevent delivering radiation dose to normal tissues for the cranial boost, particularly for sensitive structures like the middle ear.

Chemotherapy

At present, chemotherapy mainly plays a supplementary role in the treatment of medulloblastoma. As such, it is used after surgery and radiotherapy to help improve long-term disease-free survival. Results from clinical trials indicate an 85-percent disease-free survival at 5 years using cisplatin, vincristine, and CCNU, all of which can cause side effects. Nevertheless, the current standard of care involves the use of chemotherapy after surgery, CSI and boost radiotherapy. Cholesterol-lowering medications (statins) and cyclopamine (an inhibitor of the hedgehog signaling pathway) are other agents being investigated in the laboratory for the treatment of medulloblastomas. These less toxic agents may carry some promise in the treatment of this tumor.

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