Primary Brain Tumors - Physician Information

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By definition, a primary brain tumor is a growth originating within the brain. The common clinical situation is an individual who presents with a lesion in the brain and has no known source of tumor elsewhere in the body; hence there is no reason to suspect spread of tumor to the brain from elsewhere. Therefore, in contrast to the situation with metastatic tumors, surgical verification of the identity of the tumor is almost always necessary. The exception to this rule is if imaging studies have such a typical appearance that their identity may be assumed with a very high probability.

The tumors which can frequently be identified with such high probability are meningiomas, acoustic neuromas, and pituitary adenomas. These tumors can be treated with gamma knife with little likelihood that a diagnosis based on imaging is in error. These tumors also share the characteristic that they usually appear on MRI scans as discrete masses arising outside the brain, rather than from the brain tissue itself. Surgery to reduce their size or remove them completely may still be advisable, depending upon the assessment of risks inherent to the procedure and the threat that presence of the tumor causes. Most of these tumors also share the characteristic that their growth is slow.

Although the gamma knife has been in use for over two decades, the ability to control these tumors over a span of many decades has not been determined. Consequently, gamma knife is more typically used in older patients and surgery is more frequently used in the young. The effect of the gamma knife upon slowly growing tumors is gradual, and tumors threatening vision or vital functions are generally treated surgically, with gamma knife reserved for adjuvant therapy if required. Studies suggest that the long-term control of these tumors by gamma knife therapy varies between 80 and 95 per cent, depending upon tumor size, type, and length of follow up of the study.

Tumors arising from within brain tissue are different in that usually it is not possible to remove them completely by operation. This is due in part to the lack of diagnostic imaging which can accurately show the margins of the tumor, and partially because the most common examples of such tumors - astrocytoma and glioblastoma - tend to infiltrate the surrounding tissue invisibly, sometimes spreading distantly along fiber tracts. Surgery is almost always necessary to diagnose these tumors accurately, and to reduce their size to render them more susceptible to adjuvant therapies such as gamma knife, fractionated teletherapy, or chemotherapy. The tissue obtained at operation allows the pathologist to determine how aggressively the tumor is likely to behave.

Tumors are graded by pathologists from I through IV (glioblastoma), in order of worsening aggressiveness. There is a tendency over time for low grade tumors to progress to a higher grade. Average survival decreases the higher the grade of the tumor. Many factors influence survival, including age, general health, which brain functions are impaired by the tumor, and response of the tumor to treatment - whether surgical, chemotherapeutic, or radiotherapeutic.

Adjuvant therapy is almost always advisable in grade III or IV tumors, sometimes in lower grade tumors. Because of the infiltrative nature of these tumors, the mainstay of radiation therapy is usually fractionated teletherapy delivered in small doses over several weeks with a linear accelerator.

Gamma Knife can augment this by boosting the radiation dose to a localized residual portion of tumor after operation or a localized recurrence. Because the effectiveness of any adjuvant therapy depends on the volume of tumor present, patients are usually followed at intervals of 2 to 3 months with MRI scans so that progression of tumor can be detected early. Studies are in progress to assess the exact contribution that gamma knife can make to survival, but experience with individual cases suggests that sometimes the contribution can be quite significant.

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