by Steven Seung, M.D., Ph.D.
Primary malignant brain tumors are rare, occurring in about eight per 100,000 people per year and representing approximately two percent of all cancers diagnosed in the United States. A brain tumor is called malignant if it invades surrounding tissue, if it is in a critical area of the brain, or if it is life-threatening. Primary malignant brain tumors include: glioblastomas, astrocytomas, oligodendrogliomas and ependymomas.
Brain tumors are usually caused by mutations in the genetic structure (DNA). Mutated or missing genes result in abnormal cells. If abnormal cells have malignant potential, they will form a tumor when they multiply. There are many environmental and genetic factors that can cause brain tumors. However, in most cases, we just don't know what causes a brain tumor.
Tumors are graded based on their appearance under a microscope. This grade indicates the level of malignancy. Tumors are graded on their mitotic index (growth rate), vascularity (blood supply), presence of a necrotic (dead) center, invasive potential (border distinctness) and similarity to normal cells. In the World Health Organization grading system, tumor cells are given grades 1 to 4. Grade 1 tumors grow very slowly with good prognosis, and grade 4 tumors are the most malignant and invade wide areas of surrounding normal tissue. These tumors reproduce rapidly, appear very unusual under a microscope and are necrotic (have dead cells) in the center. Glioblastoma multiforme is the most common grade 4 tumor.
Surgery
Most tumors should be surgically removed whenever possible. Even partial surgical removal may alleviate symptoms and facilitate treatment of the rest of the tumor. A tumor may not be surgically removed if it is in an inaccessible location or too near to critical structures; the danger is that the removal of the tumor might cause further damage to other areas of the brain. Also, surgery may not be performed if the patient is not healthy enough to tolerate the surgery.
Radiotherapy
Radiotherapy is commonly used to treat brain tumors. With radiotherapy, the treatment is given over time to allow the normal cells an opportunity to heal between treatments. Radiotherapy may be used to kill remaining tumor cells after a surgical resection, or as a primary treatment for tumors that are very responsive to radiation. It may also be used for larger tumors for which radiosurgery is not an option. For grade 3 and 4 tumors, chemotherapy may be added to the radiotherapy to improve response rates and survival.
Stereotactic Radiosurgery
Radiosurgery may be the primary treatment for low grade 1 and 2 tumors if the patient is not highly symptomatic and does not require open-skull surgery. It is sometimes used as an immediate adjunct to standard radiotherapy for grade 3 and 4 tumors, but the benefit in survival by this strategy has not been proven. The current standard of care for grade 3 and 4 tumors is to reserve radiosurgery for situations in which conventional radiotherapy fails.
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