How are tumors graded?
The generally accepted scale for grading CNS tumors was approved by the World Health Organization in 1993. Grading is based on the tumor’s cellular makeup and location. Tumors may also be classified as low-grade (slowly growing) or high-grade (rapidly growing). Some tumors change grades as they progress, usually to a higher grade, and can become a different type of tumor. The tumor is graded by a pathologist following a biopsy or during surgery. Grade I tumors grow slowly and generally do not spread to other parts of the brain. It is often possible to surgically remove an entire grade I benign tumor, but this type of tumor may be monitored periodically, without further treatment. Grade II tumors also grow slowly, sometimes into surrounding tissue, and can become a higher-grade tumor. Treatment varies according to tumor location and may require chemotherapy, radiation, or surgery followed by close observation. Grade III tumors are malignant and can spread quickly into other CNS tissue.
Typically, primary brain tumors are graded based on how aggressive they appear under the microscope. Signs of aggressiveness include an increase in the number of cells in the tissue (“hypercellularity”), abnormal cell shape (“pleomorphism”), evidence of blood vessel growth (“angiogenesis”), and cell death (“necrosis”). The more of these features that are present, the more aggressive a tumor is likely to behave and the higher the grade. It should be kept in mind that many primary brain tumors have some regions that have only a few of these features, while other regions have more. Neuropathologists typically grade brain tumors by their most aggressive-looking regions, since this predicts likely tumor behavior.
