The answer is PET (positron emission tomography) scan. It lets doctors measure a variety of activity including blood flow, blood volume, oxygen usage, tissue pH (acidity), glucose (sugar) metabolism, and drug activity. PET is very beneficial in watching the activity of cancerous tumors. Because malignant cells grow at such a fast degree, they absorb more sugar than normal cells. Sugar can be added to a radioactive substance, and the PET scan can detect the areas of the brain through which the sugar flows. This way the doctor can give a preview into how aggressive a tumor is or how its growth is slowed by treatment remedies.