Content Summary : Overview of Tumor Modeling A brain tumor is a dynamic system in which bad cells grow and spread, eventually overwhelming good cells in the brain. Where in the brain they start, how quickly they grow, andhow they spread will all affect how quickly the cancer spreads. Additional factors of importinclude the number of cells in the tumor at a given point in time and the kill rate of particulartreatments (either singularly or combined). A brain tumor has been compared to a forest fire,because it spreads along the outer perimeter and often dies out in the center due to a lack of fuel(or, in the case of a tumor, oxygen and nutrients from the blood). Thus, tumor treatments mustalso be able to move in the brain more quickly than the tumor spreads if the treatment is toeffectively destroy the tumor entirely. Some basics of cancer and the terminology that follows will be helpful. First, brain cancer cells grow extremely fast. Second, at any point in time, only a portion of them arereplicating, and many cancer treatments only kill cells during this active phase. Models mustadjust for this constraint in determining the net tumor cell kill rates (the Wein paper does). Third, a small fraction of tumor cells (about one in a thousand) -- called clonogenic cells -- arecapable of regrowing the entire tumor. All of the clonogenic cells must be killed if the tumor isnot to grow back after treatment. Because a tumor such as gbm has so many billions of cells, nosingle treatment available is capable of such a high kill rate. Table 1 in Professor Wein's analysisdemonstrates this problem. Finally, the ability of a treatment to seek out tumor cells rather thanhealthy cells, and to move through the brain to reach the outer perimeter of the tumor, all affecthow well a treatment works.