How proton beams affect tumor cells. Ionization or excitation occurs along a proton path in tissue in a body. What occurs in a cell is schematically shown in Figure 2. Ionization or excitation in a cell produces chemically active radicals which damage DNA in a chromosome.
Cells have the capability to recovery from such damage to some extent.
However, when the damage exceeds a certain level, cells no longer proliferate. This is the reason why proton beams are used for cancer therapy. If recovery of normal tissue is greater than that of the tumor, fractionating irradiation can eliminate tumor cells with minimum damage to normal tissue surrounding the tumor. Fractionation is widely adopted not only for proton beams but also for conventional radiotherapy. Usually, the size of the fractionation is about 20.