X-ray therapy and proton therapy are both forms of external beam radiotherapy designed to kill cancer cells. In both cases, the tumor is targeted with a radiation beam made of ionizing particles that damage the DNA of cells, causing their death or preventing them to proliferate.
Cancerous cells are particularly susceptible to radiant injury and attacks on DNA because of their abnormally high rate of mitosis (division): their fast growth makes them less effective than healthy cells to repair ionization damage.
On their way to the tumor, the beam also irradiates healthy tissues, which can induce fatigue, skin irritation or specific side effects related to the irradiated organs. Secondary cancers can also develop in the long run. In order to reduce these side effects and give normal cells time to heal, radiation treatments are typically given in small daily doses, five days a week, which is called fractionation. Even so, collateral damage still occurs. The ideal treatment would therefore focus on aiming the radiation dose only at cancer cells and avoiding normal healthy ones.