In the human body, cells can grow and divide, which are parts of the normal cell cycle. DNA, the genetic material in the cell nucleus, directs and regulates this process. Occasionally, the DNA can be damaged by physical or chemical factors in the surrounding environment. At this situation, the cell responds by either repairing its DNA or initiate its own programmed death. However, the malfunction of this self-regulated mechanism fails to repair the damaged DNA, which remains in cell and causes the cell to grow and divide in an uncontrollable manner. Then, a group of this type of uncontrollable cells gathers together to form a tumor. 

To treat certain kinds of cancer, radiotherapy can be a helpful solution, since radiation can deal overwhelming damage to their DNA, and thus kill the cancer cells. Then, the tumor starts to shrink after the cells stop dividing and die. To achieve this goal, a high-energy wave, such as an X-ray beam, is generally delivered by a medical linear accelerator to the tumor tissue. Followed by the adjustment of the intensity and direction of the radiation beam by the computer so that it allows higher doses of radiation to treat the tumor while avoiding the healthy tissue around it. The doctor will keep scanning the three-dimensional shape of the tumor and make sure the effects of the radiotherapy.