When patients have radiation therapy for tumor in the hospital, it is usually performed from Monday to Friday only, with Saturday and Sunday off. Patients and their families may have doubts in their mind: Is this intentional because the medical staff needs to rest? Actually no, there is a scientific reason for this arrangement. This method was first proposed by Coutard, a Frenchman, in 1934. After decades of research, the rationality of this method is now considered as follows: (1) There are oxygenated cells and oxygen-depleted cells in tumor cells, and only oxygenated cells are sensitive to radiation, while oxygen-depleted cells are not sensitive to radiation (radiotherapy is ineffective). During fractionated radiotherapy (rest for several hours), the oxygen-depleted tumor cells become oxygenated cells. (2) Both tumor cells and normal cells have the characteristics of cell cycle. The clinical method of irradiating tumor once a day with radiotherapy aims to keep the tumor cells in the sensitive cycle phase under constant radiation strikes, and eventually the tumor is eliminated. (3) This method can make the normal cells repair their damage and re-proliferate, thus protecting the normal tissues; tumor cells have poorer damage repair ability than normal cells, thus accelerating the death of tumor cells. (4) The method achieves a better balance between tumor killing and normal tissue damage. (5) Decades of clinical practice have proved that the method has a better therapeutic effect. (6) It is convenient for the staff engaged in tumor radiotherapy.