Radiation therapy works by causing mutations in the cell’s DNA. Fortunately, our normal cells are quite good at repairing damage to DNA while cancer cells are not good at this process. Treated cancer cells do not die immediately; the damage to their DNA causes the cell to die the next time it tries to divide. Therefore, it can be several weeks between treatment and when a response would be expected. If your physician is concerned about growth of disease or other symptoms, he/she will obtain imaging (if indicated) at that time to help to determine the best course of treatment or intervention.
The Oregon Clinic