Radiotherapy or radiation therapy is used to treat cancer by sending high-intensity radio waves to kill cancer cells. Unfortunately, it kills healthy cells as well and therefore causes untoward side effects.
One article (1) discussed radiation therapy and its side effects. It noted that, since radiation therapy offers a localized effect on the tissue, most of the side effects are related to the local effects radiation has on the tissues near the tumor.
Common side effects of radiotherapy include skin problems (dryness, peeling, blistering, or itching of the skin), tiredness (feeling exhausted nearly all the time despite rest), and some long-term side effects. The major long term side effect is the development of a secondary cancer near the site of where the radiation was previously given. If radiotherapy is given to the chest, it can cause scar tissue to form in the lung tissue.
Source: 1.Radiation Therapy Effects. http://www.cancer.org/treatment/treatmentsandsideeffects/physicalsideeffects/radiationtherapyeffects/radiation-therapy-effects-landing. Accessed 8/26/16.