Radiotherapy applies ionizing irradiation to selected areas of the body with the scope of destroying cancer cells, either as part of curative therapies to remove a primary malignant tumor and to prevent tumor recurrence after surgery, or as part of palliative measures to avoid local advancement of bone and brain metastases. Intriguingly, radiotherapy does not only have local effects but may lead to the delayed regression of distant non-irradiated lesions. Most likely, these "abscopal" effects are mediated by the immune system.