Radiotherapy (or radiation therapy) is a treatment delivered from a machine with a beam aimed at the tumour. For testis cancer, radiotherapy is used occasionally to treat less advanced cases of seminoma, aimed at the lymph glands, after the testis containing the cancer has already been removed. Even though the lymph glands in the area appear normal on the CT scan, some cancer cells may already have landed there and your cancer doctor may decide to treat the area with radiotherapy. Your doctor will likely treat smaller seminoma tumour masses in the midand lower abdomen with radiotherapy. (Larger seminoma tumour masses are better treated with chemotherapy).
The radiation causes damage to the cells in the path of the beam. Both cancer cells as well as normal cells are affected and so you have side effects.