This information is for men who have early prostate cancer. It tells you about radiotherapy, a treatment used for prostate cancer that has not been detected outside the prostate gland. It is based on the best and most up-to-date research.
Does it work?
We don't know for certain. Having radiotherapy may help, but it hasn't been studied thoroughly.
We do know that between 5 in 10 and 9 in 10 men who have radiotherapy for early cancer survive their disease for at least 10 years. And around half of these men don't get any symptoms during this time. But we don't know whether the radiotherapy helped these men to live longer. They may have lived just as long without the treatment. 
We don't know how radiotherapy compares with active surveillance or surgery to remove the prostate. There haven't been any good-quality studies comparing these treatments.
A newer radiotherapy, called conformal radiotherapy, may have fewer side effects than the standard external beam radiotherapy.
The information here is for men whose cancer is at an early stage and has not been detected outside their prostate gland. Some doctors call this stage clinically localised cancer and classify the cancer as T1 or T2. For more information about how doctors classify prostate cancer, see How far has your cancer spread? The TNM system.
What is it?
Radiotherapy is a treatment that uses a machine positioned above your body to direct X-rays (a form of radiation) through your abdomen and into the cells in your prostate. This treatment is also called external beam radiotherapy.
Radiation is a source of energy that is transmitted in the form of either waves or particles. X-rays are a form of very high-energy radiation that can penetrate and destroy cells.  The aim of the treatment is to kill the cancer cells in the prostate so that the tumour can't grow and spread to other parts of your body. If the cancer spreads, you could die from prostate cancer. You will need to have radiotherapy five days a week for four to eight weeks.
It takes only a few minutes to have treatment with radiotherapy. Your doctor will carefully calculate the dose of radiation you need at each session.
The radiation is targeted at your whole prostate, because cancer cells are often scattered throughout it. Radiation may also be targeted at your seminal vesicles, just in case the cancer has spread there. Like the prostate, the seminal vesicles help produce the fluid that comes out with your sperm when you ejaculate.
As an extra form of insurance, some doctors may also aim the radiation at the lymph nodes near your prostate. Lymph nodes are small lumps made up of special cells that help protect your body against infection. Tiny tubes, called lymph vessels, drain fluid from the prostate and carry it to the nodes. This fluid contains waste material from the prostate cells. If there are any cancer cells in the fluid, they will also be carried to your lymph nodes. From here, the cancer cells may spread to other parts of your body.