The prostate gland lies up against both the bladder and the rectum (the lower part of the bowel). Normal external beam radiotherapy will always include some bladder and bowel tissue in the treatment and this can lead to side effects such as cystitis (inflammation of the bladder) and diarrhoea (due to inflammation of the bowel).

Conformal radiotherapy uses the same radiotherapy machine as the normal treatment (this is called a Linear Accelerator) but uses metal blocks placed in the path of the x-ray beam to alter the shape of the beam. This means that the metal blocks can be carefully planned for each patient to shape the treated area more precisely and so reduce the amount of bowel and bladder tissue that is irradiated.

More recently a device called a multi-leaf collimator has been produced as an alternative to the metal blocks. This consists of a number of layers of metal sheets, which are fixed to the Linear Accelerator. Each layer can be adjusted to a different position and so alters the shape and intensity of the beam of x-rays reaching the patient. This means the settings of the multi-leaf collimator can simply be changed for each patient's treatment and there is no need to make special metal blocks.

A further development has been the ability to move the layers of the multi-leaf collimator whilst the actual treatment is in progress. It has been argued that this can give even more precise shaping of the treatment areas to the contours of prostate gland (or any other tumour that is to be treated). This method of moving the collimator during the radiotherapy treatment is called intensity modulated therapy. So intensity modulated therapy is a form of conformal radiotherapy.

There is evidence that conformal treatments reduce the number of side effects. NICE (the National Institute for Clinical Excellence) produced a manual in September 2002 ('Improving outcomes in urological cancers') which recommends conformal radiotherapy as the best type of external radiotherapy for prostate cancer. At present it is not known how intensity modulated therapy compares with standard conformal treatment.