Radiotherapy is a treatment for cancer that uses radiation to kill tumour cells. There are many different ways that the radiation can be produced and targeted at a tumour and your doctor will decide which of the available methods is best for you based on the available clinical evidence and your particular circumstances. Some radiotherapy is administered by implanting short-ranged radioactive sources into the body – this is known as brachytherapy. The alternative is external beam radiotherapy, in which a focused beam of radiation is produced by a machine and targeted at the tumour.

This page explains how most external beam radiotherapy takes place, but the process can vary between hospitals and is different for some cancers (in particular for many tumours of the eye and skin).

In external beam radiotherapy, the patient lies on a movable couch which the radiation-producing treatment machine (usually a linear accelerator, also known as a linac) can rotate around. This means that the radiation can approach the tumour from the best angle. The radiation given at a particular angle is referred to as a field. It is important that the beam is aimed directly at the tumour, so patients must stay very still and sometimes they are helped by a moulded face mask or other immobilisation device. Radiation therapy is normally given in a number of daily sessions (or fractions) as this is known to cause less damage to healthy tissue.

Doctors and medical physicists work together to produce an individual treatment plan for every patient using 3D images, such as CT and MRI scans, which are taken before the radiotherapy schedule commences. A newer technology known as IGRT (Image Guided Radiation Therapy) also takes these images regularly during treatment. This technology can work well for tumours that move or shrink as the treatment goes on – if needed, doctors can change your position during treatment or adjust the radiotherapy dose.

More info: Cancer Help on IGRT

For most tumours inside the body, the radiation used in conventional radiotherapy consists of x-rays (sometimes known as high-energy photons). In order to effectively treat cancer these x-rays have more energy than those used in imaging and much more radiation is needed. The radiation dose outside the tumour is limited by delivering radiation at different angles and, for each of these directions, the beam of radiation is shaped to match the cross-section of the target. These beams are described as conforming to the tumour, and so this treatment type is often referred to as 3D conformal radiotherapy.

Conventional radiotherapy provides high doses of radiation to tumour sites, but it can sometimes damage healthy tissue near the tumour. If the cancer to be treated is close to an important organ, doctors will try and reduce the amount of radiation that is received outside of the tumour and there are a number of technologies to help them do so. These are described below.

Stereotactic radiotherapy gives radiotherapy in fewer sessions, using smaller radiation fields (narrower beams) and higher doses than 3D conformal radiotherapy. It is administered using a small linear accelerator machine on a robotic arm, which may move around the couch. The couch can also change position.

This way of giving radiotherapy allows beams to be directed from more angles than standard radiotherapy and so the treatment can target the tumour very precisely. It is used for small, well defined tumours that are deep within the body and likely to move, for example due to breathing. It is most commonly used for tumours in the lung, liver or pancreas but can also be used for some types of brain tumour.

Doctors often refer to this treatment using the name of one of the machines used to give it - the CyberKnife.

More info: Cancer Help on CyberKnife

IMRT (Intensity Modulated Radiation Therapy) can deliver larger doses of radiation to the tumour site while reducing the amount delivered to surrounding normal tissue. This limits the potential damage to the healthy tissues and so may result in fewer side effects.

In IMRT the radiation beam is shaped to the cross section of the target using hundreds of tiny devices called multi-leaf collimators. This can be the case in conventional therapy too, but in IMRT these collimators move during the treatment varying the intensity of the radiation within each field. For example, if two leaves gradually close during treatment, they will let less radiation through at the edge than the middle. Using this technique, the machine can give very precise doses to a cancer or to specific areas within the tumour.

Currently IMRT is being used to treat some cancers of the prostate, head and neck, breast, thyroid and lung, as well as some women's cancers, liver tumours, brain tumours, lymphomas, children's cancers and sarcomas.

Combining IMRT and IGRT radiotherapy has been shown to optimize the effects of radiotherapy. Using the combined precision of the two techniques, the tumour receives the required dose whilst limiting the severity of side effects caused by unwanted irradiation of surrounding normal tissue. This system of radiation therapy is recognized worldwide as the state of the art x-ray therapy for a wide range of cancers.

More info: Radiology Info on IMRT

Proton beam therapy is a newer technology for treating cancer with radiation. It uses a different type of radiation to the above techniques – protons, which are accelerated in a machine called a cyclotron or synchrotron. Protons are different to x-rays because they have an electric charge, which causes them to slow down as they travel through the body. As the protons slow down, they deposit more and more energy until they eventually run out of energy and stop. The overall result is that protons deposit much of their radiation dose at the target depth, a reduced dose along the path they took to the target and no dose at all beyond the target. The target depth can be tuned by varying the energy of the protons leaving the accelerator. By leveraging the favourable dose-depth characteristics of protons, treatment plans can be designed which irradiate healthy tissues to lower doses for the same tumour doses as x-ray treatments. As a result, side effects may be lower with proton therapy and it is possible to treat some cancers for which conventional radiotherapy may not be an option.

There is currently only one proton beam radiotherapy machine in the UK and it is only able to treat cancer of the eye. Higher energy machines are necessary to be able to treat cancers deeper in the body. Other countries, including many in Europe, are testing and using proton beam radiation for deeper cancers, including a type of spinal cord tumour called chordoma. If proton therapy is considered necessary, the NHS will pay to send British patients for treatment abroad – usually to Switzerland or France, but sometimes to the USA.

More info: Cancer Help on Proton Therapy for Chordoma
Introduction to charged particle therapy from the PTCRi, University of Oxford