Image-guided radiotherapy

In contrast to chemotherapy, radiation therapy is a means of treating cancer in a localised area. The treatment works by depositing energy or photons in the tissues thereby destroying the cancer cells. Ideally the maximum dose should be deposited in the cancer mass with minimal dose received by surrounding normal tissues. Advances in radiation therapy have been directed towards the goal of increasing the dose received by the tumour and minimising the dose received by the normal tissues.

Radiation therapy is most commonly delivered using a linear accelerator and directing the beam towards the tumour. However, movement of the patient both between each treatment delivered and during the treatment delivery, creates uncertainty regarding the position of the tumour and the surrounding critical organs. The patient’s position changes each day when lying on the treatment couch. This movement is minimised but not eliminated by the use of immobilising devices such as masks or casts and accurate positioning of the patient by the radiotherapist. Internal body motion caused by breathing, bowel movement, and bladder filling and emptying, changes the position of both the target and the structures we are trying to spare. These movements cannot be stopped but if they could be measured and tracked, it would be possible to improve treatment delivery.

Image guided radiotherapy or IGRT is a term used for technology that assists in tracking these movements in real time and thereby maximising the dose to the tumour and minimising the dose to critical, sensitive normal tissues.

The idea of visualising treatment fields during treatment is not new. When the treated area, or volume is on the surface, on the skin for example, it is easy to see the area that needs to be treated. But when the cancer is under the skin and is not visible, we need a method to visualise the area that should be treated.

In the seventies we used two-dimensional delineation of the treating volume. To ensure treatment accuracy X-rays were taken from above the patient and from the side. The volume of interest (treated volume) was in the middle. Most of the time the bony structures were used as guidelines for ensuring that the position was accurate. The field was often shaped using lead blocks to conform the treated volume. This method of treatment localisation was called simulation, and was done before the treatment started in the simulation room. Before starting the treatment the patient was positioned on the treating machine, in the same position as during the simulation, and again X-rays were taken. The patient had to wait on the treating bed, without moving until the pictures were checked and approved. If changes needed to be done, the whole procedure had to be repeated. These images are called “portal images.” This procedure was repeated once a week or every two weeks depending on the department policy to ensure that the position remained correct for each treatment.

In the 1980’s treatment planning programmes, software and treatment machines became more sophisticated and three-dimensional (3D) planning was introduced, together with more sophisticated portal imaging and verification techniques. The verification systems became part of the treating machine and fields could be visualised instantly without development of the films. This enabled correction of field placement instantly, before and even during treatment. The placement of radiopaque markers within the tumour makes this tracking easier.

As technology advanced more sophisticated and easier to interpret X-rays could be taken of the treatment position using CT scans, Kilovoltage X-rays and megavoltage imaging. These various methods become part of the treating machine, making it easier to track movement between and during treatments.

Modern radiation therapy utilises CT scans of the treatment area, which are transferred to a planning computer. The planning computer determines how the radiation fields should be placed in order to optimise the dose given to the treatment area. Once planning has been completed, the information is transferred to the treatment machine and the field placement needs to be verified on the patient. The CT scan or portal image taken in the treatment room is compared and superimposed on the CT taken for planning. If there is need for changes it is done immediately while the patient is lying on the bed. Internal body motion during treatment, such as what occurs during normal breathing, can also be tracked and the treatment can be switched on and off during the therapy, depending on real time position of the tumour and critical organs during therapy.

With these techniques the cancer can be better targeted, and the volume of normal tissue within the treatment field can be reduced safely without compromising the treatment of the tumour, thereby reducing the side-effects of treatment.

Most cancer should be treated with Image-guided Radio Therapy, especially head and neck, lung, breast, gastroesophageal, rectum, prostate and bladder.

This is a team effort and many specialities are involved, including the radiation oncologist, radiologist, planning and treating radiographers and physicist.

The benefit to the patient is that a smaller volume (smaller margins) can be treated more accurately, and more healthy tissue can be spared, reducing the acute and late side-effects of the treatment. The patient should discuss this with the treating radiation oncologist, as each case is different and unique.

Written bu Dr Edna Retter.

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