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Radiotherapy Treatment Machines

These photos show the machine most commonly used to deliver external beam radiotherapy treatment. This machine is called a linear accelerator, or linac for short. All linacs generate high energy x-rays (photons) which are then carefully aimed at the area the Consultant Oncologist wishes to treat. They can be used to treat all areas of the body from head to toe.

More advanced linacs have the capability to deliver another type of treatment, that is used to treat areas that are on, or close to the skin's surface. This treatment uses
electrons instead of high energy x-rays (photons)

The newest linacs also have a capability to treat using
Intensity modulated radiation therapy (IMRT). These linacs have Multi leaf collimators (MLC's) that are used to alter the shape of the beam. Without these, the machine can only treat square or rectangular shapes (treatment fields) without having to attach blocks of lead to the machine.

All linacs have some method that radiographers can use to ensure they are treating in the correct place. The basic form of this is a plain x-ray ( Radiograph ) Most modern machines take a digital image using the bottom arm you can see on this image. This image is called an EPI ( Electronic portal image) or PI (portal image). These images are checked against those generated during your radiotherapy planning, by the radiographers, before they deliver any treatment (verification). The number and frequency of images that are taken depends on each department's imaging protocols.

Some radiotherapy treatment machines also have an 'On Board Imager' (OBI) It consists of a Kv x-ray unit (arm to the left of the linac) and a detector (arm to the right of the linac). This system gives higher quality verification images and allows for another radiotherapy technique called image guided radiotherapy (IGRT). IGRT aims to further increase the accuracy of radiotherapy treatment, by accounting for daily changes, such as that of organ motion which in turn helps to reduce some of the associated treatment side effects.

How they work in a nut shell...

The linac generates electrons and then speeds them up to almost the speed of light using electrical fields. As the electron is accelerated its kinetic energy is increased (given by the equation KE=1/2mv2) until such time as it collides with the target and the energy is released as a photon (Bremsstrahlung radiation). This is a simplified version of events and we have left details of 'shell binding energies' and 'excitement' out of the discussion!!

When these high energy photons enter the patient's body, they aim to
break the DNA in all the cells within the treatment area. The good cells are able to mend themselves, but the 'bad cells' (Cancerous) are unable to do this and therefore die.

( wishes to thank Christopher Horn at Elekta for the technical details - Thanks Chris!)

Reviewed: March 2015