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Worldwide, all proton facilities currently planned or under construction will use scanning technology. Using this method, the beam scans the tumour in a grid-like fashion with the utmost precision, with up to 10,000 target points in the tumour.
The use of proton scanning technology means that proton therapy can now sterilise small areas anywhere in the body.
In practice this means that the limits of treatability are set by the accuracy and definition of current diagnostic methods, rather than the treatment system.
Radiotherapy uses beams of X-rays. These damage the DNA of the target tumour, but they also damage healthy tissue before and beyond it.
Various techniques have been developed in order to minimise this, including more accurate targeting, 'gating' to time the beam to meet the tumour during body movements and treating from many different angles, to spread the exposure.
The advantages of proton therapy
Serious and distressing side effects which are common in some conventional radiotherapy treatments are greatly reduced.
Proton therapy can treat tumours where X-rays cannot be used, because they would damage or destroy sensitive tissue beyond.
Unwanted radiation absorbed by healthy tissue around the target is only 1/3 to 1/5 that of X-ray therapy. This means that higher doses can safely be delivered to stubborn, deep-seated tumours - though ion beam therapy is now even better than this.
The number of days required for treatment can be much less (this varies with different cases and between centres).
The danger of the therapy itself causing cancers later in life is much less.
Particle beam therapy is treatment (usually of cancer tumours) by a stream of charged particles, rather than a beam of X-rays as in radiotherapy.
With proton therapy the charged particles are protons (hydrogen ions) whereas ion beam therapy uses heavier ions such as carbon instead of hydrogen.
New proton centres are being built in many countries but as yet there are still very few ion beam centres in operation outside Japan.
Why not use radiotherapy?
Treatments are usually given daily over several weeks and from many different angles, to allow healthy tissue time to recover between treatments, but unfortunately the tumour can also recover, which can reduce the chance of a cure.
However the same basic problem is inherent in all external X-ray based treatments.
Proton therapy has fewer side effects than radiotherapy and lower risk of inducing cancer later.
Proton beams behave very differently to the X-ray beams used in radiotherapy, as each proton beam releases the greater part of its energy at a precise depth.
This peak radiation dose is called the Bragg Peak and it is delivered within the target tumour. Beyond this point the energy of the beam falls away sharply to almost zero.
How is proton therapy any better?
The Y (vertical) axis on the left of the chart above can be thought of as the patient’s skin and the beige rectangle as a tumour inside the body.
The black line shows how an X-ray delivers a high dose initially, which then declines along its path, in order to deliver the required dose to the tumour. It then continues through the tissues beyond the tumour.
In contrast the dark blue line shows how a single proton beam delivers a much lower dose along its path, rising to a peak inside the tumour, where it then stops.
The turquoise line shows the cumulative impact of several proton beams. By adjusting the power of the beam, the peak falls at a different depth, so the total dosage can be sculpted in three dimensions to fit the shape of the tumour.
The areas shaded in gray, between the black and turquoise lines show the additional dose delivered to healthy tissue during radiotherapy, which can be avoided by proton therapy.