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It has long been known that ionizing radiation can kill cancerous tumours, but it is less well known that Nature has given a helping hand by making cancerous cells repair less-readily than healthy cells after irradiation. Conventional, multi-session, X-ray therapy relies on these facts combined with an assiduous use of geometry and intensity modulation to concentrate the radiation inside the tumour. However, the physics properties of X-ray photons invariably lead to significant dose levels in the surrounding healthy tissue. A much-improved dose conformity can be obtained by using protons or light ions.
Protons and light ions (first proposed by an American, Robert R. Wilson in 1946) benefit from the so-called Bragg peak effect that makes it possible to control the depth to which the particles penetrate. As a result: - The healthy tissue behind the tumour remains almost entirely unexposed and the radiation exposure of the healthy tissue in front of the tumour is reduced.
- Due to this exceptional protection of the healthy tissue, ion therapy can apply higher doses than photon therapy.
Radiation therapy in general also has the advantages that it is non-invasive and, in many cases, can be applied on an out-patient basis. 
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