Radiation therapy is the medical use of ionizing radiation to treat cancer. In conventional radiation therapy, beams of X rays (high energy photons) are produced by accelerated electrons and then delivered to the patient to destroy tumour cells. Using crossing beams from many angles, radiation oncologists irradiate the tumour target while trying to spare the surrounding normal tissues. Inevitably some radiation dose is always deposited in the healthy tissues.

When the irradiating beams are made of charged particles (protons and other ions, such as carbon), radiation therapy is called hadrontherapy. The strength of hadrontherapy lies in the unique physical and radiobiological properties of these particles; they can penetrate the tissues with little diffusion and deposit the maximum energy just before stopping. This allows a precise definition of the specific region to be irradiated. The peaked shape of the hadron energy deposition is called Bragg peak and has become the symbol of hadrontherapy. With the use of hadrons the tumour can be irradiated while the damage to healthy tissues is less than with X-rays.

The idea of using protons for cancer treatment was first proposed in 1946 by the physicist Robert Wilson, who later became the founder and first director of the Fermi National Accelerator Laboratory (Fermilab) near Chicago. The first patients were treated in the 1950s in nuclear physics research facilities by means of non-dedicated accelerators. Initially, the clinical applications were limited to few parts of the body, as accelerators were not powerful enough to allow protons to penetrate deep in the tissues.

In the late 1970s improvements in accelerator technology, coupled with advances in medical imaging and computing, made proton therapy a viable option for routine medical applications. However, it has only been since the beginning of the 1990s that proton facilities have been established in clinical settings, the first one being in Loma Linda, USA. Currrently about thirty proton centres are either in operation or in construction worldwide.

Although protons are used in several hospitals, the next step in radiation therapy is the use of carbon and other ions. These have some clear advantages even over protons in providing both a local control of very aggressive tumours and a lower acute or late toxicity, thus enhancing the quality of life during and after cancer treatment. Since the birth of hadrontherapy, more than 120,000 patients have been treated globally with hadrons, including 20,000 with carbon ions.

In Europe, the interest in hadrontherapy has been growing rapidly and the first dual ion (carbon and protons) clinical facility in Heidelberg, Germany started treating patients at the end of 2009. Three more such facilities are now in operation: CNAO in Pavia, MIT in Marburg, and MedAustron in Wiener Neustadt are treating patients.

Globally there is a huge momentum in particle therapy, especially treatment with protons. By 2020 it is expected there will be almost 100 centres around the world, with over 30 of these in Europe.