Paris, January 18, 2006
Proton therapy is an effective treatment against cancers located in areas which are inaccessible to the surgeon's instruments or which are hard to treat by radiotherapy, since the X-rays would damage the tissues that they travel through before reaching the tumor. This is the case for tumors of the brain, in areas close to the spinal cord, or inside the eye. Unlike X-rays, proton beams mainly release their energy when they reach their target (they do not damage the tissues they pass through). Moreover, they make it possible to target a tumor to within a millimeter.
Today, there are only two French proton therapy centers, one in Orsay and one in Nice (the latter being restricted to eye treatment). They use conventional accelerators (cyclotrons), in which a combination of magnetic and electrical fields accelerates protons up to the energies required for medical applications (60 MeV at Nice and 300 MeV at Orsay). The Orsay proton therapy center, a department of the Institut Curie, is currently initiating an expansion and modernization plan which in 2009 will give it the capacity to treat 650 patients per year. A third center, which will use carbon ions as well as protons, should be in service in
Researchers from CNRS and the CEA used an alternative technique to produce protons which can be used in proton therapy. This uses a high-intensity pulsed laser focused on a metal target. The laser is powerful enough to cause protons located at the back of the target to be torn off (1). This technique has a certain number of advantages. Firstly,it makes it possible to build compact accelerators, since it amounts to producing a microscopic linear accelerator: the protons, which are initially at rest, acquire an energy of several tens of MeV after travelling a mere 10 micrometers. Including the support systems, a proton-producing facility would thus fit inside a room (as opposed to an entire building for a cyclotron) and could be installed in a hospital. By using a set of mirrors, the laser beam could easily be “conveyed” to the patient, whereas cyclotrons require heavy equipment to convey high-energy protons over a distance of several tens of meters. Secondly, this technique would be able to substantially lower the overall cost of proton therapy facilities, by reducing the cost not only of the proton source (the research facility being built at LULI costs only a few million euros) but also of the infrastructure.
However, the protons produced in this way are not yet energetic enough: they attain 60 MeV at most, which is the minimum required for medical applications in the treatment of the eye. Other, denser, tissues require more energy (from 250 to 300 MeV). In addition, the protons are not yet produced at a high enough rate. The article published in Nature Physics is based on a large number of both theoretical and experimental results which relate the properties of the accelerated protons to the characteristics of the target and of the laser pulse. With the help of this model, the researchers have defined the parameters of the ideal laser for proton therapy, ie a pulsed laser whose pulses have an energy of around 100 joules (with ultra-short pulses) and a duration of a fraction of a picosecond, and a repetition rate of 10 Hz. A laser of this type, which is extremely powerful and able to fire at a high rate, represents a major leap forward in technology compared to current lasers. The researchers at LULI and LOA are in the process of building it (2) and expect to have completed it within two years.
1. At the Laboratoire pour l'Utilisation des Lasers Intenses (Laboratory for the Use of Intense Lasers)(LULI, UMR CNRS/CEA/Ecole Polytechnique/Paris 6), Laboratoire d'Optique Appliquée (Applied Optics Laboratory)(LOA, UMR CNRS/ENSTA/Ecole Polytechnique) and Centre de Physique Théorique (Center for Theoretical Physics) CPhT, UMR CNRS/Ecole Polytechnique)
Laser-driven proton scaling laws and new paths towards energy increase, J. Fuchs, P. Antici, E. d'Humières, E. Lefebvre, M. Borghesi, E. Brambrink, C. A. Cecchetti, M. Kaluza, V. Malka, M. Manclossi, S. Meyroneinc, P. Mora, J. Schreiber, T. Toncian, H. Pépin, P. Audebert, Nature Physics, volume 2, issue 1, pp 48-54 (2006)
2. At the Département de Physique Théorique et Appliquée de la Direction des Applications Militaires (Department of Theoretical and Applied Physics at the Military Applications Directorate), in Bruyères-le-Châtel
3. The laser ionizes the atoms in the target and pushes the electrons towards the back surface. Here, the electrons produce an electric field which ionizes and accelerates the ions located on this surface. Among these ions there are protons derived from a layer of plastic deposited on the back of the target (or simply from water vapor remaining on the target in the vacuum chamber). The protons are then separated from the heavier ions, and subsequently refocused using another innovative device: a cylindrical “micro-lens” (a recent discovery made by the same research group), whose working is also based on laser-matter interaction.
4. In this new laser, the photons are produced by amplifiers which are stimulated by laser diodes rather than by the traditional flash lamps, which solves both the problem of the energy efficiency of the device and that of the heating of the materials which amplify the laser (a limiting factor for the pulse repetition rate).
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