Laser-based accelerators

Towards compact proton therapy ?

Scientists at CNRS and CEA have defined the characteristics of future lasers that could be applied for cancer treatment by proton therapy. The first prototype is under construction and should be operational within four years.

 

Proton therapy is a very effective form of tumor treatment. Its precision–to the millimeter–makes it possible to irradiate a tumor while leaving the surrounding healthy tissues intact. This also makes treatment of difficult areas such as the eye, brain, and spinal cord, possible. Protons are energized to specific velocities, which determine how deeply in the body they will deposit their energy. A proton beam has a low entrance dose, and releases the major part of its energy within the cancerous tumor, causing maximum damage at the end of its range. Hence, proton beams have the added benefit of producing no dose beyond the tumor, unlike conventional radiation therapy. This makes it possible to target the tumor with a higher dose of radiation, while reducing the harmful aspects of conventional radiotherapy.

Specialized proton therapy centers throughout the world use particle accelerators (mainly cyclotrons and synchrotrons) in which a combination of very large electromagnets and electrical fields accelerate protons to the required energies. Due to the size of the accelerator and of the beam transport lines, an entire building (the size of a football field) is needed to house this equipment, which can weigh up to 900 tons. Only about 40 facilities worldwide have the means to deliver this sort of therapy, making their current capacity vastly outnumbered by the number of patients seeking treatment.

 

In 2000, several research groups discovered that by using a high-intensity pulsed laser irradiating a thin metallic foil, energetic ion beams could be accelerated to high energies (several tens of MeV) within a distance of just 10 micrometers. Using compact lasers could therefore greatly reduce the size of the overall accelerator. Moreover, by using a set of mirrors, the laser beam could be relayed much closer to the patient, greatly reducing the need for large and costly beam transport lines and gantry. These new methods would greatly reduce both the size and cost of the facility (current infrastructures cost more than a €100 million each), making the treatment much more accessible to patients.

However, major obstacles remain. At present, protons produced by these new microscopic linear accelerators are not energetic enough. At best they reach 60 MeV, the workable minimum for eye treatment. Denser tissues and deeper zones require more energy, between 200 and 300 MeV. Other impediments of laser-accelerated beams are their broad energy spectrum and divergence at the source. Recently, scientists at CNRS and CEA have not only defined what they consider to be ideal parameters to produce high-energy ions,¹ but they have also been able, with teams from Düsseldorf and Belfast, to line up and energy select these proton beams.²

 

The scientists at LULI³ are now building a new laser, the LUCIA project,⁴ a first step towards cost-effective and compact production of high levels of laser energy with the necessary increase of repetition rate and efficiency. It is expected to be completed within four years. Photons will be produced by amplifiers pumped by laser diodes, rather than flash lamps. This will considerably increase energy conversion efficiency, as well as reduce the thermal load of the amplifiers and improve the laser beam quality. Other projects, such as the European PROPULSE project, coordinated by the Applied Optics Laboratory,⁵ are pursuing similar goals. “A laser that can generate the required ion energies for deep-seated tumors while fitting into a CT room could be developed within 10 years, and may ultimately be priced at around €10 million,” explains Julien Fuchs, one of the leading physicists working on the project. “If beam stability and shaping requirements for therapy are met, these smaller and cheaper facilities could equip all major hospitals.” But Fuchs goes further, “in addition to medical applications, laser-based ion accelerators could be used for triggering fusion reactions, and in basic physics research.”

 

Marion Girault-Rime