Tumors Under Laser Fire

Much to the delight of oncologists, a highly energetic ion beam in an accurately defined dose that provides a pin-sharp (and cost-effective) radiation treatment of tumors has been experimentally demonstrated.

Physicists at the Munich Centre for Advanced Photonics (MAP), led by Dr. Dietrich Habs, professor at Ludwig Maximilian University, in cooperation with scientists at the Max Born Institute in Berlin, have published their results in the latest issue of Physical Review Letters. MAP is a German Research Foundation Cluster of Excellence.

According to the group, modern techniques based on intense laser pulses may in the future replace expensive conventional particle accelerators.

Carbon beams are considered to be the most effective method of cancer therapy, as tumors are destroyed permanently with minimum trauma. Conventional x-rays or electron beams, on the other hand, cause significant damage to the surrounding healthy tissue on their pathway into the body.

Like wind in a sail propelling a boat, the laser light pushes a nanometer-thin layer of electrons and ions forward. (Illustration: Andreas Henig)

Carbon beams have high biological effectiveness and can be precisely concentrated in the tumor so that they exclusively kill targeted cancer cells. This makes carbon ions an ideal tool for radiation therapy of tumors deeply situated in highly sensitive regions, such as the vicinity of the brain stem, where doctors would refuse even to contemplate surgical intervention.

Generating these beams is currently rather challenging, requiring state-of-the-art accelerator facilities that are extremely expensive to construct, operate and maintain. Unfortunately, the vast majority of today’s cancer patients are unable to benefit from this kind of treatment.

“As doctors, we are dependent on the physicists’ progress to develop a cheaper and more compact carbon beam source in order to make ion beam therapy available for everybody,” said Dr. Michael Molls, another MAP member, and a professor and director of the department of radiation oncology at Munich Technical University (TUM).

In recent years, there have been major advances in the generation of highly energetic ion beams based on compact lasers instead of large-scale accelerator facilities.

“The new technique allows an acceleration distance smaller than the diameter of a human hair,” explained Habs.

Dietrich Habs and Andreas Henig. (Photo: Thorsten Naeser)

Such small distances are sufficient to accelerate ions to high energies when highly intense laser pulses are employed. Not only the accelerator itself, but also the beam guide is being shrunken significantly, as the several tons of steering magnets can be replaced by small-sized mirrors. However, up to now no efficient method has been developed to transfer the same amount of energy from the laser to every single ion to allow for a well-defined penetration depth of the particle beam in radiation therapy. This is what Habs and his team are working on.

Andreas Henig carried out the first successful experiments together with Berlin physicists: “With the latest results, we succeeded in an efficient ion beam generation, while simultaneously reducing the energy spread of the accelerated particles. We are very happy about this experimental break-through.”

The scientists generate the high-energy ions by irradiating diamond-like carbon foils with intense laser pulses. Atoms located within the foil are split into electrons and ions by the strong electric field of the laser focus, generating a plasma. The enormous laser intensity (about 1020 times more intense than the sun) strongly heats the electrons and separates them in an expanding cloud from the heavier and therefore slower ions. A huge charge separation field builds up, accelerating ions to velocities up to a tenth of light speed. However, up to now, laser-accelerated ions exhibited a broad energy spectrum, whereas medical applications demand a well-defined particle energy to allow for a precise control of penetration depth and dose distribution in the body.

The group of Munich physicists is the first to experimentally demonstrate an acceleration process that allows all ions to fly with the same velocity. By changing the laser polarization from linear to circular and reducing the diamond-like carbon foil to only a few nanometers in thickness, an uncontrolled heating of the particles, along with subsequent foil expansion, was avoided. Instead, the laser light now pushes the electrons collectively as a nanometer-thin layer in forward direction, dragging carbon ions with it. The whole foil is driven like a sail by the light pressure of the laser – a mechanism that was predicted by theorists a long time ago.

The results provide the first experimental proof and pave the way toward a cost-saving generation of the highly promising carbon ion beams. The next challenge for the physicists in the Cluster of Excellence is to further increase the energy of the laser-accelerated ion beam.

At the moment, it is not yet sufficient to penetrate the body far enough to reach deeply situated tumors. Nonetheless, Habs is excited: “Already, in a few months from now, we will start irradiating single cells at our biomedical beamline here at the Max Planck Institute of Quantum Optics in Garching and will in parallel work hard to further enhance the parameters of the ion beam.”

For more information, visit: http://www.en.uni-muenchen.de/