Proton therapy is a promising technology that may overcome many of the side effects that are common to traditional radiation beams. This is possible because protons flying at nearly the speed of light only release their energy once they come to a stop – the tissues on the way to the target are spared the radiation exposure that’s common to gamma rays and X-rays. The real downsides to using proton therapy is that it’s very expensive to host a particle accelerator and that the installation takes up a lot of space. Getting the size and cost down to levels manageable by average hospitals can help introduce the technology to many new patients. Umar Masood, a medical physicist PhD candidate at Dresden University of Technology, and colleagues has proposed a new approach to building small scale proton accelerators that can be made two to three times smaller than current models.
The new technology harnesses laser-driven particle acceleration to get ions moving, a radically different approach from Ernest Lawrence’s cyclotron that’s been the basis for most accelerators. The laser powered approach produces protons at a wide range of energies, normally a disadvantage since a coherent beam focused on a predefined spot is predictable in its therapeutic action. Additionally, the beam is wider, another potential disadvantage that may limit accuracy and precision. Nevertheless, these characteristics can be harnessed positively via software if they are considered before therapy begins.
From the announcement:
Given the wide energy window during radiation using laser-accelerated protons, a large portion of protons has to be eliminated from the beam to reach a comparatively narrower energy window. This compromises efficiency. However, Umar Masood has come up with an innovative solution to this dilemma: Not only does he use the wider energy distribution but also the proton beam’s naturally larger diameter, which thus emits its dose within a larger volume. This translates to a larger number of cancer cells that are simultaneously irradiated within the same unit of time. Technische Universität München is currently in the process of developing a special kind of software to help with calculating dose deposition in the patient during planning of the treatment optimized for laser-accelerated proton beams.
Another property of laser-accelerated protons lies with the fact that we are not looking at a continuous particle beam here but instead at individual particle pulses. In the case of pulsed beams, more powerful magnets can be used to guide the beam from accelerator to patient – an important prerequisite for decreasing the beamline’s and, more importantly, the massive gantry’s overall size. Dresden is banking on pulsed magnets since the HZDR’s Dresden High Magnetic Field Laboratory has extensive experience with these.
Umar Masood had to run tests on a number of different incarnations of his idea in order to be able to come up with a concept for guiding laser-accelerated proton beams in the first place. First, a magnetic coil fashions a beam from protons that were accelerated directly inside the gantry using intense laser light. Thereafter, a dipole magnet directs the beam around a 90-degree curve while ensuring that protons with an unsuitable energy window are cut off. A group of focusing magnets called quadrupoles, which are also only switched on for about 100 milliseconds at a time, keep the beam on track. Transport of a pulsed beam with a wide energy distribution can be tricky as there are at least six dimensions to be taken care of. A second dipole magnet redirects the beam in a direction opposite to the original acceleration towards the patient table located at the center of the gantry.
In spite of the fact that now, for the first time ever, an entire facility was modeled on the basis of a laser accelerator, there are yet many obstacles to be overcome before this kind of a facility can become a reality. As such, the various pulsed magnets have to first be developed and tested. Also, at this time, the laser-accelerated protons’ energies are falling short of targeting deeper-lying tumors within the patient’s body. Which is why the HZDR’s own laser DRACO is currently undergoing an upgrade and is also getting a new big sister, PENELOPE, which, at a performance of 1 petawatt will take its place among the World’s most powerful lasers. Professor Ulrich Schramm, head of the HZDR laser-particle acceleration group, is quite certain that “following some five years of intense research using DRACO we’re thinking we will at last be able to realize the necessary parameters for patient radiation therapy.”
Study in Applied Physics B: A compact solution for ion beam therapy with laser accelerated protons…
