gms | German Medical Science

48th Meeting of the Particle Therapy Co-Operative Group

Particle Therapy Co-Operative Group (PTCOG)

28.09. - 03.10.2009, Heidelberg

Modifying proton fluence spectra to generate spread-out Bragg peaks with laser accelerated proton beams

Meeting Abstract

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  • S. Schell - Department of Radiation Oncology, Technische Universität München, Klinikum rechts der Isar, München
  • J. J. Wilkens - Department of Radiation Oncology, Technische Universität München, Klinikum rechts der Isar, München

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog177

DOI: 10.3205/09ptcog177, URN: urn:nbn:de:0183-09ptcog1777

Published: September 24, 2009

© 2009 Schell et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.



Background: Laser based acceleration of protons is a promising new technology that might become a possibility to utilize the superior properties of protons in radiation therapy with reduced costs compared to conventional acceleration methods with cyclotrons or synchrotrons. However, besides the fact that the maximal energy is still too low for patient treatments, energy spectra of laser accelerated proton beams are currently far from being monoenergetic. To compensate for the latter, energy selection systems using magnetic fields have been proposed to single out particles with the desired energy. These systems allow choosing protons between a lowest and a highest energy, but they do not change the transmitted part of the spectrum. To produce spread-out Bragg peaks with a steep lateral dose fall-off within the patient, multiple laser shots with a very small energy spread in the proton spectrum are required. This means that many particles have to be absorbed in the energy selection system which generates a lot of secondary radiation.

Material and methods: We investigate a slight modification of such energy selection systems that allows us to influence the relative number of particles per energy bin. This change of the spectrum can be achieved by inserting suitably formed scattering material at the central plane of the energy selection system where the particles are separated in space depending on their energy. Depending on the thickness of the scatterer, a portion of the protons is scattered so that they will not reach the exit of the energy selection system. Since a spread-out Bragg peak requires more high energy protons than low energy ones, the scattering material is chosen to be thicker at the position that is traversed by low energy protons. Thus, these particles are scattered the most which reduces their relative population compared to high energy protons in the final beam. Furthermore, there is a gap for high energy particles that enables them to pass the system without any scattering. With the help of Monte Carlo simulations we analyzed both simple wedge geometries and various stacks of lead slices with a maximal total thickness of 600 µm.

Results: We found that these configurations can provide energy spectra that naturally produce spread-out Bragg peaks within one laser shot. For a stack of ten lead slices a spread-out Bragg peak with a range of 25.2 cm and modulation width of 7.4 cm could be produced with a broad initial spectrum whose particle number was decreasing with energy.

Conclusion: The modification of the energy selection system with additional scattering material allows the shaping of the transmitted spectrum in such a way that it corresponds to a full spread-out Bragg peak delivered simultaneously. This reduces the treatment time and increases the particle efficiency of the system.

Acknowledgment: Supported by DFG Cluster of Excellence: Munich-Centre for Advanced Photonics.