Online characterisation of laser-driven beams is an urgent but still unsolved problem worldwide. Biomedical applications of these beams will rely on careful monitoring of a well characterised beam in front of a biological sample. In addition, safe tumour treatment requires online control of the applied dose. Here we will address these issues by developing techniques for reliable online characterisation, monitoring and dosimetry of laser-driven ion beams. We will pursue three major objectives:
First, ion beam characterisation will be addressed in close collaboration with the source development. Up to ~25 MeV / u, pixelated absorption filters combined with a scintillator and wide-angle spectrometers can be used. At higher energies, scintillator stacks or a volumetric scintillator imaged from various sides can be used for a tomographic reconstruction of ion distributions.
The second objective is to investigate beam monitoring by pixel detectors, which can handle ion pulses containing 107-109 particles / cm2 due to their small pixel size, yielding sub-mm spatial resolution. Concerning radiation hardness, instrumentation for harsh radiation environments, such as the Medipix system, will be addressed in collaboration with detector development for X-rays. Finally, online dosimetry will be developed by employing the detection of prompt y rays from nuclear reactions in the irradiated volume. Precise localisation of the ion beam track will be achieved by Compton tracking of photons and Compton electrons. A precise position resolution significantly beyond presently available capabilities will be a major benefit of laserdriven particle therapy.
Detection of laser-accelerated particles
The development of new particle and light sources for therapy and diagnostic of tumors is one of the major goals of the DFG cluster of excellence Munich-Centre for Advanced Photonics (MAP). Radiation therapy with ions experienced a growing interest in the past years because of their favourable physical depth dose distribution. Ions deposit the majority of their energy in the so-called Bragg peak at the end of their range which allows a highly conformal radiation therapy of deep-seated tumor and optimal sparing of surounding healthy tissue. However, ion acceleration to therapeutically relevant energies (90-250 MeV in case of protons) and a beam transport systems that allows patient irradiation from any direction (i.e. rotating gantry) requires an expensive infrastructure. As a consequence only few ion beam therapy centers exist these days world wide.
Laser-based particle acceleration could offer an economic alternative for compact particle accelerators and beam transport lines for radiation therapy in future. The special acceleration process generates short and intense ions pulses, with typical pulse duration in the order of few ns and fluence exceeding 107 particles/cm2. The pulse intensity is, thus, several orders of magnitude (105) higher than in a conventional particle accelerators (cyclotron, synchrotron). The detection of such high intense, ultra-short ion pulses is a challenge we meet in our research group.
Medical imaging - Development of a Compton camera system as a versatile tool for position-sensitive photon detection
a) Principle of a Compton camera
Due to the kinematics of the Compton scattering process and subsequent photon absorption, a Compton camera allows for reconstructing the origin of a primary photon on the surface of the 'Compton cone'. So a Compton camera typically consists of two parts: a scatter detector and an absorber detector. The left figure shows a sketch of such an arrangement, indicating the Compton scattering interaction and the Compton cone reconstructed from a measurement of energies and interaction positions in the respective deetctor modules. Two modes of operation can be distinguished: if a thick scatter detector is used, the generated Compton electron will be absorbed and cannot add to the kinematical information. If, however, the scatter detector is built from a layer of segmented double-sided Silicon strip detectors, electron tracking will become possible (at least in cases where the initioa photon energy was high eneough to create Compton electrons that can be tracked over several layers for the Si detectors). In this case, electron tracking will allow to restrict the Compton cone to an arc segment.
b) Detection of multi-MeV photons
One motivation for our development of a Compton Camera aims at the position-resolved detection of prompt photons emitted from the interaction of laser-accelerated proton eams with biological samples (or equivalent phantoms). The generation of such proton beams (up to 100 MeV) is one of the goals pursued within the MAP Cluster of Excellence at the future CALA facility (Center for Advanced laser Applications) in Garching. Here we expect the emission of energetic prompt photons that carry the information of the beam trajectory and particularly of the position of the Bragg peak in the sample. Verifying the range of the proton beam in the sample is addressing one of the key issues of modern hadron therapy. The Figure below shows a sketch of our Compton Camera geometry, designed to allow for gamma and electron tracking. We have chosen to use the scintillator material LaBr3 for the absorber of our Compton camera due to its excellent timing properties while still mainaining a very good energy resolution.
c) Introducing the gamma-PET-technique based on a Compton camera
A detector system based on several Compton Camera modules can also contribute to improve the performance of the PET (positron emission tomography) technique, where positron-emitting unstable nuclides are used to reconstruct the image of the source via the detection of the diametral detection of the two 511 keV annihilation photons. The position-sensitive detection of these photons defines a "line of respose" (LOR), while the superposition of many of such LOR's allows for a localization of the emission source. However, there exists a number of beta+ emitters, where in addition a third, prompt, photon is emitted from an excited nuclear state in the daughter nucleus. Our goal is to use a system of Compton Cameras to register this aditional third prompt photon in coincidence with the two 511 keV photons. Intersecting the LOR with the reconstructed trajectory of the third photon allows for a much increased sensitivity, and thus to either shorter PET examination times or much reduced radioactive dose applied to the patients for a PET examination.
d) Status of our prototype development
We are currently characterizing our LaBr3 absorber crystal (50x50x30 mm3), which is read out by a multi-anode photomultiplier. Currently we are using 64 pixels (6x6 mm2), with an option to upgrade soon to 256 pixels (3x3 mm2). The photograph shows this detector.
Our silicon strip detectors are currently being manufactured. We will receive 6 DSSSD modules with an active area of 50x50 mm2, segmented to 128 strips on each side (pitch size 390 micron). The thickness of the modules is 0.5 mm. The resulting 1534 detector channels will be read out by an ASIC-based electronics, connected to the VME data acquisition system.
Use of Monte Carlo Simulations in ion beam therapy
Monte Carlo (MC) methods are increasingly being utilized to support several aspects of ion beam therapy. Two areas of MC applications are of major interest. On the one hand Monte Carlo simulations are essential tools to complement analytical treatment planning system (TPS) for validation of absorbed dose in arbitrary complex geometries as well as for assessment of different quantities related to the biological effectiveness of the radiation.
On the other hand the development and implementation of imaging techniques based on the detection of emerging secondary radiation (e.g., positron emission tomography and prompt prompt gamma) or transmitted high energetic ions rely on detailed Monte Carlo calculations of all the interactions of the radiation with the tissue and the detector. In our group, we use both simulation codes FLUKA and Geant4 to tackle several aspects of the above mentioned research areas, in strong connection with ion beam therapy centers and research institutions.