Two dimensional silicon-based detectors for ion beam therapy
M. Martisikova 1*, S. Brons 2, C. Granja 3, J. Jakubek 3, J. Telsemeyer 1,4, B. Hartmann 1,4, L. Huber 1, B. Hesse 1, O. Jakel 1,2
1 German Cancer Research Center, Heidelberg, Germany
2 Heidelberg Ion-beam Therapy Center, Heidelberg, Germany
3 Institute of Experimental and Applied Physics, Czech Technical University in Prague, Prague, Czech Republic
4 Heidelberg University Clinic, Heidelberg, Germany
Within the field of radiation therapy there is an increasing interest in ion beam therapy, which is caused by the promising clinical results reached during the last decades. As ion beams traverse material, they cause the highest ionization density at the end of their path, known as the Bragg-Peak. Due to this property, ion beams enable higher dose conformation to the tumor and increased sparing of the surrounding tissue, in comparison to the standard radiation therapy using high energy photons. Ions heavier than protons offer in addition lower scattering and increased biological effectiveness.
The Heidelberg Ion Beam Therapy Center (HIT) is a state-of-the-art ion beam therapy facility and the first hospital-based facility in Europe. It provides proton and carbon ion treatments. For dose delivery to the patient, narrow pencil-like beams are scanned over the target volume. A synchrotron is used for ion acceleration. Two of the treatment rooms are equipped with horizontal beam lines. In the third room the worldwide first carbon ion beam gantry provides rotation of the beam around the patient to increase the choice of beam directions.
Within the Heavy Ion Project Group at the DKFZ in Heidelberg we investigate the potential of various detectors to further improve this high precision therapy. Our studies involve both amorphous and crystalline silicon-based detectors, which offer high spatial and time resolution as well as online readout. The experiments are performed at HIT.
The commercial flat-panel detector RID 256L (Perkin Elmer, Wiesbaden, Germany) was originally designed for medical imaging in photon beams. It consists of an array of 256 x 256 amorphous silicon photodiodes with a pitch of 800 μm. We investigate its capabilities to image fluence distributions of ion fields as well as to image objects using ion beam radiography. We have shown that 2D absolute fluence measurements in the plateau region of the Bragg curve are feasible with this detector. Such a method can be employed for the verification of patient plans before the treatment and can be used for both, proton and carbon ion patient plans. This technique provides a 150-times higher density of measured points than the currently used ionization chamber array. Therefore it is especially beneficial for strongly inhomogeneous dose distributions. Since it does not require a water phantom, it can be conveniently used with a gantry.
The flat-panel detector shows also a high potential for ion beam radiography. It is capable to image metal seeds implanted in soft tissue, which can be exploited not only to check the patient positioning before the beam application, but also to monitor the movement of the target during the treatment. Given by the steepness of the Bragg-curve, ion beams can provide high contrast radiographies. In our group, online images with high soft tissue contrast were obtained using the flat-panel detector. An unambiguous separation of soft tissues, represented by tissue equivalents of 1cm thickness, was achieved.
During carbon ion therapy, a variety of ion species is created by nuclear fragmentation processes of the primary beam. Since they differ in their biological effectiveness, it is of large interest to measure the ion spectra created under different conditions. In cooperation with the CTU in Prague we investigate the pixel detector Timepix, developed by the Medipix Collaboration, for its applications in ion beam spectroscopy. It is a state-of-the-art crystalline silicon detector with high sensitivity and an enhanced signal-to-noise ratio. The high spatial resolution (55 um pixel pitch) allows its operation as an active nuclear emulsion, registering single ions online. Using the energy calibration of each pixel, the detector was found to provide accurate measurements of energy loss for high energy ions in silicon. In addition, the size of the pixel-cluster activated by a single particle shows a strong dependence on the particle type and is thus the second promising parameter to distinguish ions. Due to the small size of the detector, it is suitable for measurements within phantoms and for real patient plans. This project is carried out in framework of the Medipix Collaboration.