New measurement techniques for ion beam therapy purposes

M. Martišíková^1,2,  J. Jakubek³, C. Granja³, B. Hartmann^1,2, K. Gwosch², P. Soukup³, S. Pospíšil³, and O. Jäkel^1,2,4

¹ Heidelberg University Hospital, Heidelberg, Germany

² German Cancer Research Center, Heidelberg, Germany

³ Institute of Experimental and Applied Physics, Czech Technical University in Prague, Prague, Czech Republic

⁴ Heidelberg Ion-Beam Therapy Center, Heidelberg, Germany

Ion beams are used for radiotherapywhen critical organs close to the irradiated target volume need to be spared from radiation.  Due to the finite range of ions in material and the presence of the Bragg-peak at the end of their range, ion beams provide highly localized dose distributions.  Ions heavier than protons offer in addition lower scattering and increased biological effectiveness.  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. 

We investigate the potential of silicon-based pixelated detectors to further improve this high precision therapy by measurements of ion distributions and by ion identification.  In this presentation we concentrate on the research with the Timepix detector, developed by the Medipix Collaboration based at CERN.  This research is conducted in cooperation with the Czech Technical Universty in Prague. Timepix is a state-of-the-art crystalline silicon detector with 55 um pixel pitch and a sensitive area of 1.4 x 1.4 cm².  Its high sensitivity and the high spatial resolution allow registering single ions online.

The experiments are performed at the Heidelberg Ion-Beam Therapy Center (HIT), which is the first European hospital-based ion beam therapy facility.  Both proton and carbon ion treatments are provided.  A synchrotron is used for ion acceleration.  For dose delivery to the patient, narrow pencil-like beams are scanned over the target volume.  Two of the treatment rooms are equipped with fixed horizontal beam lines.  In the third room the worldwide first carbon ion beam gantry will provide rotation of the beam around the patient to increase the number of possible beam directions.

During carbon ion therapy, a variety of ion species is created by nuclear fragmentation processes of the primary beam.  Since these ions differ in their biological effectiveness, it is of large interest to measure the ion spectra created under different conditions.  Up to now large experimental setups were used to perform ion spectroscopy exploiting dE-E and time-of-flight techniques.  We investigate the Timepix detector for its application in ion beam spectroscopy.  Due to the small size of the detector, it is suitable for measurements directly within phantoms. Using the energy calibration of all 65535 pixels, the detector provides measurements of the energy loss of ions in silicon.  In addition, the characteristic tracks of single particles are dependent on the particle type, energy and direction.  A detailed pattern recognition enables to differentiate between the primary carbon ions and secondary particles such as protons, helium and heavier fragments and allows to measure their spatial distributions.

Due to the lower charge, the range of secondary light ions can be much larger than that of the primary carbon ions and they may leave the patient.  The measurement of the spatial distributions of secondary ions around the patient was suggested for a non-invasive monitoring of the beam delivery in the patient to assess the quality of the delivered dose distribution.  We will present that the Timepix detector is capable to measure secondary ions leaving a homogeneous phantom during an irradiation. Directions of those ions were recorded by a multi-layered array of detectors (a voxel detector).  The distributions of the secondary ion tracks were found to be sensitive on the beam range differences in the phantom down to 1.7 mm as well as on the beam width.

In summary, pixelated silicon-based detectors, up to now largely unexploited in ion beam therapy, provide large potential to improve this highly precise radiotherapy.

This work has been carried out in frame of the Medipix Collaboration.