Mária Martišíková

Three dimensional reconstruction of therapeutic carbon ion beams in phantoms using single secondary ion tracks

Carbon ion beam radiotherapy enables a very localised dose deposition. However, even small changes in the patient geometry or positioning errors can significantly distort the dose distribution. A live, non-invasive monitoring system of the beam delivery within the patient is therefore highly desirable, and could improve patient treatment. We present a novel three-dimensional method for imaging the beam in the irradiated object, exploiting the measured tracks of single secondary ions emerging under irradiation. The secondary particle tracks are detected with a TimePix stack-a set of parallel pixelated semiconductor detectors. We developed a three-dimensional reconstruction algorithm based on maximum likelihood expectation maximization. We demonstrate the applicability of the new method in the irradiation of a cylindrical PMMA phantom of human head size with a carbon ion pencil beam of [Formula: see text] MeV u-1. The beam image in the phantom is reconstructed from a set of nine discrete detector positions between [Formula: see text] and [Formula: see text] from the beam axis. Furthermore, we demonstrate the potential to visualize inhomogeneities by irradiating a PMMA phantom with an air gap as well as bone and adipose tissue surrogate inserts. We successfully reconstructed a three-dimensional image of the treatment beam in the phantom from single secondary ion tracks. The beam image corresponds well to the beam direction and energy. In addition, cylindrical inhomogeneities with a diameter of [Formula: see text] cm and density differences down to [Formula: see text] g cm-3 to the surrounding material are clearly visualized. This novel three-dimensional method to image a therapeutic carbon ion beam in the irradiated object does not interfere with the treatment and requires knowledge only of single secondary ion tracks. Even with detectors with only a small angular coverage, the three-dimensional reconstruction of the fragmentation points presented in this work was found to be feasible.

Dynamics of charge collection in pixelated semiconductor sensor studied with heavy ions and Timepix

This paper presents a novel technique allowing for the measurement and visualization of the spatial distribution and time evolution of the charge collection process in semiconductor sensors of ionizing radiation. The study was carried out with a pixelated high resistivity silicon sensor bump-bonded to the Timepix readout chip (256 × 256 pixels, with pitch of 55 μm). The sensor was irradiated with energetic protons (132 MeV) and carbon ions (240 MeV/u) entering the sensor at shallow angles. Such ions penetrate the full sensor thickness ionizing and depositing charge along their tracks. The charge deposited is collected by individual pixels of the Timepix chip operated in Time mode. The overall accuracy of these measurements was enhanced by averaging many particle tracks. The time accuracy is in order of nanoseconds and the position accuracy is about 5 μm. The purpose of this work is to demonstrate the accurate measurement that may be used with the mathematical model to investigate the electric field profile in a semiconductor sensor.

A novel method for assessment of fragmentation and beam-material interactions in helium ion radiotherapy with a miniaturized setup

Radiotherapy with protons and carbon ions enables to deliver dose distributions of high conformation to the target. Treatment with helium ions has been suggested due to their physical and biological advantages. A reliable benchmarking of the employed physics models with experimental data is required for treatment planning. However, experimental data for helium interactions is limited, in part due to the complexity and large size of conventional experimental setups. We present a novel method for the investigation of helium interactions with matter using miniaturized instrumentation based on highly integrated pixel detectors. The versatile setup consisted of a monitoring detector in front of the PMMA phantom of varying thickness and a detector stack for investigation of outgoing particles. The ion type downstream from the phantom was determined by high-resolution pattern recognition analysis of the single particle signals in the pixelated detectors. The fractions of helium and hydrogen ions behind the used targets were determined. As expected for the stable helium nucleus, only a minor decrease of the primary ion fluence along the target depth was found. E.g. the detected fraction of hydrogen ions on axis of a 220MeV/u 4He beam was below 6% behind 24.5cm of PMMA. Monte-Carlo simulations using Geant4 reproduce the experimental data on helium attenuation and yield of helium fragments qualitatively, but significant deviations were found for some combinations of target thickness and beam energy. The presented method is promising to contribute to the reduction of the uncertainty of treatment planning for helium ion radiotherapy.

Experimental verification of ion range calculation in a treatment planning system using a flat-panel detector

Heavy ion-beam therapy is a highly precise radiation therapy exploiting the characteristic interaction of ions with matter. The steep dose gradient of the Bragg curve allows the irradiation of targets with high-dose and a narrow dose penumbra around the target, in contrast to photon irradiation. This, however, makes heavy ion-beam therapy very sensitive to minor changes in the range calculation of the treatment planning system, as it has a direct influence on the outcome of the treatment. Our previous study has shown that ion radiography with an amorphous silicon flat-panel detector allows the measurement of the water equivalent thickness (WET) of an imaging object with good accuracy and high spatial resolution. In this study, the developed imaging technique is used to measure the WET distribution of a patient-like phantom, and these results are compared to the WET calculation of the treatment planning system. To do so, a measured two-dimensional map of the WET of an anthropomorphic phantom was compared to WET distributions based on x-ray computed tomography images as used in the treatment planning system. It was found that the WET maps agree well in the overall shape and two-dimensional distribution of WET values. Quantitatively, the ratio of the two-dimensional WET maps shows a mean of 1.004 with a standard deviation of 0.022. Differences were found to be concentrated at high WET gradients. This could be explained by the Bragg-peak degradation, which is measured in detail by ion radiography with the flat-panel detector, but is not taken into account in the treatment planning system. Excluding pixels exhibiting significant Bragg-peak degradation, the mean value of the ratio was found to be 1.000 with a standard deviation of 0.012. Employment of the amorphous silicon flat-panel detector for WET measurements allows us to detect uncertainties of the WET determination in the treatment planning process. This makes the investigated technique a very helpful tool to study the WET determination of critical and complex phantom cases.

Study of the capabilities of the Timepix detector for Ion Beam radiotherapy applications

The finite range of ion beams in matter and the presence of the Bragg-peak at the end of their range enable to create highly localized dose distributions. This advantage represents at the same time a challenge - namely to control all factors influencing the dose distribution with high precision. To ensure the required quality of the dose distribution applied to the patient, a number of measurement procedures is conducted. Many of these techniques were adopted from photon beam radiotherapy. Improvements in terms of the information amount gained are expected from new generations of dedicated ion detection techniques. One of the possible research directions is given by semiconductor-based detectors, which are at present rarely used in ion beam radiotherapy. In this contribution we report on the studies of the capabilities of the Timepix detector. This hybrid semiconductor pixelated detector was developed by the Medipix Collaboration at CERN. Its high spatial resolution allows an online registration of single ions. Experiments were performed at the Heidelberg Ion Beam Therapy Center in Germany using carbon ion beams. The signal pattern of single ions in the detector signal was shown to provide separation of the primary and different types of secondary ions. The direction of the secondary ions emerging from different irradiated phantoms were measured using 3D voxel detector containing several Timepix layers. The analysis of the recorded ion tracks shows a correlation with the beam range, position and width, which are all of high interest for monitoring of the beam within the patient during the therapy delivery. The results of the discussed studies show that the Timepix detector offers attractive capabilities for the needs of carbon ion beam therapy.

Monitoring of ion beam energy by tracking of secondary ions: First measurements in a patient-like phantom

Due to the finite range of ions in matter and the presence of the Bragg-peak, ion beams provide highly localized dose distributions. In radiation therapy with ion beams, unpredictable changes within the patient can deteriorate the quality of the dose distribution in the target. Therefore it is desired to monitor the beam within the patient in a non-invasive way. In this contribution the information carried by secondary ions, which are emerging from a patient during carbon ion beam irradiation, is studied. In a framework of investigations on the feasibility of beam monitoring exploiting secondary ions, the aim of this study was to show the possibility of distinction of two different primary carbon ion beam energies solely from the analysis of the secondary ion directions measured. The situation was complicated by realistic tissue inhomogeneities simulated by using a patient-like phantom. Experiments were performed at the Heidelberg Ion Beam Therapy Center in Germany using narrow carbon ion beams. The emerging secondary ions were registered by the Timepix detector. To determine the direction of the particles, a multi-layered detector (3D voxel detector) was employed. Clear differences between the application of the two beams with different energies (213 and 250 MeV!u) could be observed in the distributions of the measured secondary ions, despite the inherent tissue inhomogeneities. This result was achieved in both brain (homogeneous) and skull base regions (containing inhomogeneities). Differences between the energies could be observed with the detector placed on different positions around the head. The performed first experiments towards 12C beam monitoring in a patient-like geometry exploiting tracking of secondary ions with a 3D voxel detector show, that the information about the therapeutic carbon ion beam which is carried by secondary ions is promising and worthwhile to be investigated further.

Investigation of mixed ion fields in the forward direction for 220.5 MeV/u helium ion beams: comparison between water and PMMA targets.

Currently there is a rising interest in helium ion beams for radiotherapy. For benchmarking of the physical beam models used in treatment planning, there is a need for experimental data on the composition and spatial distribution of mixed ion fields. Of particular interest are the attenuation of the primary helium ion fluence and the build-up of secondary hydrogen ions due to nuclear interactions. The aim of this work was to provide such data with an enhanced precision. Moreover, the validity and limits of the mixed ion field equivalence between water and PMMA targets were investigated. Experiments with a 220.5 MeV/u helium ion pencil beam were performed at the Heidelberg Ion-Beam Therapy Center in Germany. The compact detection system used for ion tracking and identification was solely based on Timepix position-sensitive semiconductor detectors. In comparison to standard techniques, this system is two orders of magnitude smaller, and provides higher precision and flexibility. The numbers of outgoing helium and hydrogen ions per primary helium ion as well as the lateral particle distributions were quantitatively investigated in the forward direction behind water and PMMA targets with 5.2-18 cm water equivalent thickness (WET). Comparing water and PMMA targets with the same WET, we found that significant differences in the amount of outgoing helium and hydrogen ions and in the lateral particle distributions arise for target thicknesses above 10 cm WET. The experimental results concerning hydrogen ions emerging from the targets were reproduced reasonably well by Monte Carlo simulations using the FLUKA code. Concerning the amount of outgoing helium ions, significant differences of 3-15% were found between experiments and simulations. We conclude that if PMMA is used in place of water in dosimetry, differences in the dose distributions could arise close to the edges of the field, in particular for deep seated targets.

Visualization of air and metal inhomogeneities in phantoms irradiated by carbon ion beams using prompt secondary ions

PURPOSE Non-invasive methods for monitoring of the therapeutic ion beam extension in the patient are desired in order to handle deteriorations of the dose distribution related to changes of the patient geometry. In carbon ion radiotherapy, secondary light ions represent one of potential sources of information about the dose distribution in the irradiated target. The capability to detect range-changing inhomogeneities inside of an otherwise homogeneous phantom, based on single track measurements, is addressed in this paper. METHODS Air and stainless steel inhomogeneities, with PMMA equivalent thickness of 10mm and 4.8mm respectively, were inserted into a PMMA-phantom at different positions in depth. Irradiations of the phantom with therapeutic carbon ion pencil beams were performed at the Heidelberg Ion Beam Therapy Center. Tracks of single secondary ions escaping the phantom under irradiation were detected with a pixelized semiconductor detector Timepix. The statistical relevance of the found differences between the track distributions with and without inhomogeneities was evaluated. RESULTS Measured shifts of the distal edge and changes in the fragmentation probability make the presence of inhomogeneities inserted into the traversed medium detectable for both, 10mm air cavities and 1mm thick stainless steel. Moreover, the method was shown to be sensitive also on their position in the observed body, even when localized behind the Bragg-peak. CONCLUSIONS The presented results demonstrate experimentally, that the method using distributions of single secondary ion tracks is sensitive to the changes of homogeneity of the traversed material for the studied geometries of the target.

3D beam monitoring for 12 C radiotherapy by tracking of secondary ions using the timepix detector

Radiotherapy with narrow 12C ion beams enables treatment of tumors with high precision while sparing the surrounding healthy tissue. Unpredictable changes in the patients geometry can alter the ion range and result in changes in the dose distribution. Therefore, it is desirable to verify the actual dose delivery in the patient, preferably in real-time and in a non-invasive manner. Currently, the only technically feasible dose delivery monitoring method is based on tissue activation measurements by means of positron emission tomography (PET). As an alternative to PET-based measurements, beam range monitoring exploiting the detection of prompt secondary ions has been suggested. This modality is expected to allow for real-time monitoring, thereby reducing the influences of physiological signal wash-out known from PET-based techniques. In this contribution, the potential of monitoring narrow 12C ion beams in a head-sized PMMA phantom by tracking of secondary ions is investigated experimentally. Experiments with therapeutic carbon ion beams were performed at the Heidelberg Ion Beam Therapy Center (HIT), Germany. The Timepix detector was used to track secondary ions emerging from the irradiated phantom. Analysis of the secondary ion directions was used to monitor the range, width and position of pencil-like 12C ion beams in therapy relevant conditions. Clear dependences of the secondary ion track distribution on the investigated beam settings were found. Detectable were beam range differences down to about 2 mm and differences in the beam width of 1.4 mm. Furthermore, lateral shifts of the beam position by 1 mm were measurable. The presented experiments show the potential of secondary ion tracking for monitoring therapeutic carbon ion beams.

Towards fragment distinction in therapeutic carbon ion beams: A novel experimental approach using the Timepix detector

Radiotherapy with carbon ion beams is a highly precise method for cancer treatment. This is due to the finite range and the relatively low lateral scattering of the carbon ions in comparison to protons. On their path through tissue, the carbon ions can undergo nuclear fragmentation, resulting in lighter projectile fragments. Since the biological effect of the fragments differs from the primary particles, it is important to consider fragmentation in beam models used for therapy planning. Until now, large apparatus have been utilized for ion spectroscopic measurements. Employing a small detector would allow investigations directly within therapy relevant phantoms. In this contribution we report on the development of such a method using the Timepix detector. Its high spatial resolution enables to visualize and distinguish tracks of single particles. Experiments were performed at the Heidelberg Ion-Beam Therapy Center, using a carbon ion pencil beam of E=271 MeV/u. To investigate fragments in different material depths, the detector was placed perpendicular to the beam with 12 to 45 cm thick PMMA slabs in front of it. The pixel-wise energy calibration allows to directly determine the particle energy loss in the sensitive layer. Charge released in the detector by an ion spreads out during charge collection and is collected by several adjacent pixels forming signal clusters. Pattern recognition analysis of the signal shows a clear dependence of cluster parameter distributions on the PMMA depth. Based on this information the particular particle species in the obtained spectra could be identified. In this way, discrimination between primary carbon ions and hydrogen, helium and heavier fragments is possible. The presented novel method enables fragment distinction in mixed particle fields. Its main advantage lies in the flexibility and small size of the set-up.