Scanned Particle Beams Research Articles

Motion compensation with a scanned ion beam: a technical feasibility study

BackgroundIntrafractional motion results in local over- and under-dosage in particle therapy with a scanned beam. Scanned beam delivery offers the possibility to compensate target motion by tracking with the treatment beam.MethodsLateral motion components were compensated directly with the beam scanning system by adapting nominal beam positions according to the target motion. Longitudinal motion compensation to mitigate motion induced range changes was performed with a dedicated wedge system that adjusts effective particle energies at isocenter.ResultsLateral compensation performance was better than 1% for a homogeneous dose distribution when comparing irradiations of a stationary radiographic film and a moving film using motion compensation. The accuracy of longitudinal range compensation was well below 1 mm.ConclusionMotion compensation with scanned particle beams is technically feasible with high precision.

TU-EE-A2-03: Target Motion Tracking with a Scanned Particle Beam: Calculation and Experimental Validation of Biologically Effective Doses in the Presence of Motion

Purpose: A beam tracking system for scanned ion beams was developed to compensate intrafractional motion. A novel method to calculate biologically effective doses for charged particles (especially carbon ions) in the presence of motion was implemented and experimentally validated. Method and Materials: The tracking system adapts pencil beam positions according to target motion in quasi real time. Lateral position adaptation is performed with the beam scanning system, longitudinal range compensation with a dedicated energy modulation system that includes absorber material of adjustable thickness. To validate the calculation of biologically effective dose distributions in the presence of motion, a medium‐filled container with Chinese hamster ovary cells was irradiated on a sliding table (3D biological dosimetry). Cell survival was measured for three irradiation schemes: 1. stationary target, 2.moving target without tracking, 3. moving target with tracking. Measured cell survival was converted to biologically effective doses and compared to calculations. Furthermore, measurements of the stationary setup and tracking irradiation under motion were compared directly to validate the functionality of the beam tracking system.Results: The average relative differences between measured and calculated biologically effective doses D (Dcalc/Dmeas‐1) in the target area were −5%±9%, 5%±9% and −6%±8% for the stationary, moving target and tracking irradiation, respectively. Differences between calculations and measurements were comparable to relative variations in biologically effective doses of the experimental data of 11% (individual measurements). The difference between stationary and tracking irradiation (1‐Dcomp/Dstat) was 6%±9%. Conclusion: The performance of the beam tracking system was succesfully validated with 3D biological dosimetry by cell survival measurements. Results of the novel method to calculate biologcially effective dose distributions are in good agreement with measured data. Conflict of Interest: Research sponsored in part by Siemens Healthcare.

Quantification of interplay effects of scanned particle beams and moving targets

Scanned particle beams and target motion interfere. This interplay leads to deterioration of the dose distribution. Experiments and a treatment planning study were performed to investigate interplay. Experiments were performed with moving radiographic films for different motion parameters. Resulting dose distributions were analyzed for homogeneity and dose coverage. The treatment planning study was based on the time-resolved computed tomography (4DCT) data of five lung tumor patients. Treatment plans with margins to account for respiratory motion were optimized, and resulting dose distributions for 108 different motion parameters for each patient were calculated. Data analysis for a single fraction was based on dose–volume histograms and the volume covered with 95% of the planned dose. Interplay deteriorated dose conformity and homogeneity (1-standard deviation/mean) in the experiments as well as in the treatment-planning study. The homogeneity on radiographic films was below ≈80% for motion amplitudes of ≈15 mm. For the treatment-planning study based on patient data, the target volume receiving at least 95% of the prescribed dose was on average (standard deviation) 71.0% (14.2%). Interplay of scanned particle beams and moving targets has severe impact on the resulting dose distributions. Fractionated treatment delivery potentially mitigates at least parts of these interplay effects. However, especially for small fraction numbers, e.g. hypo-fractionation, treatment of moving targets with scanned particle beams requires motion mitigation techniques such as rescanning, gating, or tracking.

Target motion tracking with a scanned particle beam

Treatment of moving targets with scanned particle beams results in local over- and under-dosage due to interplay of beam and target motion. To mitigate the impact of respiratory motion, a motion tracking system has been developed and integrated in the therapy control system at Gesellschaft für Schwerionenforschung. The system adapts pencil beam positions as well as the beam energy according to target motion to irradiate the planned position. Motion compensation performance of the tracking system was assessed by measurements with radiographic films and a 3D array of 24 ionization chambers. Measurements were performed for stationary detectors and moving detectors using the tracking system. Film measurements showed comparable homogeneity inside the target area. Relative differences of 3D dose distributions within the target volume were 1 +/- 2% with a maximum of 4%. Dose gradients and dose to surrounding areas were in good agreement. The motion tracking system successfully preserved dose distributions delivered to moving targets and maintained target conformity.

Quantifying lateral tissue heterogeneities in hadron therapy

In radiotherapy with scanned particle beams, tissue heterogeneities lateral to the beam direction are problematic in two ways: they pose a challenge to dose calculation algorithms, and they lead to a high sensitivity to setup errors. In order to quantify and avoid these problems, a heterogeneity number H(i) as a method to quantify lateral tissue heterogeneities of single beam spot i is introduced. To evaluate this new concept, two kinds of potential errors were investigated for single beam spots: First, the dose calculation error has been obtained by comparing the dose distribution computed by a simple pencil beam algorithm to more accurate Monte Carlo simulations. The resulting error is clearly correlated with H(i). Second, the analysis of the sensitivity to setup errors of single beam spots also showed a dependence on H(i). From this data it is concluded that H(i) can be used as a criterion to assess the risks of a compromised delivered dose due to lateral tissue heterogeneities. Furthermore, a method how to incorporate this information into the inverse planning process for intensity modulated proton therapy is presented. By suppressing beam spots with a high value of H(i), the unfavorable impact of lateral tissue heterogeneities can be reduced, leading to treatment plans which are more robust to dose calculation errors of the pencil beam algorithm. Additional possibilities to use the information of H(i) are outlined in the discussion.

Simulations to design an online motion compensation system for scanned particle beams

Respiration-induced target motion is a major problem in intensity-modulated radiation therapy. Beam segments are delivered serially to form the total dose distribution. In the presence of motion, the spatial relation between dose deposition from different segments will be lost. Usually, this results in over- and underdosage. Besides such interplay effects between target motion and dynamic beam delivery as known from photon therapy, changes in internal density have an impact on delivered dose for intensity-modulated charged particle therapy. In this study, we have analysed interplay effects between raster scanned carbon ion beams and target motion. Furthermore, the potential of an online motion strategy was assessed in several simulations. An extended version of the clinical treatment planning software was used to calculate dose distributions to moving targets with and without motion compensation. For motion compensation, each individual ion pencil beam tracked the planned target position in the lateral as well as longitudinal direction. Target translations and rotations, including changes in internal density, were simulated. Target motion simulating breathing resulted in severe degradation of delivered dose distributions. For example, for motion amplitudes of ±15 mm, only 47% of the target volume received 80% of the planned dose. Unpredictability of resulting dose distributions was demonstrated by varying motion parameters. On the other hand, motion compensation allowed for dose distributions for moving targets comparable to those for static targets. Even limited compensation precision (standard deviation ∼2 mm), introduced to simulate possible limitations of real-time target tracking, resulted in less than 3% loss in dose homogeneity.

3D Online compensation of target motion with scanned particle beam

Intensity modulated, active beam delivery, like magnetic raster scanning, strongly relies on target immobilisation. If target motion cannot be avoided, interferences between the scanning procedure and the tumour displacement destroy the volume conformity. A prototype setup for 3D online motion compensation (3D-OMC) with a scanned particle beam is described, transferring the full potential of volume conformal irradiation to moving targets.

Online compensation for target motion with scanned particle beams: simulation environment

Target motion is one of the major limitations of each high precision radiation therapy. Using advanced active beam delivery techniques, such as the magnetic raster scanning system for particle irradiation, the interplay between time-dependent beam and target position heavily distorts the applied dose distribution. This paper presents a simulation environment in which the time-dependent effect of target motion on heavy-ion irradiation can be calculated with dynamically scanned ion beams. In an extension of the existing treatment planning software for ion irradiation of static targets (TRiP) at GSI, the expected dose distribution is calculated as the sum of several sub-distributions for single target motion states. To investigate active compensation for target motion by adapting the position of the therapeutic beam during irradiation, the planned beam positions can be altered during the calculation. Applying realistic parameters to the planned motion-compensation methods at GSI, the effect of target motion on the expected dose uniformity can be simulated for different target configurations and motion conditions. For the dynamic dose calculation, experimentally measured profiles of the beam extraction in time were used. Initial simulations show the feasibility and consistency of an active motion compensation with the magnetic scanning system and reveal some strategies to improve the dose homogeneity inside the moving target. The simulation environment presented here provides an effective means for evaluating the dose distribution for a moving target volume with and without motion compensation. It contributes a substantial basis for the experimental research on the irradiation of moving target volumes with scanned ion beams at GSI which will be presented in upcoming papers.

A new measurement of the 7Be(p, γ) 8B cross-section with an implanted 7Be target

The 7Be(p, γ) 8B capture reaction is of major importance to the physics of the sun and the issues of the “solar neutrino puzzle”. We report here on a new determination of the absolute cross section of this reaction, using a novel method which overcomes some of the major experimental uncertainties of previous measurements. We utilize a 7Be target implanted into a Cu substrate and a uniformly scanned particle beam larger than the target spot, eliminating issues of target homogeneity and backscattering loss of 8B reaction products. The target was produced using a beam of 1.8·10 10/s 7Be nuclei extracted at ISOLDE (CERN) from a graphite target bombarded by 1 GeV protons in a two-step resonant laser ionization source. The 7Be nuclei were directly implanted into a copper substrate to obtain a target of 2 mm diameter with a total of 3.10 15 atoms. The measurement of the 8B production cross section was carried out at the Van de Graaff laboratory of the Weizmann Institute of Science. We obtain for the S factor of the reaction: S 17( E cm = 1.09 MeV) = 22.7(1.2) eV. barn and S 17( E cm = 1.29 MeV) = 23.8(1.5) eV. barn, somewhat higher than other recent measurements. The present results can serve as a benchmark for further measurements of this cross section.

X-Ray Imaging

Element mapping has been so far an analytical field covered by those investigation methods using a scanned particle beam on the excitation side of the specimen. These methods include localized interaction processes, as in the electron microprobe, secondary ion mass spectrometer and Auger electron spectrometer. On the other hand, methods which use a stationary radiation beam on the input side (non-localized interaction) usually give summarized information about the sample area (X-ray fluorescence spectrometry, photoelectron spectrometry, …). This paper deals with an approach to extending the application field of X-ray fluorescence analysis (XRFA) to element mapping. Fig. 1 shows the main components of the X-ray imaging system. Since modern SRF systems often work under computer control, the only additional hardware components of the system are a scanned sample holder and an X-ray source producing a line-shaped X-ray beam. The computer is usually not used for ease, speed or convenience of operation, but is used to generate the element image and process all the collected fluorescence data.