Die Konzeption der Heidelberger Ionentherapieanlage HICAT

High-energy proton imaging for biomedical applications

The charged particle community is looking for techniques exploiting proton interactions instead of X-ray absorption for creating images of human tissue. Due to multiple Coulomb scattering inside the measured object it has shown to be highly non-trivial to achieve sufficient spatial resolution. We present imaging of biological tissue with a proton microscope. This device relies on magnetic optics, distinguishing it from most published proton imaging methods. For these methods reducing the data acquisition time to a clinically acceptable level has turned out to be challenging. In a proton microscope, data acquisition and processing are much simpler. This device even allows imaging in real time. The primary medical application will be image guidance in proton radiosurgery. Proton images demonstrating the potential for this application are presented. Tomographic reconstructions are included to raise awareness of the possibility of high-resolution proton tomography using magneto-optics.

Beam Delivery Systems for Particle Radiation Therapy: Current Status and Recent Developments

An overview is given of different techniques of dose delivery applied in currently operating and planned particle therapy systems. Their advantages and disadvantages will be compared and consequences of the methods for the rest of the instrumentation will be discussed. The interrelationship between beam delivery at the patient and the accelerator system is shown by means of several concrete examples. Apart from a description of several subsystems in a particle therapy facility, design rules for optimizing the reliability of an accelerator and beam delivery system will be discussed, as well as some remarks concerning how to deal with future developments.

Respiratory motion management in particle therapy

Clinical outcomes of charged particle therapy are very promising. Currently, several dedicated centers that use scanning beam technology are either close to clinical use or under construction. Since scanned beam treatments of targets that move with respiration most likely result in marked local over- and underdosage due to interplay of target motion and dynamic beam application, dedicated motion mitigation techniques have to be employed. To date, the motion mitigation techniques, rescanning, beam gating, and beam tracking, have been proposed and tested in experimental studies. Rescanning relies on repeated irradiations of the target with the number of particles reduced accordingly per scan to statistically average local misdosage. Specific developments to prohibit temporal correlation between beam scanning and target motion will be required to guarantee adequate averaging. For beam gating, residual target motion within gating windows has to be mitigated in order to avoid local misdosage. Possibly the most promising strategy is to increase the overlap of adjacent particle pencil beams laterally as well as longitudinally to effectively reduce the sensitivity against small residual target motion. The most conformal and potentially most precise motion mitigation technique is beam tracking. Individual particle pencil beams have to be adapted laterally as well as longitudinally according to the target motion. Within the next several years, it can be anticipated that rescanning as well as beam gating will be ready for clinical use. For rescanning, treatment planning margins that incorporate the full extent of target motion as well as motion induced density variations in the beam paths will result in reduced target conformity of the applied dose distributions. Due to the limited precision of motion monitoring devices, it seems likely that beam gating will be used initially to mitigate interplay effects only but not to considerably decrease treatment planning margins. Then, in the next step, beam gating, based on more accurate motion monitoring systems, provides the possibility to restore target conformity as well as steep dose gradients due to reduced treatment planning margins. Accurate motion monitoring systems will be required for beam tracking. Even though beam tracking has already been successfully tested experimentally, full clinical implementation requires direct feedback of the actual target position in quasireal time to the treatment control system and can be anticipated to be several more years ahead.

Medical physics aspects of particle therapy

Charged particle beams offer an improved dose conformation to the target volume when compared with photon radiotherapy, with better sparing of normal tissue structures close to the target. In addition, beams of heavier ions exhibit a strong increase of the linear energy transfer in the Bragg peak when compared with the entrance region. These physical and biological properties make ion beams more favourable for radiation therapy of cancer than photon beams. As a consequence, particle therapy with protons and heavy ions has gained increasing interest worldwide. This contribution summarises the physical and biological principles of charged particle therapy with ion beams and highlights some of the developments in the field of beam delivery, the principles of treatment planning and the determination of absorbed dose in ion beams. The clinical experience gathered so far with carbon ion therapy is briefly reviewed.

Speed and accuracy of a beam tracking system for treatment of moving targets with scanned ion beams

The technical performance of an integrated three-dimensional carbon ion pencil beam tracking system that was developed at GSI was investigated in phantom studies. Aim of the beam tracking system is to accurately treat tumours that are subject to respiratory motion with scanned ion beams. The current system provides real-time control of ion pencil beams to track a moving target laterally using the scanning magnets and longitudinally with a dedicated range shifter. The system response time was deduced to be approximately 1 ms for lateral beam tracking. The range shifter response time has been measured for various range shift amounts. A value of 16 +/- 2 ms was achieved for a water equivalent shift of 5 mm. An additional communication delay of 11 +/- 2 ms was taken into account in the beam tracking process via motion prediction. Accuracy of the lateral beam tracking was measured with a multi-wire position detector to < or =0.16 mm standard deviation. Longitudinal beam tracking accuracy was parameterized based on measured responses of the range shifter and required time durations to maintain a specific particle range. For example, 5 mm water equivalence (WE) longitudinal beam tracking results in accuracy of 1.08 and 0.48 mm WE in root mean square for time windows of 10 and 50 ms, respectively.

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 approximately 80% for motion amplitudes of approximately 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.

4D treatment planning for scanned ion beams

At Gesellschaft für Schwerionenforschung (GSI) more than 330 patients have been treated with scanned carbon ion beams in a pilot project. To date, only stationary tumors have been treated. In the presence of motion, scanned ion beam therapy is not yet possible because of interplay effects between scanned beam and target motion which can cause severe mis-dosage. We have started a project to treat tumors that are subject to respiratory motion. A prototype beam application system for target tracking with the scanned pencil beam has been developed and commissioned.

To facilitate treatment planning for tumors that are subject to organ motion, we have extended our standard treatment planning system TRiP to full 4D functionality. The 4D version of TRiP allows to calculate dose distributions in the presence of motion. Furthermore, for motion mitigation techniques tracking, gating, rescanning, and internal margins optimization of treatment parameters has been implemented. 4D calculations are based on 4D computed tomography data, deformable registration maps, organ motion traces, and beam scanning parameters.

We describe the methods of our 4D treatment planning approach and demonstrate functionality of the system for phantom as well as patient data.

European developments in radiotherapy with beams of large radiobiological effectiveness

This paper reviews the European activities in the field of tumour therapy with beams which have a Radio Biological Effectiveness (RBE) larger than 1. Initially neutron beams have been used. Then charged pions promised better cure rates so that their use was pursued in the framework of the ;Piotron' project at the Paul Scherrer Institute (Switzerland). However both approaches did not meet the expectations and in the 80s the EULIMA project became the flagship of these attempts to improve the effects of the delivery of radiation doses of large RBE with respect to photons, electrons and even protons. The EULIMA ion accelerator was never built and it took more than ten years to see the approval, in Heidelberg and Pavia, of the construction of the HIT and CNAO ;dual' centres for carbon ions and protons. In 2008 they will start treating patients. The developments that brought to these construction projects are described together with the special features of these two facilities. The third European dual centre is being built by Siemens Medical Systems in Marburg, Germany, while other facilities have been approved but not yet fully financed in Wiener Neustadt (Austria), Lyon (France) and Uppsala (Sweden). Finally the collaboration activities of the European Network ENLIGHT are presented together with the recent involvements of European industries in the construction of turn-key dual centres and the development of a new accelerator concept for hadrontherapy, the ;cyclinac'.

Specifying carbon ion doses for radiotherapy: the heidelberg approach

There are currently no guidelines for prescribing and reporting radiation therapy (RT) with ion beams. In this paper an overview over some technical aspects and their implication on ion RT are reported. This includes a discussion of the difference in the treatment planning systems currently used for active and passive beam shaping systems, aspects of patient positioning and target definition and dose prescription. Special emphasis is put on the questions arising from the use of the beam scanning methods in combination with biological treatment plan optimization, which is used in the German heavy ion therapy facility at GSI and will also be introduced at the hospital based facility in Heidelberg. Furthermore, the Heidelberg approach for the clinical dose prescription is compared with the methods developed at HIMAC in Chiba, Japan.

Radiation tolerance of the rat spinal cord after 6 and 18 fractions of photons and carbon ions: Experimental results and clinical implications

PURPOSE

The tolerance of the rat spinal cord to photon and carbon ion irradiations was investigated to determine the relative biologic effectiveness (RBE) of carbon ions ((12)C) in the plateau region and in a 1 cm spread-out Bragg-peak.

METHODS AND MATERIALS

The cranial part of the cervical and thoracic spinal cord of 336 rats was irradiated with 6 or 18 fractions (Fx) of photons or (12)C-ions, respectively. Animals were followed up for 300 days for the onset of paresis grade II and dose-response curves were calculated.

RESULTS

The D(50)-values (dose at 50% complication probability) were 42.9 +/- 0.5 Gy, 62.2 +/- 0.9 Gy (6 and 18 Fx, (12)C-plateau) and 19.2 +/- 0.2 Gy, 17.6 +/- 0.2 Gy (6 and 18 Fx (12)C-peak), respectively. For photons, the D(50)-values were 57.0 +/- 0.7 Gy for 6 and 88.6 +/- 0.7 Gy for 18 Fx. The corresponding RBE-values were 1.33 +/- 0.02, 1.42 +/- 0.02 (6 and 18 Fx, (12)C-plateau) and 2.97 +/- 0.05, 5.04 +/- 0.08 (6 and 18 Fx (12)C-peak), respectively. Including data of a previously performed experiment for 1 and 2 Fx (1) the parameter alpha/beta of the LQ-model was found to be 2.8 +/- 0.4 Gy, 2.1 +/- 0.4 Gy and 37.0 +/- 5.3 Gy for photon-, (12)C-plateau- and (12)C-peak irradiations, respectively.

CONCLUSIONS

Carbon ion irradiations of the spinal cord are significantly more effective in the peak than in the plateau region. The alpha/beta-values indicate a significant fractionation effect only for the plateau irradiations. In the Bragg-peak, the applied RBE-model correctly describes the main features although it generally underestimates the RBE by 25%. In the plateau region, maximum deviations of up to 20% were found. The acquired data contribute significantly to the validation of the applied RBE-model.

Distributions of positron-emitting nuclei in proton and carbon-ion therapy studied with GEANT4

Depth distributions of positron-emitting nuclei in PMMA phantoms are calculated within a Monte Carlo model for heavy-ion therapy (MCHIT) based on the GEANT4 toolkit (version 8.0). The calculated total production rates of (11)C, (10)C and (15)O nuclei are compared with experimental data and with corresponding results of the FLUKA and POSGEN codes. The distributions of e(+) annihilation points are obtained by simulating radioactive decay of unstable nuclei and transporting positrons in the surrounding medium. A finite spatial resolution of the positron emission tomography (PET) is taken into account in a simplified way. Depth distributions of beta(+)-activity as seen by a PET scanner are calculated and compared to available data for PMMA phantoms. The obtained beta(+)-activity profiles are in good agreement with PET data for proton and (12)C beams at energies suitable for particle therapy. The MCHIT capability to predict the beta(+)-activity and dose distributions in tissue-like materials of different chemical composition is demonstrated.