A beam source model for scanned proton beams

A beam source model, i.e. a model for the initial phase space of the beam, for scanned proton beams has been developed. The beam source model is based on parameterized particle sources with characteristics found by fitting towards measured data per individual beam line. A specific aim for this beam source model is to make it applicable to the majority of the various proton beam systems currently available or under development, with the overall purpose to drive dose calculations in proton beam treatment planning. The proton beam phase space is characterized by an energy spectrum, radial and angular distributions and deflections for the non-modulated elementary pencil beam. The beam propagation through the scanning magnets is modelled by applying experimentally determined focal points for each scanning dimension. The radial and angular distribution parameters are deduced from measured two-dimensional fluence distributions of the elementary beam in air. The energy spectrum is extracted from a depth dose distribution for a fixed broad beam scan pattern measured in water. The impact of a multi-slab range shifter for energy modulation is calculated with an own Monte Carlo code taking multiple scattering, energy loss and straggling, non-elastic and elastic nuclear interactions in the slab assembly into account. Measurements for characterization and verification have been performed with the scanning proton beam system at The Svedberg Laboratory in Uppsala. Both in-air fluence patterns and dose points located in a water phantom were used. For verification, dose-in-water was calculated with the Monte Carlo code GEANT 3.21 instead of using a clinical dose engine with approximations of its own. For a set of four individual pencil beams, both with the full energy and range shifted, 96.5% (99.8%) of the tested dose points satisfied the 1%/1 mm (2%/2 mm) gamma criterion.     

Dosimetric consequences of tumour motion due to respiration for a scanned proton beam

A method for simulating spot-scanned delivery to a moving tumour was developed which uses patient-specific image and plan data. The magnitude of interplay effects was investigated for two patient cases under different fractionation and respiratory motion variation scenarios. The use of volumetric rescanning for motion mitigation was also investigated. For different beam arrangements, interplay effects lead to severely distorted dose distributions for a single fraction delivery. Baseline shift variations for single fraction delivery reduced the dose to the clinical target volume (CTV) by up to 14.1 Gy. Fractionated delivery significantly reduced interplay effects; however, local overdosage of 12.3% compared to the statically delivered dose remained for breathing period variations. Variations of the tumour baseline position and respiratory period were found to have the largest influence on target inhomogeneity; these effects were reduced with fractionation. Volumetric rescanning improved the dose homogeneity. For the CTV, underdosage was improved by up to 34% in the CTV and overdosage to the lung was reduced by 6%. Our results confirm that rescanning potentially increases the dose homogeneity; however, it might not sufficiently compensate motion-induced dose distortions. Other motion mitigation techniques may be required to additionally treat lung tumours with scanned proton beams.     

Dose-volume delivery guided proton therapy using beam on-line PET system

Proton therapy is one form of radiotherapy in which the irradiation can be concentrated on a tumor using a scanned or modulated Bragg peak. Therefore, it is very important to evaluate the proton-irradiated volume accurately. The proton-irradiated volume can be confirmed by detection of pair annihilation gamma rays from positron emitter nuclei generated by the target nuclear fragment reaction of irradiated proton nuclei and nuclei in the irradiation target using a positron emission tomography (PET) apparatus, and dose-volume delivery guided proton therapy (DGPT) can thereby be achieved using PET images. In the proton treatment room, a beam ON-LINE PET system (BOLPs) was constructed so that a PET apparatus of the planar-type with a high spatial resolution of about 2 mm was mounted with the field of view covering the isocenter of the beam irradiation system. The position and intensity of activity were measured using the BOLPs immediately after the proton irradiation of a gelatinous water target containing 16O nuclei at different proton irradiation energy levels. The change of the activity-distribution range against the change of the physical range was observed within 2 mm. The experiments of proton irradiation to a rabbit and the imaging of the activity were performed. In addition, the proton beam energy used to irradiate the rabbit was changed. When the beam condition was changed, the difference between the two images acquired from the measurement of the BOLPs was confirmed to clearly identify the proton-irradiated volume.     

Dose delivery error detection by a computer-controlled linear accelerator

A commercial dual photon energy, computer-controlled linear accelerator has a complex collimation and beam delivery system. For accurate dose delivery, six separate motorized elements must be properly positioned for a given beam selection. Experimentally instituted misalignments of the primary electron scattering foils, the primary collimator, the flattening filter, the scattering foil carousel, and the backscatter shield or shutter produced significant dose delivery errors. The ability of the Mylar window monitor chamber to detect these errors was examined for x-ray beams. The fault detection system failed to interrupt dose delivery in a number of situations where the error in dose per monitor unit delivered ranged from -6% to +270% of the calibrated value.     

Quality assurance for a treatment planning system in scanned ion beam therapy

Conformal radiation therapy using dynamic beam delivery systems like scanned ion beams requires concise quality assurance procedures for the complete treatment planning process. For the heavy ion therapy facility at GSI, Darmstadt, a quality assurance program for the treatment planning system (TPS) has been developed. It covers the development and updating of software, data protection and safety, and the application of soft- and hardware. The tests also apply to the geometrical precision of imaging devices and the geometrical and dosimetrical verification of dose distributions in different phantoms. The quality assurance program addresses acceptance and constancy tests of the treatment planning program. Results of the acceptance tests served as a basis for its governmental approval. Two main results of the acceptance tests are representative for the overall performance of the system. (1) The geometrical uncertainty that could be achieved for the target point definition, setup accuracy, field contouring, and field alignment is typically 1.5 mm. The uncertainty for the setup verification using digitally reconstructed radiographs (DRR's) is limited to 2 mm. (2) The mean deviations between measured and planned dose values is 3% for standardized cases in a water phantom and up to 6% for more complicated treatment configurations.     

Adjustment of the lateral and longitudinal size of scanned proton beam spots using a pre-absorber to optimize penumbrae and delivery efficiency

In scanned-beam proton therapy, the beam spot properties, such as the lateral and longitudinal size and the minimum achievable range, are influenced by beam optics, scattering media and drift spaces in the treatment unit. Currently available spot scanning systems offer few options for adjusting these properties. We investigated a method for adjusting the lateral and longitudinal spot size that utilizes downstream plastic pre-absorbers located near a water phantom. The spot size adjustment was characterized using Monte Carlo simulations of a modified commercial scanned-beam treatment head. Our results revealed that the pre-absorbers can be used to reduce the lateral full width at half maximum (FWHM) of dose spots in water by up to 14 mm, and to increase the longitudinal extent from about 1 mm to 5 mm at residual ranges of 4 cm and less. A large factor in manipulating the lateral spot sizes is the drift space between the pre-absorber and the water phantom. Increasing the drift space from 0 cm to 15 cm leads to an increase in the lateral FWHM from 2.15 cm to 2.87 cm, at a water-equivalent depth of 1 cm. These findings suggest that this spot adjustment method may improve the quality of spot-scanned proton treatments.     

Reference dosimetry in a scanned pulsed proton beam using ionisation chambers and a Faraday cup

In order to give the correct dose to a patient, the monitor chamber for a proton scanning system has to be calibrated. As recombination of ion pairs occurs in the monitor chamber, the relation between the number of particles traversing it per time unit and the ionization chamber signal is not linear. A method developed for a scanned pulsed proton beam taking the nonlinear monitor signal into account is described. A vital part of the reference dosimetry procedure is to determine the absorbed dose under reference conditions, which is recommended to be done with an ionization chamber. For a scanned pulsed proton beam, the recombination in the ionization chamber is not negligible and the signal from the ionization chamber has to be corrected. In this work, it is shown that although the pulse length is comparable to the ion transit time the beam can be considered as continuously scanned if the applied high voltage is not too small. Also shown is that the two-voltage formula for a continuous beam is under some conditions applicable for a continuous scanned beam as well.     

Parametrization and application of scatter kernels for modelling scanned proton beam collimator scatter dose

Collimators are routinely used in proton radiotherapy to laterally confine the field and improve the penumbra. Collimator scatter contributes up to 15% of the local dose and is therefore important to include in treatment planning dose calculation. We present a method for reconstruction of the collimator scatter phase space based on the parametrization of pre-calculated scatter kernels. Collimator scatter distributions, generated by the Monte Carlo (MC) package GEANT4.8.2, were scored differential in direction and energy. The distributions were then parametrized so as to enable a fast reconstruction by sampling. MC calculated dose distributions in water based on the parametrized phase space were compared to full MC simulations that included the collimator in the simulation geometry, as well as to experimental data. The experiments were performed at the scanned proton beam line at the The Svedberg Laboratory (TSL) in Uppsala, Sweden. Dose calculations using the parametrization of this work and the full MC for isolated typical cases of collimator scatter were compared by means of the gamma index. The result showed that in total 96.7% (99.3%) of the voxels fulfilled the gamma 2.0%/2.0 mm (3.0%/3.0 mm) criterion. The dose distribution for a collimated field was calculated based on the phase space created by the collimator scatter model incorporated into the generation of the phase space of a scanned proton beam. Comparing these dose distributions to full MC simulations, including particle transport in the MLC, yielded that in total for 18 different collimated fields, 99.1% of the voxels satisfied the gamma 1.0%/1.0 mm criterion and no voxel exceeded the gamma 2.6%/2.6 mm criterion. The dose contribution of collimator scatter along the central axis as predicted by the model showed good agreement with experimental data.     

Dose calculation models for proton treatment planning using a dynamic beam delivery system: an attempt to include density heterogeneity effects in the analytical dose calculation

The gantry for proton radiotherapy at the Paul Scherrer Institute (PSI) is designed specifically for the spot-scanning technique. Use of this technique to its full potential requires dose calculation algorithms which are capable of precisely simulating each scanned beam individually. Different specialized analytical dose calculations have been developed, which attempt to model the effects of density heterogeneities in the patient's body on the dose. Their accuracy has been evaluated by a comparison with Monte Carlo calculated dose distributions in the case of a simple geometrical density interface parallel to the beam and typical anatomical situations. A specialized ray casting model which takes range dilution effects (broadening of the spectrum of proton ranges) into account has been found to produce results of good accuracy. This algorithm can easily be implemented in the iterative optimization procedure used for the calculation of the optimal contribution of each individual scanned pencil beam. In most cases an elemental pencil beam dose calculation has been found to be most accurate. Due to the long computing time, this model is currently used only after the optimization procedure as an alternative method of calculating the dose.     

A prototype beam delivery system for the proton medical accelerator at Loma Linda.

A variable energy proton accelerator was commissioned at Fermi National Accelerator Laboratory for use in cancer treatment at the Loma Linda University Medical Center. The advantages of precise dose localization by proton therapy, while sparing nearby healthy tissue, are well documented [R. R. Wilson, Radiology 47, 487 (1946); M. Wagner, Med. Phys. 9, 749 (1982); M. Goitein and F. Chen, Med. Phys. 10, 831 (1983)]. One of the components of the proton therapy facility is a beam delivery system capable of delivering precise dose distributions to the target volume in the patient. To this end, a prototype beam delivery system was tested during the accelerator's commissioning period. The beam delivery system consisted of a beam spreading device to produce a large, uniform field, a range modulator to generate a spread out Bragg peak (SOBP), and various beam detectors to measure intensity, beam centering, and dose distributions. The beam delivery system provided a uniform proton dose distribution in a cylindrical volume of 20-cm-diam area and 9-cm depth. The dose variations throughout the target volume were found to be less than +/- 5%. Modifications in the range modulator should reduce this considerably. The central axis dose rate in the region of the SOBP was found to be 0.4 cGy/spill with an incident beam intensity of 6.7 x 10(9) protons/spill. With an accelerator repetition rate of 30 spills/min and expected intensity of 2.5 x 10(10) protons/spill for patient treatment, this system can provide 50 cGy/min for a 20-cm-diam field and 9-cm range modulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Respiratory liver motion estimation and its effect on scanned proton beam therapy

Proton therapy with active scanning beam delivery has significant advantages compared to conventional radiotherapy. However, so far only static targets have been treated in this way, since moving targets potentially lead to interplay effects. For 4D treatment planning, information on the target motion is needed to calculate time-resolved dose distributions. In this study, respiratory liver motion has been extracted from 4D CT data using two deformable image registration algorithms. In moderately moving patient cases (mean motion range around 6 mm), the registration error was no more than 3 mm, while it reached 7 mm for larger motions (range around 13 mm). The obtained deformation fields have then been used to calculate different time-resolved 4D treatment plans. Averaged over both motion estimations, interplay effects can increase the D?-D?? value for the clinical target volume (CTV) from 8.8% in a static plan to 23.4% when motion is considered. It has also been found that the different deformable registration algorithms can provide different motion estimations despite performing similarly for the selected landmarks, which in turn can lead to differing 4D dose distributions. Especially for single-field treatments where no motion mitigation is used, a maximum (mean) dose difference (averaged over three cases) of 32.8% (2.9%) can be observed. However, this registration ambiguity-induced uncertainty can be reduced if rescanning is applied or if the treatment plan consists of multiple fields, where the maximum (mean) difference can decrease to 15.2% (0.57%). Our results indicate the necessity to interpret 4D dose distributions for scanned proton therapy with some caution or with error bars to reflect the uncertainties resulting from the motion estimation. On the other hand, rescanning has been found to be an appropriate motion mitigation technique and, furthermore, has been shown to be a robust approach to also deal with these motion estimation uncertainties.     

Dose distributions of a proton beam for eye tumor therapy: hybrid pencil-beam ray-tracing calculations

For the case of eye tumor therapy with protons, improvements are introduced compared to the standard dose calculation which implies straight-line optics and the constant-density assumption for the eye and its surrounding. The progress consists of (i) taking account of the lateral scattering of the protons in tissue by folding the entrance fluence distribution with the pencil beam distribution widening with growing depth in the tissue, (ii) rescaling the spread-out Bragg peak dose distribution in water with the radiological path length calculated voxel by voxel on ray traces through a realistic density matrix for the treatment geometry, yielding a trajectory dependence of the geometrical range. Distributions calculated for some specific situations are compared to measurements and/or standard calculations, and differences to the latter are discussed with respect to the requirements of therapy planning. The most pronounced changes appear for wedges placed in front of the eye, causing additional widening of the lateral falloff. The more accurate prediction of the dose dependence at the field borders is of interest with respect to side effects in the risk organs of the eye.     

Particle selection and beam collimation system for laser-accelerated proton beam therapy

In a laser-accelerated proton therapy system, the initial protons have broad energy and angular distributions, which are not suitable for direct therapeutic applications. A compact particle selection and collimation device is needed to deliver small pencil beams of protons with desired energy spectra. In this work, we characterize a superconducting magnet system that produces a desired magnetic field configuration to spread the protons with different energies and emitting angles for particle selection. Four magnets are set side by side along the beam axis; each is made of NbTi wires which carry a current density of approximately 10(5) A/cm2 at 4.2 K, and produces a magnetic field of approximately 4.4 T in the corresponding region. Collimation is applied to both the entrance and the exit of the particle selection system to generate a desired proton pencil beam. In the middle of the magnet system, where the magnetic field is close to zero, a particle selection collimator allows only the protons with desired energies to pass through for therapy. Simulations of proton transport in the presence of the magnetic field show that the selected protons have successfully refocused on the beam axis after passing through the magnetic field with the optimal magnet system. The energy spread for any given characteristic proton energy has been obtained. It is shown that the energy spread is a function of the magnetic field strength and collimator size and reaches the full width at half maximum of 25 MeV for 230 MeV protons. Dose distributions have also been calculated with the GEANT3 Monte Carlo code to study the dosimetric properties of the laser-accelerated proton beams for radiation therapy applications.     

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.     

A calibration procedure for beam monitors in a scanned beam of heavy charged particles

An international code of practice (CoP) for dosimetry based on standards of absorbed dose to water has recently been published by the IAEA [Technical Report Series No. 398, 2000] (TRS-398). This new CoP includes procedures for proton and heavy ion beams as well as all other beam qualities. In particular it defines reference conditions to which dose measurements should refer to. For proton and ion beams these conditions include dose measurements in the center of all possible modulated Bragg peaks. The recommended reference conditions in general are used also for the calibration of beam monitors. For a dynamic beam delivery system using beam scanning in combination with energy variation, like, e.g., at the German carbon ion radiotherapy facility, this calibration procedure is not appropriate. We have independently developed a different calibration procedure. Similar to the IAEA CoP this procedure is based on the measurement of absorbed dose to water. This is translated in terms of fluence which finally results in an energy-dependent calibration of the beam monitor in units of particle number per monitor unit, which is unique for all treatment fields. In contrast to the IAEA CoP, the reference depth is chosen to be very small. The procedure enables an accurate and reliable determination of calibration factors. In a second step, the calibration is verified by measurements of absorbed dose in various modulated Bragg peaks by comparing measured against calculated doses. The agreement between measured and calculated doses is usually better than 1% for homogeneous fields and the mean deviation for more inhomogeneous treatment fields, as they are used for patient treatments, is within 3%. It is proposed that the CoP in general, and in particular the IAEA TRS-398 should include explicit recommendations for the beam monitor calibration. These recommendations should then distinguish between systems using static and dynamic beams.     

Development and verification of the pulsed scanned proton beam at The Svedberg Laboratory in Uppsala

In this paper we present the recent developments made for the scanning system for proton beams at TSL in Uppsala, showing that this system is now fully functional being able to produce conformal intensity modulated scan patterns with sufficient accuracy. A new control and supervising system handling the beam delivery including the control of the synchrocyclotron and the scanning system is developed and described in detail. A complete dosimetry system with transmission ionization chambers and a multi-wire ionization chamber for monitoring of the beam during scanning has been constructed. The details of the dose monitors and the position sensitive multi-wire ionization chamber are presented in this work. Furthermore, we have established procedures for verification measurements to ensure the quality of the beam and also methods for calibration of the beam monitors and relative and absolute dosimetry for complex scanned beams.     

Monte Carlo dose calculations for spot scanned proton therapy

Density heterogeneities can have a profound effect on dose distributions for proton therapy. Although analytical calculations in homogeneous media are relatively straightforward, the modelling of the propagation of the beam through density heterogeneities can be more problematical. At the Paul Scherrer Institute, an in-house dedicated Monte Carlo (MC) code has been used for over a decade to assess the possible deficiencies of the analytical calculations in patient geometries. The MC code has been optimized for speed, and as such traces primary protons only through the treatment nozzle and patient's CT. Contributions from nuclear interactions are modelled analytically with no tracing of secondary particles. The MC code has been verified against measured data in water and experimental proton radiographs through a heterogeneous anthropomorphic phantom. In comparison to the analytical calculation, the MC code has been applied to both spot scanned and intensity modulated proton therapy plans, and to a number of cases containing titanium metal implants. In summary, MC-based dose calculations could provide an invaluable tool for independently verifying the calculated dose distribution within a patient geometry as part of a comprehensive quality assurance protocol for proton treatment plans.     

Treatment planning for a scanned carbon beam with a modified microdosimetric kinetic model

We describe a method to calculate the relative biological effectiveness in mixed radiation fields of therapeutic ion beams based on the modified microdosimetric kinetic model (modified MKM). In addition, we show the procedure for integrating the modified MKM into a treatment planning system for a scanned carbon beam. With this procedure, the model is fully integrated into our research version of the treatment planning system. To account for the change in radiosensitivity of a cell line, we measured one of the three MKM parameters from a single survival curve of the current cells and used the parameter in biological optimization. Irradiation of human salivary gland tumor cells was performed with a scanned carbon beam in the Heavy Ion Medical Accelerator in Chiba (HIMAC), and we then compared the measured depth-survival curve with the modified MKM predicted survival curve. Good agreement between the two curves proves that the proposed method is a candidate for calculating the biological effects in treatment planning for ion irradiation.     

一种新型质子治疗剂量递送系统的设计研究及模拟验证

目的:针对激光等离子体加速的质子束流特性,设计用于剂量递送的新型紧凑治疗头系统,并通过模拟计算验证该方法的有效性与适用性。方法:基于实验上已实现的激光质子束流参数,利用散射体设计软件NEU（Nozzles with Everything Upstream）进行流线型散射体设计。通过散角选择和能散调制进一步优化剂量递送效率,并利用蒙特卡罗模拟计算软件TOPAS（TOol for PArticle Simulation）及底层的Geant4（GEometry ANd Tracking）计算引擎分析并验证激光质子通过此剂量递送方法后水模体中的剂量分布。结果:在直径6 cm、高5 cm的圆柱形靶区内,深度剂量分布平坦度在±1%以内,横向剂量分布在±3%以内。结论:此剂量递送方法及系统适用于现阶段激光质子束流特性,水模体靶区内剂量递送均匀、高效且稳定。