A beam model for focused proton pencil beams

IntroductionWe present a beam model for Monte Carlo simulations of the IBA pencil beam scanning dedicated nozzle installed at the Skandion Clinic. Within the nozzle, apart from entrance and exit windows and the two ion chambers, the beam traverses vacuum, allowing for a beam that is convergent downstream of the nozzle exit.Materials and methodsWe model the angular, spatial and energy distributions of the beam phase space at the nozzle exit with single Gaussians, controlled by seven energy dependent parameters. The parameters were determined from measured profiles and depth dose distributions. Verification of the beam model was done by comparing measured and GATE acquired relative dose distributions, using plan specific log files from the machine to specify beam spot positions and energy.ResultsGATE-based simulations with the acquired beam model could accurately reproduce the measured data. The gamma index analysis comparing simulated and measured dose distributions resulted in >95% global gamma index pass rates (3%/2 mm) for all depths.ConclusionThe developed beam model was found to be sufficiently accurate for use with GATE e.g. for applications in quality assurance (QA) or patient motion studies with the IBA pencil beam scanning dedicated nozzles.     

Intelligence-guided beam angle optimization in treatment planning of intensity-modulated radiation therapy

An intelligence guided approach based on fuzzy inference system (FIS) was proposed to automate beam angle optimization in treatment planning of intensity-modulated radiation therapy (IMRT). The model of FIS is built on inference rules in describing the relationship between dose quality of IMRT plan and irradiated region of anatomical structure. Dose quality of IMRT plan is quantified by the difference between calculated and constraint doses of the anatomical structures in an IMRT plan. Irradiated region of anatomical structure is characterized by the metric, covered region of interest, which is the region of an anatomical structure under radiation field while beam’s eye-view is conform to target volume. Initially, an IMRT plan is created with a single beam. The dose difference is calculated for the input of FIS and the output of FIS is obtained with processing of fuzzy inference. Later, a set of candidate beams is generated for replacing the current beam. This process continues until no candidate beams is found. Then the next beam is added to the IMRT plan and optimized in the same way as the previous beam. The new beam keeps adding to the IMRT plan until the allowed beam number is reached. Two spinal cases were investigated in this study. The preliminary results show that dose quality of IMRT plans achieved by this approach is better than those achieved by the default approach with equally spaced beam setting. It is effective to find the optimal beam combination of IMRT plan with the intelligence-guided approach.     

Influence of beam incidence and irradiation parameters on stray neutron doses to healthy organs of pediatric patients treated for an intracranial tumor with passive scattering proton therapy

PurposeIn scattering proton therapy, the beam incidence, i.e. the patient’s orientation with respect to the beam axis, can significantly influence stray neutron doses although it is almost not documented in the literature.MethodsMCNPX calculations were carried out to estimate stray neutron doses to 25 healthy organs of a 10-year-old female phantom treated for an intracranial tumor. Two beam incidences were considered in this article, namely a superior (SUP) field and a right lateral (RLAT) field. For both fields, a parametric study was performed varying proton beam energy, modulation width, collimator aperture and thickness, compensator thickness and air gap size.ResultsUsing a standard beam line configuration for a craniopharyngioma treatment, neutron absorbed doses per therapeutic dose of 63 μGy Gy⁻¹ and 149 μGy Gy⁻¹ were found at the heart for the SUP and the RLAT fields, respectively. This dose discrepancy was explained by the different patient’s orientations leading to changes in the distance between organs and the final collimator where external neutrons are mainly produced. Moreover, investigations on neutron spectral fluence at the heart showed that the number of neutrons was 2.5 times higher for the RLAT field compared against the SUP field. Finally, the influence of some irradiation parameters on neutron doses was found to be different according to the beam incidence.ConclusionBeam incidence was thus found to induce large variations in stray neutron doses, proving that this parameter could be optimized to enhance the radiation protection of the patient.     

On the parametrization of lateral dose profiles in proton radiation therapy

PurposeThe accurate evaluation of the lateral dose profile is an important issue in the field of proton radiation therapy. The beam spread, due to Multiple Coulomb Scattering (MCS), is described by the Molière's theory. To take into account also the contribution of nuclear interactions, modern Treatment Planning Systems (TPSs) generally approximate the dose profiles by a sum of Gaussian functions. In this paper we have compared different parametrizations for the lateral dose profile of protons in water for therapeutical energies: the goal is to improve the performances of the actual treatment planning.MethodsWe have simulated typical dose profiles at the CNAO (Centro Nazionale di Adroterapia Oncologica) beamline with the FLUKA code and validated them with data taken at CNAO considering different energies and depths. We then performed best fits of the lateral dose profiles for different functions using ROOT and MINUIT.ResultsThe accuracy of the best fits was analyzed by evaluating the reduced χ², the number of free parameters of the functions and the calculation time. The best results were obtained with the triple Gaussian and double Gaussian Lorentz–Cauchy functions which have 6 parameters, but good results were also obtained with the so called Gauss–Rutherford function which has only 4 parameters.ConclusionsThe comparison of the studied functions with accurate and validated Monte Carlo calculations and with experimental data from CNAO lead us to propose an original parametrization, the Gauss–Rutherford function, to describe the lateral dose profiles of proton beams.     

Technical challenges for FLASH proton therapy

There is growing interest in the radiotherapy community in the application of FLASH radiotherapy, wherein the dose is delivered to the entire treatment volume in less than a second. Early pre-clinical evidence suggests that these extremely high dose rates provide significant sparing of healthy tissue compared to conventional radiotherapy without reducing the damage to cancerous cells. This interest has been reflected in the proton therapy community, with early tests indicating that the FLASH effect is also present with high dose rate proton irradiation.In order to deliver clinically relevant doses at FLASH dose rates significant technical hurdles must be overcome in the accelerator technology before FLASH proton therapy can be realised. Of these challenges, increasing the average current from the present clinical range of 1–10 nA to in excess of 100 nA is at least feasible with existing technology, while the necessity for rapid energy adjustment on the order of a few milliseconds is much more challenging, particularly for synchrotron-based systems. However, the greatest challenge is to implement full pencil beam scanning, where scanning speeds 2 orders of magnitude faster than the existing state-of-the-art will be necessary, along with similar improvements in the speed and accuracy of associated dosimetry. Hybrid systems utilising 3D-printed patient specific range modulators present the most likely route to clinical delivery. However, to correctly adapt and develop existing technology to meet the challenges of FLASH, more pre-clinical studies are needed to properly establish the beam parameters that are necessary to produce the FLASH effect.     

Preliminary analysis for integration of spot-scanning proton beam therapy and real-time imaging and gating

PurposeSpot-scanning proton beam therapy (PBT) can create good dose distribution for static targets. However, there exists larger uncertainty for tumors that move due to respiration, bowel gas or other internal circumstances within the patients. We have developed a real-time tumor-tracking radiation therapy (RTRT) system that uses an X-ray linear accelerator gated to the motion of internal fiducial markers introduced in the late 1990s. Relying on more than 10 years of clinical experience and big log data, we established a real-time image gated proton beam therapy system dedicated to spot scanning.Materials and methodsUsing log data and clinical outcomes derived from the clinical usage of the RTRT system since 1999, we have established a library to be used for in-house simulation for tumor targeting and evaluation. Factors considered to be the dominant causes of the interplay effects related to the spot scanning dedicated proton therapy system are listed and discussed.Results/conclusionsTotal facility design, synchrotron operation cycle, and gating windows were listed as the important factors causing the interplay effects contributing to the irradiation time and motion-induced dose error. Fiducial markers that we have developed and used for the RTRT in X-ray therapy were suggested to have the capacity to improve dose distribution. Accumulated internal motion data in the RTRT system enable us to improve the operation and function of a Spot-scanning proton beam therapy (SSPT) system. A real-time-image gated SSPT system can increase accuracy for treating moving tumors. The system will start clinical service in early 2014.     

Beam characterisation studies of the 62 MeV proton therapy beamline at the Clatterbridge Cancer Centre

The Clatterbridge Cancer Centre (CCC) in the United Kingdom is the world’s first hospital proton beam therapy facility, providing treatment for ocular cancers since 1989. A 62 MeV beam of protons is produced by a Scanditronix cyclotron and transported through a passive delivery system. In addition to the long history of clinical use, the facility supports a wide programme of experimental work and as such, an accurate and reliable simulation model of the treatment beamline is highly valuable. However, as the facility has seen several changes to the accelerator and beamline over the years, a comprehensive study of the CCC beam dynamics is needed to firstly examine the beam optics. An extensive analysis was required to overcome facility related constraints to determine fundamental beamline parameters and define an optical lattice written with the Methodical Accelerator Design (MAD-X) and the particle tracking Beam Delivery Simulation (BDSIM) code. An optimised case is presented and simulated results of the optical functions, beam distribution, losses and the transverse rms beam sizes along the beamline are discussed. Corresponding optical and beam information was used in TOPAS to simulate transverse beam profiles and compared to EBT3 film measurements. We provide an overview of the magnetic components, beam transport, cyclotron, beam and treatment related parameters necessary for the development of a present day optical model of the facility. This work represents the first comprehensive study of the CCC facility to date, as a basis to determine input beam parameters to accurately simulate and completely characterise the beamline.     

Geometry optimisation of graphite energy degrader for proton therapy

Introduction: Cyclotron-based proton therapy facilities use an energy degrader of variable thickness to deliver beams of the different energies required by a patient treatment plan; scattering and straggling in the degrader give rise to an inherent emittance increase and subsequent particle loss in the downstream energy-selection system (ESS). Here we study alternative graphite degrader geometries and examine with Monte-Carlo simulations the induced emittance growth and consequent particle transmission.Methods: We examined the conventional multiple-wedge degrader used in the Paul Scherrer Institute PROSCAN proton therapy system, the equivalent parallel-sided degrader, and a single block degrader of equivalent thickness. G4Beamline Monte-Carlo tracking of protons was benchmarked against measurements of the existing degrader for proton energies from 75 to 230 MeV, and used to validate simulations of the alternative geometries.Results: Using a careful calculation of the beam emittance growth, we determined that a single-block degrader placed close to the collimators of the ESS is expected to deliver significantly larger transmission, up to 17% larger at 150 MeV. At the lowest deliverable of 75 MeV there is still a clear improvement in beam transmission.Conclusions: Whilst dose rates are not presently limited on the PROSCAN system at higher energies, a single-block degrader offers the ability to access either lower energies for treatment or a larger dose rate at 75 MeV in case transmission optimisation is desired. Single-block degraders should be considered for the delivery of low-energy protons from a cyclotron-based particle therapy system.     

FLUKA simulation of target fragmentation in proton therapy

In proton therapy, secondary fragments are created in nuclear interactions of the beam with the target nuclei. The secondary fragments have low kinetic energies and high atomic numbers as compared to primary protons. Fragments have a high LET and deposit all their energy close to the generation point. For their characteristics, secondary fragments can alter the dose distribution and lead to an increase of RBE for the same delivered physical dose. Moreover, the radiobiological impact of target fragmentation is significant mostly in the region before the Bragg peak, where generally healthy tissues are present, and immediately after Bragg peak. Considering the high biological impact of those particles, especially in the case of healthy tissues or organs at risk, the inclusion of target fragmentation processes in the dose calculation of a treatment planning system can be relevant to improve the treatment accuracy and for this reason it is one of the major tasks of the MoVe IT project.In this study, Monte Carlo simulations were employed to fully characterize the mixed radiation field generated by target fragmentation in proton therapy. The dose averaged LET has been evaluated in case of a Spread Out Bragg Peak (SOBP). Starting from LET distribution, RBE has been evaluated with two different phenomenological models. In order to characterize the mixed radiation field, the production cross section has been evaluated by means of the FLUKA code. The future development of present work is to generate a MC database of fragments fluence to be included in TPS.     

Accurate proton treatment planning for pencil beam crossing titanium fixation implants

PurposeTo present a planning strategy for proton pencil-beam scanning when titanium implants need to be crossed by the beam.MethodsWe addressed three issues: the implementation of a CT calibration curve to assign to titanium the correct stopping power; the effect of artefacts on CT images and their reduction by a dedicated algorithm; the differences in dose computation depending on the dose engine, pencil-beam vs Monte-Carlo algorithms. We performed measurement tests on a simple cylinder phantom and on a real implant. These phantoms were irradiated with three geometries (single spots, uniform mono-energetic layer and uniform box), measuring the exit dose either by radio-chromic film or multi-layer ionization chamber. The procedure was then applied on two patients treated for chordoma.ResultsWe had to set in the calibration curve a mass density equal to 4.37 g/cm³ to saturated Hounsfield Units, in order to have the correct stopping power assigned to titanium in TPS. CT artefact reduction algorithm allowed a better reconstruction of the shape and size of the implant. Monte-Carlo resulted accurate in computing the dose distribution whereas the pencil-beam algorithm failed due to sharp density interfaces between titanium and the surrounding material. Finally, the treatment plans obtained on two patients showed the impact of the dose engine algorithm, with 10–20% differences between pencil-beam and Monte-Carlo in small regions distally to the titanium screws.ConclusionThe described combination of CT calibration, artefacts reduction and Monte-Carlo computation provides a reliable methodology to compute dose in patients with titanium implants.     

A GATE/Geant4 beam model for the MedAustron non-isocentric proton treatment plans quality assurance

PurposeTo present a reference Monte Carlo (MC) beam model developed in GATE/Geant4 for the MedAustron fixed beam line. The proposed model includes an absolute dose calibration in Dose-Area-Product (DAP) and it has been validated within clinical tolerances for non-isocentric treatments as routinely performed at MedAustron.Material and MethodsThe proton beam model was parametrized at the nozzle entrance considering optic and energy properties of the pencil beam. The calibration in terms of absorbed dose to water was performed exploiting the relationship between number of particles and DAP by mean of a recent formalism. Typical longitudinal dose distribution parameters (range, distal penumbra and modulation) and transverse dose distribution parameters (spot sizes, field sizes and lateral penumbra) were evaluated. The model was validated in water, considering regular-shaped dose distribution as well as clinical plans delivered in non-isocentric conditions.ResultsSimulated parameters agree with measurements within the clinical requirements at different air gaps. The agreement of distal and longitudinal dose distribution parameters is mostly better than 1 mm. The dose difference in reference conditions and for 3D dose delivery in water is within 0.5% and 1.2%, respectively. Clinical plans were reproduced within 3%.ConclusionA full nozzle beam model for active scanning proton pencil beam is described using GATE/Geant4. Absolute dose calibration based on DAP formalism was implemented. The beam model is fully validated in water over a wide range of clinical scenarios and will be inserted as a reference tool for research and for independent dose calculation in the clinical routine.     

Multi-strip silicon sensors for beam array monitoring in micro-beam radiation therapy

We present here the latest results from tests performed at the ESRF ID17 and ID21 beamlines for the characterization of novel beam monitors for Microbeam Radiation Therapy (MRT), which is currently being implemented at ID17. MRT aims at treating solid tumors by exploiting an array of evenly spaced microbeams, having an energy spectrum distributed between 27 and 600 keV and peaking at 100 keV. Given the high instantaneous dose delivered (up to 20 kGy/s), the position and the intensity of the microbeams has to be precisely and instantly monitored. For this purpose, we developed dedicated silicon microstrip beam monitors. We have successfully characterized them, both with a microbeam array at ID17, and a submicron scanning beam at ID21. We present here the latest results obtained in recent tests along with an outlook on future developments.     

Simulated annealing applied to IMRT beam angle optimization: A computational study

Electing irradiation directions to use in IMRT treatments is one of the first decisions to make in treatment planning. Beam angle optimization (BAO) is a difficult problem to tackle from the mathematical optimization point of view. It is highly non-convex, and optimization approaches based on gradient descent methods will probably get trapped in one of the many local minima. Simulated Annealing (SA) is a local search probabilistic procedure that is known to be able to deal with multimodal problems. SA for BAO was retrospectively applied to ten clinical examples of treated cases of head-and neck tumors signalized as complex cases where proper target coverage and organ sparing proved difficult to achieve. The number of directions to use was considered fixed and equal to 5 or 7. It is shown that SA can lead to solutions that significantly improve organ sparing, even considering a reduced number of angles, without jeopardizing tumor coverage.     

Beamline characterization of a dielectric-filled reentrant cavity resonator as beam current monitor for a medical cyclotron facility

At PSI (Paul Scherrer Institute), Switzerland, a superconducting cyclotron called “COMET” delivers proton beam of 250 MeV pulsed at 72.85 MHz for proton radiation therapy. Measuring proton beam currents (0.1–10nA) is of crucial importance for the treatment safety and is usually performed with invasive monitors such as ionisation chambers (ICs) which degrade the beam quality. A new non-invasive beam current monitor working on the principle of electromagnetic resonance is built to replace ICs in order to preserve the beam quality delivered. The fundamental resonance frequency of the resonator is tuned to 145.7 MHz, which is the second harmonic of the pulse rate, so it provides signals proportional to beam current. The cavity resonator installed in the beamline of the COMET is designed to measure beam currents for the energy range 238–70 MeV. Good agreement is reached between expected and measured resonator response over the energy range of interest. The resonator can deliver beam current information down to 0.15 nA for a measurement integration time of 1 s. The cavity resonator might be applied serving as a safety monitor to trigger interlocks within the existing domain of proton radiation therapy. Low beam currents limit the abilities to detect sufficiently, however, with the potential implementation of FLASH proton therapy, the application of cavity resonator as an online beam-monitoring device is feasible.     

Wax boluses and accuracy of EBT and RTQA radiochromic film detectors in radiotherapy with the JINR Phasotron proton beam

Aim

To present the results obtained using radiochromic films EBT and RTQA 1010P for the reconstruction the dose distributions for targets irradiated by proton beam and modified by wax boluses.

Background

In Medico-Technical Complex at the Joint Institute for Nuclear Research in Dubna implemented technology of wax boluses.

Materials and methods

Wax boluses are easier to make and they give better dose distributions than boluses made from modeling clay previously used at our center. We irradiated two imaginary targets, one shaped as a cylinder and the other one as two cuboids. The evaluated calibration curve was used for calculation of the dose distributions measured by the EBT and RTQA radiochromic film. In both cases, the measured dose distributions were compared to the dose distributions calculated by the treatment planning system (TPS). We also compared dose distributions using three different conformity indices at a 95% isodose.

Results

Better target coverage and better compliance of measurements (semiconductor detectors and radiochromic films) with calculated doses was obtained for cylindrical target than for cuboidal target. The 95% isodose covered well the tumor for both target shapes, while for cuboidal target larger volume around the target received therapeutic dose, due to the complicated target shape. The use wax boluses provided to be effective tool in modifying proton beam to achieve appropriate shape of isodose distribution.

Conclusion

EBT film yielded the best visual matching. Both EBT and RTQA films confirmed good conformity between calculated and measured doses, thus confirming that wax boluses used to modify the proton beam resulted in good dose distributions.     

Cancer risk after breast proton therapy considering physiological and radiobiological uncertainties

BackgroundThe reduced normal tissue dose burden from protons can reduce the risk of second cancer for breast cancer patients. Breathing motion and the impact of variable relative biological effectiveness (RBE) are however concerns for proton dose distributions. This study aimed to quantify the impact of these factors on risk predictions from proton and photon therapy.Materials and methodsTwelve patients were planned in free breathing with protons and photons to deliver 50 Gy (RBE) in 25 fractions (assuming RBE = 1.1 for protons) to the left breast. Second cancer risk was evaluated with several models for the lungs, contralateral breast, heart and esophagus as organs at risk (OARs). Plans were recalculated on CT-datasets acquired in extreme phases to account for breathing motion. Proton plans were also recalculated assuming variable RBE for a range of radiobiological parameters.ResultsThe OARs received substantially lower doses from protons compared to photons. The highest risks were for the lungs (average second cancer risks of 0.31% and 0.12% from photon and proton plans, respectively). The reduced risk with protons was maintained, even when breathing and/or RBE variation were taken into account. Furthermore, while the total risks from the photon plans were seen to increase with the integral dose, no such correlation was observed for the proton plans.ConclusionsProtons have an advantage over the photons with respect to the induction of cancer. Uncertainties in physiological movements and radiobiological parameters affected the absolute risk estimates, but not the general trend of lower risk associated with proton therapy.     

Validation and practical implementation of seated position radiotherapy in a commercial TPS for proton therapy

PurposeThis work aims to validate new 6D couch features and their implementation for seated radiotherapy in RayStation (RS) treatment planning system (TPS).Materials and methodsIn RS TPS, new 6D couch features are (i) chair support device, (ii) patient treatment option of “Sitting: face towards the front of the chair”, and (iii) patient support pitch and roll capabilities. The validation of pitch and roll was performed by comparing TPS generated DRRs with planar x-rays. Dosimetric tests through measurement by 2D ion chamber array were performed for beams created with varied scanning and treatment orientation and 6D couch rotations. For the implementation of 6D couch features for treatments in a seated position, the TPS and oncology information system (Mosaiq) settings are described for a commercial chair. An end-to-end test using an anthropomorphic phantom was performed to test the complete workflow from simulation to treatment delivery.ResultsThe 6D couch features were found to have a consistent implementation that met IEC 61712 standard. The DRRs were found to have an acceptable agreement with planar x-rays based on visual inspection. For dose map comparison between measured and calculated, the gamma index analysis for all the beams was >95% at a 3% dose-difference and 3 mm distance-to-agreement tolerances. For an end-to end-testing, the phantom was successfully set up at isocenter in the seated position and treatment was delivered.ConclusionsChair-based treatments in a seated position can be implemented in RayStation through the use of newly released 6D couch features.