Characterization and validation of a Monte Carlo code for independent dose calculation in proton therapy treatments with pencil beam scanning

Evaluation of monte carlo to support commissioning of the treatment planning system of new pencil beam scanning proton therapy facilities

Objective. To demonstrate the potential of Monte Carlo (MC) to support the resource-intensive measurements that comprise the commissioning of the treatment planning system (TPS) of new proton therapy facilities.Approach. Beam models of a pencil beam scanning system (Varian ProBeam) were developed in GATE (v8.2), Eclipse proton convolution superposition algorithm (v16.1, Varian Medical Systems) and RayStation MC (v12.0.100.0, RaySearch Laboratories), using the beam commissioning data. All models were first benchmarked against the same commissioning data and validated on seven spread-out Bragg peak (SOBP) plans. Then, we explored the use of MC to optimise dose calculation parameters, fully understand the performance and limitations of TPS in homogeneous fields and support the development of patient-specific quality assurance (PSQA) processes. We compared the dose calculations of the TPSs against measurements (DDTPSvs.Meas.) or GATE (DDTPSvs.GATE) for an extensive set of plans of varying complexity. This included homogeneous plans with varying field-size, range, width, and range-shifters (RSs) (n= 46) and PSQA plans for different anatomical sites (n= 11).Main results. The three beam models showed good agreement against the commissioning data, and dose differences of 3.5% and 5% were found for SOBP plans without and with RSs, respectively. DDTPSvs.Meas.and DDTPSvs.GATEwere correlated in most scenarios. In homogeneous fields the Pearson's correlation coefficient was 0.92 and 0.68 for Eclipse and RayStation, respectively. The standard deviation of the differences between GATE and measurements (±0.5% for homogeneous and ±0.8% for PSQA plans) was applied as tolerance when comparing TPSs with GATE. 72% and 60% of the plans were within the GATE predicted dose difference for both TPSs, for homogeneous and PSQA cases, respectively.Significance. Developing and validating a MC beam model early on into the commissioning of new proton therapy facilities can support the validation of the TPS and facilitate comprehensive investigation of its capabilities and limitations.

A systematic approach for calibrating a Monte Carlo code to a treatment planning system for obtaining dose, LET, variable proton RBE and out-of-field dose

Objective. While integration of variable relative biological effectiveness (RBE) has not reached full clinical implementation, the importance of having the ability to recalculate proton treatment plans in a flexible, dedicated Monte Carlo (MC) code cannot be understated . Here we provide a step-wise method for calibrating dose from a MC code to a treatment planning system (TPS), to obtain required parameters for calculating linear energy transfer (LET), variable RBE and in general enabling clinical realistic research studies beyond the capabilities of a TPS.Approach. Initially, Pristine Bragg peaks (PBP) were calculated in both the Eclipse TPS and the FLUKA MC code. A rearranged Bortfeld energy-range relation was applied to the initial energy of the beam to fine-tune the range of the MC code at 80% dose level distal to the PBP. The energy spread was adapted by dividing the TPS range by the MC range for dose level 80%-20% distal to the PBP. Density and relative proton stopping power were adjusted by comparing the TPS and MC for different Hounsfield units. To find the relationship of dose per primary particle from the MC to dose per monitor unit in the TPS, integration was applied to the area of the Bragg curve. The calibration was validated for spread-out Bragg peaks (SOBP) in water and patient treatment plans. Following the validation, variable RBE were calculated using established models.Main results.The PBPs ranges were within ±0.3mm threshold, and a maximum of 5.5% difference for the SOBPs was observed. The patient validation showed excellent dose agreement between the TPS and MC, with the greatest differences for the lung tumor patient.Significance. Aprocedure for calibrating a MC code to a TPS was developed and validated. The procedure enables MC-based calculation of dose, LET, variable RBE, advanced (secondary) particle tracking and more from treatment plans.

Development and validation of an optimal GATE model for proton pencil-beam scanning delivery

OBJECTIVE

To develop and validate a versatile Monte Carlo (MC)-based dose calculation engine to support MC-based dose verification of treatment planning systems (TPSs) and quality assurance (QA) workflows in proton therapy.

METHODS

The GATE MC toolkit was used to simulate a fixed horizontal active scan-based proton beam delivery (SIEMENS IONTRIS). Within the nozzle, two primary and secondary dose monitors have been designed to enable the comparison of the accuracy of dose estimation from MC simulations with respect to physical QA measurements. The developed beam model was validated against a series of commissioning measurements using pinpoint chambers and 2D array ionization chambers (IC) in terms of lateral profiles and depth dose distributions. Furthermore, beam delivery module and treatment planning has been validated against the literature deploying various clinical test cases of the AAPM TG-119 (c-shape phantom) and a prostate patient.

RESULTS

MC simulations showed excellent agreement with measurements in the lateral depth-dose parameters and spread-out Bragg peak (SOBP) characteristics within a maximum relative error of 0.95 mm in range, 1.83% in entrance to peak ratio, 0.27% in mean point-to-point dose difference, and 0.32% in peak location. The mean relative absolute difference between MC simulations and measurements in terms of absorbed dose in the SOBP region was 0.93% ± 0.88%. Clinical phantom studies showed a good agreement compared to research TPS (relative error for TG-119 planning target volume PTV-D95 ∼ 1.8%; and for prostate PTV-D95 ∼ -0.6%).

CONCLUSION

We successfully developed a MC model for the pencil beam scanning system, which appears reliable for dose verification of the TPS in combination with QA information, prior to patient treatment.

Extended in-field and out-of-field validation of a compact Monte Carlo model of an IBA PROTEUS®ONE proton beam in TOPAS/GEANT4

Objective:This study evaluates a compact Monte Carlo (MC) model of a pencil beam scanning clinical proton beam using TOPAS to estimate the dose out-of-field (OOF). Compact modelling means that the model starts from a pristine proton beam at the nozzle exit, customised based on acceptance and commissioning data, instead of modelling the full treatment head and room.Approach: First, in-field validation tests were performed. Then, the OOF dose was validated in an RW3 phantom with bubble detectors for personal neutron dosimetry (measuring the neutron dose equivalent) and thermoluminiescent detectors (measuring the absorbed dose by protons and gammas). Measurements were performed at 15 and 35 cm from the distal edge of the field for five different irradiation plans, covering different beam orientations, proton energies and a 40 mm range shifter. TOPAS simulations were performed with QGSP Binary Cascade HP (BIC) and QGSP Bertini HP (Bertini) hadron physics lists.Main results: In-field validation shows that MC simulations agree with point dose measurements within -2.5 % and +1.5 % at locations on- and off-axis and before, in and after the Bragg peak or plateau. The gamma passing rate 2%/3mm of four simulated treatment plans compared to the dose distribution calculated by the TPS exceeds 97 % agreement score. OOF dose simulations showed an average overestimation of 27 % of the neutron dose equivalent for the BIC hadron physics list and an average underestimation of 20 % for the Bertini hadron physics list. The simulated absorbed dose of protons and gammas showed a systematic underestimation which was on average 21 % and 51 % for BIC and Bertini respectively.Significance: Our study demonstrates that a compact MC model can reliably produce in-field data, while out-of-field dose data are within the uncertainties of the detector systems and MC simulations nuclear models, and do so with shorter modelling and faster calculation time.

A Geant4 based simulation platform of the HollandPTC R&D proton beamline for radiobiological studies

A Geant4 based simulation platform of the Holland Proton Therapy Centre (HollandPTC, Netherlands) R&D beamline (G4HPTC-R&D) was developed to enable the planning, optimisation and advanced dosimetry for radiobiological studies. It implemented a six parameter non-symmetrical Gaussian pencil beam surrogate model to simulate the R&D beamline in both a pencil beam and passively scattered field configuration. Three different experimental proton datasets (70 MeV, 150 MeV, and 240 MeV) of the pencil beam envelope evolution in free air and depth-dose profiles in water were used to develop a set of individual parameter surrogate functions to enable the modelling of the non-symmetrical Gaussian pencil beam properties with only the ProBeam isochronous cyclotron mean extraction proton energy as input. This refined beam model was then benchmarked with respect to three independent experimental datasets of the R&D beamline operating in both a pencil beam configuration at 120 and 200 MeV, and passively scattered field configuration at 150 MeV. It was shown that the G4HPTC-R&D simulation platform can reproduce the pencil beam envelope evolution in free air and depth-dose profiles to within an accuracy on the order of ±5% for all tested energies, and that it was able to reproduce the 150 MeV passively scattered field to the specifications need for clinical and radiobiological applications.

Commissioning dose computation model for proton source in pencil beam scanning therapy by convolution neural networks

Objective. Proton source model commissioning (PSMC) is critical for ensuring accurate dose calculation in pencil beam scanning (PBS) proton therapy using Monte Carlo (MC) simulations. PSMC aims to match the calculated dose to the delivered dose. However, commissioning the 'nominal energy' and 'energy spread' parameters in PSMC can be challenging, as these parameters cannot be directly obtained from solving equations. To efficiently and accurately commission the nominal energy and energy spread in a proton source model, we developed a convolution neural network (CNN) named 'PSMC-Net.'Methods. The PSMC-Net was trained separately for 33 energies (E, 70-225 MeV with a step of 5 MeV plus 226.09 MeV). For eachE, a dataset was generated consisting of 150 source model parameters (15 nominal energies ∈ [E,E+ 1.5 MeV], ten spreads ∈ [0, 1]) and the corresponding 150 MC integrated depth doses (IDDs). Of these 150 data pairs, 130 were used for training the network, 10 for validation, and 10 for testing.Results. The source model, built by 33 measured IDDs and 33 PSMC-Nets (cost 0.01 s), was used to compute the MC IDDs. The gamma passing rate (GPRs, 1 mm/1%) between MC and measured IDDs was 99.91 ± 0.12%. However, when no commissioning was made, the corresponding GPR was reduced to 54.11 ± 22.36%, highlighting the tremendous significance of our CNN commissioning method. Furthermore, the MC doses of a spread-out Bragg peak and 20 patient PBS plans were also calculated, and average 3D GPRs (2 mm/2% with a 10% threshold) were 99.89% and 99.96 ± 0.06%, respectively.Significance. We proposed a nova commissioning method of the proton source model using CNNs, which made the PSMC process easy, efficient, and accurate.

Does variable RBE affect toxicity risks for mediastinal lymphoma patients? NTCP-based evaluation after proton therapy treatment

INTRODUCTION

Mediastinal lymphoma (ML) is a solid malignancy affecting young patients. Modern combined treatments allow obtaining good survival probability, together with a long life expectancy, and therefore with the need to minimize treatment-related toxicities. We quantified the expected toxicity risk for different organs and endpoints in ML patients treated with intensity-modulated proton therapy (IMPT) at our centre, accounting also for uncertainties related to variable RBE.

METHODS

Treatment plans for ten ML patients were recalculated with a TOPAS-based Monte Carlo code, thus retrieving information on LET and allowing the estimation of variable RBE. Published NTCP models were adopted to calculate the toxicity risk for hypothyroidism, heart valve defects, coronary heart disease and lung fibrosis. NTCP was calculated assuming both constant (i.e. 1.1) and variable RBE. The uncertainty associated with individual radiosensitivity was estimated by random sampling α/β values before RBE evaluation.

RESULTS

Variable RBE had a minor impact on hypothyroidism risk for 7 patients, while it led to significant increase for the remaining three (+24% risk maximum increase). Lung fibrosis was slightly affected by variable RBE, with a maximum increase of ≅ 1%. This was similar for heart valve dysfunction, with the exception of one patient showing an about 10% risk increase, which could be explained by means of large heart volume and D₁ increase.

DISCUSSION

The use of NTCP models allows for identifying those patients associated with a higher toxicity risk. For those patients, it might be worth including variable RBE in plan evaluation.

Commissioning a beam line for MR-guided particle therapy assisted by in silico methods

BACKGROUND

Radiation therapy is continuously moving towards more precise dose delivery. The combination of online MR imaging and particle therapy, for example, radiation therapy using protons or carbon ions, could enable the next level of precision in radiotherapy. In particle therapy, research towards a combination of MR and particle therapy is well underway, but still far from clinical systems. The combination of high magnetic fields with particle therapy delivery poses several challenges for treatment planning, treatment workflow, dose delivery, and dosimetry.

PURPOSE

To present a workflow for commissioning of a light ion beam line with an integrated dipole magnet to perform MR-guided particle therapy (MRgPT) research, producing not only basic beam data but also magnetic field maps for accurate dose calculation. Accurate dose calculation in magnetic field environments requires high-quality magnetic field maps to compensate for magnetic-field-dependent trajectory changes and dose perturbations.

METHODS

The research beam line at MedAustron was coupled with a resistive dipole magnet positioned at the isocenter. Beam data were measured for proton and carbon ions with and without an applied magnetic field of 1 T. Laterally integrated depth-dose curves (IDC) as well as beam profiles were measured in water while beam trajectories were measured in air. Based on manufacturer data, an in silico model of the magnet was created, allowing to extract high-quality 3D magnetic field data. An existing GATE/Geant4 Monte Carlo (MC) model of the beam line was extended with the generated magnetic field data and benchmarked against experimental data.

RESULTS

A 3D magnetic field volume covering fringe fields until 50 mT was found to be sufficient for an accurate beam trajectory modeling. The effect on particle range retraction was found to be 2.3 and 0.3 mm for protons and carbon ions, respectively. Measured lateral beam offsets in water agreed within 0.4 and -0.5 mm with MC simulations for protons and carbon ions, respectively. Experimentally determined in-air beam trajectories agreed within 0.4 mm in the homogeneous magnetic field area.

CONCLUSION

The presented approach based on in silico modeling and measurements allows to commission a beam line for MRgPT while providing benchmarking data for the magnetic field modeling, required for state-of-the art dose calculation methods.

Development and validation of an automatic commissioning tool for the Monte Carlo dose engine in myQA iON

Independent dose verification with Monte Carlo (MC) simulations is an important feature of proton therapy quality assurance (QA). However, clinical integration of such tools often generates an additional and complex workload for medical physicists. The preparation of the necessary clinical inputs, such as the machine beam model, should therefore be automated. In this work, a methodology for automatic MC commissioning has been devised, validated, and developed into a MATLAB tool for the users of myQA iON, the recent QA platform of IBA Dosimetry. With this workflow, all necessary parameters can easily be tuned using dedicated optimization methods. For the geometrical beam parameters (phase space), the assumption of a single or double Gaussian is made. To model the energy spectrum, a Gaussian function is assumed and parameters are optimized using either MC simulations or a library of pre-computed Bragg peaks. For the absolute dose calibration, commissioning fields can be reproduced with the dose engine to retrieve the necessary parameters. We discuss in a first time the tool efficiency and show that one can optimize all parameters in less than 4 min per energy with excellent accuracy. We then validate a beam model obtained with the tool by simulating homogeneous spread-out Bragg peaks (SOBPs) and patient QA plans previously measured in water. An average range agreement of 0.29 ± 0.34 mm is achieved for the SOBPs while 3%/3 mm local gamma passing rates reach 99.3% on average over all 62 measured patient QA planes, which is well within clinical tolerances.

Future technological developments in proton therapy - A predicted technological breakthrough

In the current spectrum of cancer treatments, despite high costs, a lack of robust evidence based on clinical outcomes or technical and radiobiological uncertainties, particle therapy and in particular proton therapy (PT) is rapidly growing. Despite proton therapy being more than fifty years old (first proposed by Wilson in 1946) and more than 220,000 patients having been treated with in 2020, many technological challenges remain and numerous new technical developments that must be integrated into existing systems. This article presents an overview of on-going technical developments and innovations that we felt were most important today, as well as those that have the potential to significantly shape the future of proton therapy. Indeed, efforts have been done continuously to improve the efficiency of a PT system, in terms of cost, technology and delivery technics, and a number of different developments pursued in the accelerator field will first be presented. Significant developments are also underway in terms of transport and spatial resolution achievable with pencil beam scanning, or conformation of the dose to the target: we will therefore discuss beam focusing and collimation issues which are important parameters for the development of these techniques, as well as proton arc therapy. State of the art and alternative approaches to adaptive PT and the future of adaptive PT will finally be reviewed. Through these overviews, we will finally see how advances in these different areas will allow the potential for robust dose shaping in proton therapy to be maximised, probably foreshadowing a future era of maturity for the PT technique.

Fast MCsquare-Based Independent Dose Verification Platform for Pencil Beam Scanning Proton Therapy

PURPOSE

To commission MCsquare (a multi-cores CPU-based dose calculation engine) for pencil beam scanning (PBS) proton therapy, integrate it into RayStation treatment plan system (TPS) to create a dedicated platform for fast independent dose verification.

METHOD

A MCsquare-based independent dose verification platform (MC2InRS) was developed to realize automatic dose re-calculation for clinical use, including data preparation, dose calculation, 2D/3D gamma analysis. MCsquare was commissioned based on in-air lateral dose profiles, integrated depth dose, and the absolute dose of different beam energies for Proteus®ONE. MC2InRS was validated with measurement data using various targets and depths in a water phantom. This study also investigated 15 clinical cases to demonstrate the feasibility and effectiveness of MC2InRS platform in clinic practice.

RESULTS

Between simulation and measurement, the distal range differences at 80% (R80) and 20% (R20) dose levels for each energy were below 0.05 mm, and 0.1 mm, respectively, and the absolute dose differences were below 0.5%. 29 out of 36 QA planes reached a 100% gamma passing rate (GPR) for 2%/2mm criteria, and a minimum of 98.3% gamma was obtained in water phantom between simulation and measurement. For the 15 clinical cases investigated, the average 2D GPR (2%/2mm) was 95.4%, 99.3% for MCsquare vs. measurement, MCsquare vs. TPS, respectively. The average 3D GPR (2%/2mm) was 98.9%, 95.3% for MCsquare vs. TPS in water, and computed tomography (CT), respectively.

CONCLUSION

MC2InRS, a fast, independent dose verification platform, has been developed to perform dose verification with high accuracy and efficiency for Pencil Bream Scanning (PBS). Its potential to be applied in routine clinical practice has also been discussed.

Producing a Beam Model of the Varian ProBeam Proton Therapy System using TOPAS Monte Carlo Toolkit

PURPOSE

A Geant4-based TOPAS Monte Carlo toolkit was utilized to model a Varian ProBeam proton therapy system, with the aim of providing an independent computational platform for validating advanced dosimetric methods.

MATERIALS AND METHODS

The model was tested for accuracy of dose and linear energy transfer (LET) prediction relative to the commissioning data, which included integral depth dose (IDD) in water and spot profiles in air measured at varying depths (for energies of 70 to 240 MeV in increments of 10 MeV, and 242 MeV), and absolute dose calibration. Emittance was defined based on depth-dependent spot profiles and Courant-Snyder's particle transport theory, which provided spot size and angular divergence along the inline and crossline plane. Energy spectra were defined as Gaussian distributions that best matched the range and maximum dose of the IDD. The validity of the model was assessed based on measurements of range, dose to peak difference, mean point to point difference, spot sizes at different depths, and spread-out Bragg peak (SOBP) IDD and was compared to the current treatment planning software (TPS).

RESULTS

Simulated and commissioned spot sizes agreed within 2.5%. The single spot IDD range, maximum dose, and mean point to point difference of each commissioned energy agreed with the simulated profiles generally within 0.07 mm, 0.4%, and 0.6%, respectively. A simulated SOBP plan agreed with the measured dose within 2% for the plateau region. The protons/MU and absolute dose agreed with the current TPS to within 1.6% and exhibited the greatest discrepancy at higher energies.

CONCLUSIONS

The TOPAS model agreed well with the commissioning data and included inline and crossline asymmetry of the beam profiles. The discrepancy between the measured and TOPAS-simulated SOBP plan may be due to beam modeling simplifications of the current TPS and the nuclear halo effect. The model can compute LET, and motivates future studies in understanding equivalent dose prediction in treatment planning, and investigating scintillation quenching.

Gradient corrections for reference dosimetry using Farmer-type ionization chambers in single-layer scanned proton fields

PURPOSE

The local depth dose gradient and the displacement correction factor for Farmer-type ionization chambers are quantified for reference dosimetry at shallow depth in single-layer scanned proton fields.

METHOD

Integrated radial profiles as a function of depth (IRPDs) measured at three proton therapy centers were smoothed by polynomial fits. The local relative depth dose gradient at measurement depths from 1 to 5 cm were derived from the derivatives of those fits. To calculate displacement correction factors, the best estimate of the effective point of measurement was derived from reviewing experimental and theoretical determinations reported in the literature. Displacement correction factors for the use of Farmer-type ionization chambers with their reference point (at the center of the cavity volume) positioned at the measurement depth were derived as a ratio of IRPD values at the measurement depth and at the effective point of measurement.

RESULTS

Depth dose gradients are as low as 0.1-0.4% per mm at measurement depths from 1 to 5 cm in the highest clinical proton energies (with residual ranges higher than 15 cm) and increase to 1% per mm at a residual range of 4 cm and become larger than 3% per mm for residual ranges lower than 2 cm. The literature review shows that the effective point of measurement of Farmer-type ionization chambers is, similarly as for carbon ion beams, located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. If a maximum displacement correction of 2% is deemed acceptable to be included in calculated beam quality correction factors, Farmer-type ICs can be used at measurements depths from 1 to 5 cm for which the residual range is 4 cm or larger. If one wants to use the same beam quality correction factors as applicable to the conventional measurement point for scattered beams, located at the center of the SOBP, the relative standard uncertainty on the assumption that the displacement correction factor is unity can be kept below 0.5% for measurement depths of at least 2 cm and for residual ranges of 15 cm or higher.

CONCLUSION

The literature review confirmed that for proton beams the effective point of measurement of Farmer-type ionization chambers is located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. Based on the findings in this work, three options can be recommended for reference dosimetry of scanned proton beams using Farmer-type ionization chambers: (a) positioning the effective point of measurement at the measurement depth, (b) positioning the reference point at the measurement depth and applying a displacement correction factor, and (c) positioning the reference point at the measurement depth without applying a displacement correction factor. Based on limiting the acceptable uncertainty on the gradient correction factor to 0.5% and the maximum deviation of the displacement perturbation correction factor from unity to 2%, the first two options can be allowed for residual ranges of at least 4 cm while the third option only for residual ranges of at least 15 cm.

Proton pencil beam scanning reduces secondary cancer risk in breast cancer patients with internal mammary chain involvement compared to photon radiotherapy

Purpose

Proton pencil beam scanning (PBS) represents an interesting option for the treatment of breast cancer (BC) patients with nodal involvement. Here we compare tangential 3D-CRT and VMAT to PBS proton therapy (PT) in terms of secondary cancer risk (SCR) for the lungs and for contralateral breast.

Methods

Five BC patients including supraclavicular (SVC) nodes in the target (Group 1) and five including SVC plus internal-mammary-nodes (IMNs, Group 2) were considered. The Group 1 patients were planned by PT versus tangential 3D-CRT in free-breathing (FB). The Group 2 patients were planned by PT versus VMAT considering both FB and deep-inspiration breath hold (DIBH) irradiation. The prescription dose to the target volume was 50 Gy (2 Gy/fraction). A constant RBE = 1.1 was assumed for PT. The SCR was evaluated with the excess absolute risk (EAR) formalism, considering also the age dependence. A cumulative EAR was finally computed.

Results

According to the linear, linear-exponential and linear-plateau dose response model, the cumulative EAR for Group 1 patients after PT was equal to 45 ± 10, 17 ± 3 and 15 ± 3, respectively. The corresponding relative increase for tangential 3D-CRT was equal to a factor 2.1 ± 0.5, 2.1 ± 0.4 and 2.3 ± 0.4. Group 2 patients showed a cumulative EAR after PT in FB equal to 65 ± 3, 21 ± 1 and 20 ± 1, according to the different models; the relative risk obtained with VMAT increased by a factor 3.5 ± 0.2, 5.2 ± 0.3 and 5.1 ± 0.3. Similar values emerge from DIBH plans. Contrary to photon radiotherapy, PT appears to be not sensitive to the age dependence due to the very low delivered dose.

Conclusions

PBS PT is associated to significant SCR reduction in BC patients compared to photon radiotherapy. The benefits are maximized for young patients with both SVC and IMNs involvement. When combined with the improved sparing of the heart, this might contribute to the establishment of effective patient-selection criteria for proton BC treatments.

Automated Monte-Carlo re-calculation of proton therapy plans using Geant4/Gate: implementation and comparison to plan-specific quality assurance measurements

OBJECTIVES

Software re-calculation of proton pencil beam scanning plans provides a method of verifying treatment planning system (TPS) dose calculations prior to patient treatment. This study describes the implementation of AutoMC, a Geant4 v10.3.3/Gate v8.1 (Gate-RTion v1.0)-based Monte-Carlo (MC) system for automated plan re-calculation, and presents verification results for 153 patients (730 fields) planned within year one of the proton service at The Christie NHS Foundation Trust.

METHODS

A MC beam model for a Varian ProBeam delivery system with four range-shifter options (none, 2 cm, 3 cm, 5 cm) was derived from beam commissioning data and implemented in AutoMC. MC and TPS (Varian Eclipse v13.7) calculations of 730 fields in solid-water were compared to physical plan-specific quality assurance (PSQA) measurements acquired using a PTW Octavius 1500XDR array and PTW 31021 Semiflex 3D ion chamber.

RESULTS

TPS and MC showed good agreement with array measurements, evaluated using γ analyses at 3%, 3 mm with a 10% lower dose threshold:>94% of fields calculated by the TPS and >99% of fields calculated by MC had γ ≤ 1 for>95% of measurement points within the plane. TPS and MC also showed good agreement with chamber measurements of absolute dose, with systematic differences of <1.5% for all range-shifter options.

CONCLUSIONS

Reliable independent verification of the TPS dose calculation is a valuable complement to physical PSQA and may facilitate reduction of the physical PSQA workload alongside a thorough delivery system quality assurance programme.

ADVANCES IN KNOWLEDGE

A Gate/Geant4-based MC system is thoroughly validated against an extensive physical PSQA dataset for 730 clinical fields, showing that clinical implementation of MC for PSQA is feasible.

Development of a Monte Carlo beam model for raster scanning proton beams and dosimetric comparison

PURPOSE

To develop a Monte Carlo (MC) beam model for raster scanning proton beams for dose verification purposes.

METHODS AND MATERIALS

MC program FLUKA was used in the model. The nominal energy, momentum spread and beam angular distribution in the model were determined by matching the simulation profiles with the measured integral depth dose (IDD) and in air spot size. Dosimetric comparison was done by comparing the measured and simulated dose distributions. The 1 D dose profile of cubic Spread Out Bragg Peak (SOBP) plans, and the 2 D dose distribution of previously treated breast cancer patients' clinical plans were measured by using Pinpoint chambers and 2 D array ionization chambers, respectively. Corresponding DICOM plan information was utilized for MC simulation.

RESULTS

The MC results showed good agreement with measurements for the SOBP plans. The absolute comparison of the absorbed dose difference between the MC and the measurement was 0.93%±0.88%. For the patient plans, the overall passing rate of the gamma index analysis (γ-PR) between the MC simulation and measurement with the 2%-2 mm criteria was 97.78%, and only 1 case had a γ-PR less than 90%. With the 3%-3 mm criteria, γ-PR was never below 99% for all cases with and without the range shifter.

CONCLUSIONS

This work described a method for adapting a MC simulation model for a raster scanning proton beam. The good concordance between the simulations and measurements shows that the MC model is an accurate and reliable method. It has the potential to be used for patient specific quality assurance (PSQA) to reduce the beam time for the measurements in water.

DICOM-RT Ion interface to utilize MC simulations in routine clinical workflow for proton pencil beam radiotherapy

To adopt Monte Carlo (MC) simulations as an independent dose calculation method for proton pencil beam radiotherapy, an interface that converts the plan information in DICOM format into MC components such as geometries and beam source is a crucial element. For this purpose, a DICOM-RT Ion interface (https://github.com/topasmc/dicom-interface) has been developed and integrated into the TOPAS MC code to perform such conversions on-the-fly. DICOM-RT objects utilized in this interface include Ion Plan (RTIP), Ion Beams Treatment Record (RTIBTR), CT image, and Dose. Beamline geometries, gantry and patient coordinate systems, and fluence maps are determined from RTIP and/or RTIBTR. In this interface, DICOM information is processed and delivered to a MC engine in two steps. A MC model, which consists of beamline geometries and beam source, to represent a treatment machine is created by a DICOM parser of the interface. The complexities from different DICOM types, various beamline configurations and source models are handled in this step. Next, geometry information and beam source are transferred to TOPAS on-the-fly via the developed TOPAS extensions. This interface with two treatment machines was successfully deployed into our automated MC workflow which provides simulated dose and LET distributions in a patient or a water phantom automatically when a new plan is identified. The developed interface provides novel features such as handling multiple treatment systems based on different DICOM types, DICOM conversions on-the-fly, and flexible sampling methods that significantly reduce the burden of handling DICOM based plan or treatment record information for MC simulations.

First proton minibeam radiation therapy treatment plan evaluation

Proton minibeam radiation therapy (pMBRT) is a novel dose delivery method based on spatial dose fractionation. pMBRT has been shown to be promising in terms of reduced side effects and superior tumour control in high-grade glioma-bearing rats compared to standard irradiation. These findings, together with the recent optimized implementation of pMBRT in a clinical pencil beam scanning system, have triggered reflection on the possible application to patient treatments. In this context, the present study was designed to conduct a first theoretical investigation of the clinical potential of this technique. For this purpose, a dedicated dose engine was developed and used to evaluate two clinically relevant patient treatment plans (high-grade glioma and meningioma). Treatment plans were compared with standard proton therapy plans assessed by means of a commercial treatment planning system (ECLIPSE-Varian Medical systems) and Monte Carlo simulations. A multislit brass collimator consisting of 0.4 mm wide slits separated by a centre-to-centre distance of 4 or 6 mm was placed between the nozzle and the patient to shape the planar minibeams. For each plan, spread-out Bragg peaks and homogeneous dose distributions (±7% dose variations) can be obtained in target volumes. The Peak-to-Valley Dose Ratios (PVDR) were evaluated between 9.2 and 12.8 at a depth of 20 mm for meningioma and glioma, respectively. Dose volume histograms (DVHs) for target volumes and organs at risk were quantitatively compared, resulting in a slightly better target homogeneity with standard PT than with pMBRT plans, but similar DVHs for deep-seated organs-at-risk and lower average dose for shallow organs. The proposed delivery method evaluated in this work opens the way to an effective treatment for radioresistant tumours and will support the design of future clinical research.

The TOPAS tool for particle simulation, a Monte Carlo simulation tool for physics, biology and clinical research

PURPOSE

This paper covers recent developments and applications of the TOPAS TOol for PArticle Simulation and presents the approaches used to disseminate TOPAS.

MATERIALS AND METHODS

Fundamental understanding of radiotherapy and imaging is greatly facilitated through accurate and detailed simulation of the passage of ionizing radiation through apparatus and into a patient using Monte Carlo (MC). TOPAS brings Geant4, a reliable, experimentally validated MC tool mainly developed for high energy physics, within easy reach of medical physicists, radiobiologists and clinicians. Requiring no programming knowledge, TOPAS provides all of the flexibility of Geant4.

RESULTS

After 5 years of development followed by its initial release, TOPAS was subsequently expanded from its focus on proton therapy physics to incorporate radiobiology modeling. Next, in 2018, the developers expanded their user support and code maintenance as well as the scope of TOPAS towards supporting X-ray and electron therapy and medical imaging. Improvements have been achieved in user enhancement through software engineering and a graphical user interface, calculational efficiency, validation through experimental benchmarks and QA measurements, and either newly available or recently published applications. A large and rapidly increasing user base demonstrates success in our approach to dissemination of this uniquely accessible and flexible MC research tool.

CONCLUSIONS

The TOPAS developers continue to make strides in addressing the needs of the medical community in applications of ionizing radiation to medicine, creating the only fully integrated platform for four-dimensional simulation of all forms of radiotherapy and imaging with ionizing radiation, with a design that promotes inter-institutional collaboration.

Pitfalls in the beam modelling process of Monte Carlo calculations for proton pencil beam scanning

OBJECTIVE

Monte Carlo (MC) simulations substantially improve the accuracy of predicted doses. This study aims to determine and quantify the uncertainties of setting up such a MC system.

METHODS

Doses simulated with two Geant4-based MC calculation codes, but independently tuned to the same beam data, have been compared. Different methods of MC modelling of a pre-absorber have been employed, either modifying the beam source parameters (descriptive) or adding the pre-absorber as a physical component (physical).

RESULTS

After the independent beam modelling of both systems in water (resulting in excellent range agreement) range differences of up to 3.6/4.8 mm (1.5% of total range) in bone/brain-like tissues were found, which resulted from the use of different mean water ionisation potentials during the energy tuning process. When repeating using a common definition of water, ranges in bone/brain agreed within 0.1 mm and gamma-analysis (global 1%,1mm) showed excellent agreement (>93%) for all patient fields. However, due to a lack of modelling of proton fluence loss in the descriptive pre-absorber, differences of 7% in absolute dose between the pre-absorber definitions were found.

CONCLUSION

This study quantifies the influence of using different water ionisation potentials during the MC beam modelling process. Furthermore, when using a descriptive pre-absorber model, additional Faraday cup or ionisation chamber measurements with pre-absorber are necessary.

ADVANCES IN KNOWLEDGE

This is the first study quantifying the uncertainties caused by the MC beam modelling process for proton pencil beam scanning, and a more detailed beam modelling process for MC simulations is proposed to minimise the influence of critical parameters.

Benchmarking a GATE/Geant4 Monte Carlo model for proton beams in magnetic fields

Purpose

Magnetic resonance guidance in proton therapy (MRPT) is expected to improve its current performance. The combination of magnetic fields with clinical proton beam lines poses several challenges for dosimetry, treatment planning and dose delivery. Proton beams are deflected by magnetic fields causing considerable changes in beam trajectories and also a retraction of the Bragg peak positions. A proper prediction and compensation of these effects is essential to ensure accurate dose calculations. This work aims to develop and benchmark a Monte Carlo (MC) beam model for dose calculation of MRPT for static magnetic fields up to 1 T.

Methods

Proton beam interactions with magnetic fields were simulated using the GATE/Geant4 toolkit. The transport of charged particle in custom 3D magnetic field maps was implemented for the first time in GATE. Validation experiments were done using a horizontal proton pencil beam scanning system with energies between 62.4 and 252.7 MeV and a large gap dipole magnet (B = 0–1 T), positioned at the isocenter and creating magnetic fields transverse to the beam direction. Dose was measured with Gafchromic EBT3 films within a homogeneous PMMA phantom without and with bone and tissue equivalent material slab inserts. Linear energy transfer (LET) quenching of EBT3 films was corrected using a linear model on dose‐averaged LET method to ensure a realistic dosimetric comparison between simulations and experiments. Planar dose distributions were measured with the films in two different configurations: parallel and transverse to the beam direction using single energy fields and spread‐out Bragg peaks. The MC model was benchmarked against lateral deflections and spot sizes in air of single beams measured with a Lynx PT detector, as well as dose distributions using EBT3 films. Experimental and calculated dose distributions were compared to test the accuracy of the model.

Results

Measured proton beam deflections in air at distances of 465, 665, and 1155 mm behind the isocenter after passing the magnetic field region agreed with MC‐predicted values within 4 mm. Differences between calculated and measured beam full width at half maximum (FWHM) were lower than 2 mm. For the homogeneous phantom, measured and simulated in‐depth dose profiles showed range and average dose differences below 0.2 mm and 1.2%, respectively. Simulated central beam positions and widths differed <1 mm to the measurements with films. For both heterogenous phantoms, differences within 1 mm between measured and simulated central beam positions and widths were obtained, confirming a good agreement of the MC model.

Conclusions

A GATE/Geant4 beam model for protons interacting with magnetic fields up to 1 T was developed and benchmarked to experimental data. For the first time, the GATE/Geant4 model was successfully validated not only for single energy beams, but for SOBP, in homogeneous and heterogeneous phantoms. EBT3 film dosimetry demonstrated to be a powerful dosimetric tool, once the film response function is LET corrected, for measurements in‐line and transverse to the beam direction in magnetic fields. The proposed MC beam model is foreseen to support treatment planning and quality assurance (QA) activities toward MRPT.

Methodology paper: a novel phantom setup for commissioning of scanned ion beam delivery and TPS

BACKGROUND

Commissioning of treatment planning systems (TPS) and beam delivery for scanned light ion beams is an important quality assurance task. This requires measurement of large sets of high quality dosimetric data in anthropomorphic phantoms to benchmark the TPS and dose delivery under realistic conditions.

METHOD

A novel measurement setup is described, which allows for an efficient collection of a large set of accurate dose data in complex phantom geometries. This setup allows dose measurements based on a set of 24 small volume ionization chambers calibrated in dose to water and mounted in a holder, which can be freely positioned in a water phantom with various phantoms mounted in front of the water tank. The phantoms can be scanned in a CT and a CT-based treatment planning can be performed for a direct benchmark of the dose calculation algorithm in various situations.

RESULTS

The system has been used for acceptance testing in scanned light ion beam therapy at Heidelberg Ion Beam Therapy Center for scanned proton and carbon ion beams. It demonstrated to be useful to collect large amounts of high quality data for comparison with the TPS calculation using various phantom geometries.

CONCLUSION

The setup is an efficient tool for commissioning and verification of treatment planning systems. It is especially suited for dynamic beam delivery, as many data points can be obtained during a single plan delivery, but can be adapted also for other dynamic therapies, like rotational IMRT.

STUDY FOR A PASSIVE SCATTERING LINE DEDICATED TO RADIOBIOLOGY EXPERIMENTS AT THE TRENTO PROTON THERAPY CENTER

The recent worldwide spread of Proton Therapy centers paves the way to new opportunities for basic and applied research related to the use of accelerated proton beams. Clinical centers make use of proton beam energies up to about 230 MeV. This represents an interesting energy range for a large spectrum of applications, including detector testing, radiation shielding and space research. Additionally, radiobiology research might benefit for a larger availability of proton beams, especially in those centers where a room dedicated to research activities also exists. Here, we describe the initial activities for the setup of a radiobiology irradiation facility at the Trento Proton Therapy Center. Data referring to the characterization of the beam in air are essential to that purpose and will be presented. A basic setup for large field irradiation will be also proposed, which is needed for the majority of in vitro and in vivo radiobiology experiments.

FLUKA particle therapy tool for Monte Carlo independent calculation of scanned proton and carbon ion beam therapy

While Monte Carlo (MC) codes are considered as the gold standard for dosimetric calculations, the availability of user friendly MC codes suited for particle therapy is limited. Based on the FLUKA MC code and its graphical user interface (GUI) Flair, we developed an easy-to-use tool which enables simple and reliable simulations for particle therapy. In this paper we provide an overview of functionalities of the tool and with the presented clinical, proton and carbon ion therapy examples we demonstrate its reliability and the usability in the clinical environment and show its flexibility for research purposes. The first, easy-to-use FLUKA MC platform for particle therapy with GUI functionalities allows a user with a minimal effort and reduced knowledge about MC details to apply MC at their facility and is expected to enhance the popularity of the MC for both research and clinical quality assurance and commissioning purposes.

Evaluation of the ray-casting analytical algorithm for pencil beam scanning proton therapy

For pencil beam scanned (PBS) proton therapy, analytical dose calculation engines are still typically used for the optimisation process, and often for the final evaluation of the plan. Recently however, the suitability of analytical calculations for planning PBS treatments has been questioned. Conceptually, the two main approaches for these analytical dose calculations are the ray-casting (RC) and the pencil-beam (PB) method. In this study, we compare dose distributions and dosimetric indices, calculated on both the clinical dose calculation grid and as a function of dose grid resolution, to Monte Carlo (MC) calculations. The analysis is done using a comprehensive set of clinical plans which represent a wide choice of treatment sites. When analysing dose difference histograms for relative treatment plans, pencil beam calculations with double grid resolution perform best, with on average 97.7%/91.9% (RC), 97.9%/92.7% (RC, double grid resolution), 97.6%/91.0% (PB) and 98.6%/94.0% (PB, double grid resolution) of voxels agreeing within  ±5%/±  3% between the analytical and the MC calculations. Even though these point-to-point dose comparison shows differences between analytical and MC calculations, for all algorithms, clinically relevant dosimetric indices agree within  ±4% for the PTV and within  ±5% for critical organs. While the clinical agreement depends on the treatment site, there is no substantial difference of indices between the different algorithms. The pencil-beam approach however comes at a higher computational cost than the ray-casting calculation. In conclusion, we would recommend using the ray-casting algorithm for fast dose optimization and subsequently combine it with one MC calculation to scale the absolute dose and assure the quality of the treatment plan.

Log file based Monte Carlo calculations for proton pencil beam scanning therapy

Patient specific quality assurance is crucial to guarantee safety in proton pencil beam scanning. In current clinical practice, this requires extensive, time consuming measurements. Additionally, these measurements do not consider the influence of density heterogeneities in the patient and are insensitive to delivery errors. In this work, we investigate the use of log file based Monte Carlo calculations for dose reconstructions in the patient CT, which takes the combined influence of calculational and delivery errors into account. For one example field, 87%/90% of the voxels agree within  ±3% when taking either calculational or delivery uncertainties into account (analytical versus Monte Carlo calculation/Monte Carlo from planned versus Monte Carlo from log file). 78% agree when considering both uncertainties simultaneously (nominal field versus Monte Carlo from log files). We then show the application of the log file based Monte Carlo calculations as a patient specific quality assurance tool for a set of five patients (16 fields) treated for different indications. For all fields, absolute dose scaling factors based on the log file Monte Carlo agree within  ±3% to the measurement based absolute dose scaling. Relative comparison shows that more than 90% of the voxels agree within  ±  5% between the analytical calculated plan and the Monte Carlo based on log files. The log file based Monte Carlo approach is an end-to-end test incorporating all requirements of patient specific quality assurance. It has the potential to reduce the workload and therefore to increase the patient throughput, while simultaneously enabling more accurate dose verification directly in the patient geometry.

A pre-absorber optimization technique for pencil beam scanning proton therapy treatments

PURPOSE

To implement a new proton therapy planning method for the treatment of shallow lesions with PBS and to compare it to the standard method.

METHODS AND MATERIALS

In order to treat shallow lesions, a pre-absorber, usually called range-shifter (RS), is needed: it is used to degrade the beam energy and treat tumors shallower than the minimum range available. Its use is associated to dose calculation uncertainties and plan quality degradation which should be minimized. We studied five tumor localizations requiring RS and created three plans for each case: a) standard method with the RS close to the patient surface, b) with the RS used only for the shallow part of the tumor (when strictly needed) and completely retracted and c) as the b) approach but with the RS close to the patient. We called these two approaches 'Range Shifter Optimization' (RSO) techniques. We compared those plans in terms of dose distribution quality, delivery time and patient-specific-QA results.

RESULTS

In most cases a good dose reduction to OARs with no significant loss in terms of target coverage was obtained when the RSO techniques were used. Patient-specific-QA gave very good results in terms of γ-Passing-Rate (PR) (3%, 3 mm) for both RSO techniques (mean 98.09%), while the standard had some very low PR (minimum 81.09%). The delivery time increased (5.0 min on average per treatment) but was still acceptable in terms of patient compliance.

CONCLUSION

We developed a new planning technique for shallow lesions and we demonstrated its superiority in terms of both plan quality and patient-specific-QA results with respect to the standard method. This technique is routinely used to treat patients in our center.

Implementation of planar proton minibeam radiation therapy using a pencil beam scanning system: A proof of concept study

PURPOSE

Proton minibeam radiation therapy (pMBRT) is an innovative approach that combines the advantages of minibeam radiation therapy with the more precise ballistics of protons to further reduce the side effects of radiation. One of the main challenges of this approach is the generation of very narrow proton pencil beams with an adequate dose-rate to treat patients within a reasonable treatment time (several minutes) in existing clinical facilities. The aim of this study was to demonstrate the feasibility of implementing pMBRT by combining the pencil beam scanning (PBS) technique with the use of multislit collimators. This proof of concept study of pMBRT with a clinical system is intended to guide upcoming biological experiments.

METHODS

Monte Carlo simulations (TOPAS v3.1.p2) were used to design a suitable multislit collimator to implement planar pMBRT for conventional pencil beam scanning settings. Dose distributions (depth-dose curves, lateral profiles, Peak-to-Valley Dose Ratio (PVDR) and dose-rates) for different proton beam energies were assessed by means of Monte Carlo simulations and experimental measurements in a water tank using commercial ionization chambers and a new p-type silicon diode, the IBA RAZOR. An analytical intensity-modulated dose calculation algorithm designed to optimize the weight of individual Bragg peaks composing the field was also developed and validated.

RESULTS

Proton minibeams were then obtained using a brass multislit collimator with five slits measuring 2 cm × 400 μm in width with a center-to-center distance of 4 mm. The measured and calculated dose distributions (depth-dose curves and lateral profiles) showed a good agreement. Spread-out Bragg peaks (SOBP) and homogeneous dose distributions around the target were obtained by means of intensity modulation of Bragg peaks, while maintaining spatial fractionation at shallow depths. Mean dose-rates of 0.12 and 0.09 Gy/s were obtained for one iso-energy layer and a SOBP conditions in the presence of multislit collimator.

CONCLUSIONS

This study demonstrates the feasibility of implementing pMBRT on a PBS system. It also confirms the reliability of RAZOR detector for pMBRT dosimetry. This newly developed experimental methodology will support the design of future preclinical research with pMBRT.

Clinical implementations of 4D pencil beam scanned particle therapy: Report on the 4D treatment planning workshop 2016 and 2017

In 2016 and 2017, the 8th and 9th 4D treatment planning workshop took place in Groningen (the Netherlands) and Vienna (Austria), respectively. This annual workshop brings together international experts to discuss research, advances in clinical implementation as well as problems and challenges in 4D treatment planning, mainly in spot scanned proton therapy. In the last two years several aspects like treatment planning, beam delivery, Monte Carlo simulations, motion modeling and monitoring, QA phantoms as well as 4D imaging were thoroughly discussed. This report provides an overview of discussed topics, recent findings and literature review from the last two years. Its main focus is to highlight translation of 4D research into clinical practice and to discuss remaining challenges and pitfalls that still need to be addressed and to be overcome.

Beam parameters optimization and characterization for a TUrning LInac for Protontherapy

TULIP (TUrning LInac for Protontherapy) is a novel compact accelerator system for protontherapy mounted on a rotating gantry (Amaldi et al., 2013, 2010, 2009). Its high-energy Linac has the unique property of being able to modulate the beam energy from one pulse to the next, in only a couple of milliseconds. The main purpose of this study is to optimize the properties of the beam exiting the Linac to make them compatible to medical therapy and to characterize their medical physics properties for later implementation in a Treatment Planning System. For this purpose, multi-particle tracking and Monte Carlo (MC) simulations are used to follow the particles through their path up to the treatment isocenter, following the so-called phase-space method. The data compiled includes particle fluences in air and depth-dose curves and provides the basis for a specific model of the TULIP beam.

Validation and clinical implementation of an accurate Monte Carlo code for pencil beam scanning proton therapy

Monte Carlo (MC)‐based dose calculations are generally superior to analytical dose calculations (ADC) in modeling the dose distribution for proton pencil beam scanning (PBS) treatments. The purpose of this paper is to present a methodology for commissioning and validating an accurate MC code for PBS utilizing a parameterized source model, including an implementation of a range shifter, that can independently check the ADC in commercial treatment planning system (TPS) and fast Monte Carlo dose calculation in opensource platform (MCsquare). The source model parameters (including beam size, angular divergence and energy spread) and protons per MU were extracted and tuned at the nozzle exit by comparing Tool for Particle Simulation (TOPAS) simulations with a series of commissioning measurements using scintillation screen/CCD camera detector and ionization chambers. The range shifter was simulated as an independent object with geometric and material information. The MC calculation platform was validated through comprehensive measurements of single spots, field size factors (FSF) and three‐dimensional dose distributions of spread‐out Bragg peaks (SOBPs), both without and with the range shifter. Differences in field size factors and absolute output at various depths of SOBPs between measurement and simulation were within 2.2%, with and without a range shifter, indicating an accurate source model. TOPAS was also validated against anthropomorphic lung phantom measurements. Comparison of dose distributions and DVHs for representative liver and lung cases between independent MC and analytical dose calculations from a commercial TPS further highlights the limitations of the ADC in situations of highly heterogeneous geometries. The fast MC platform has been implemented within our clinical practice to provide additional independent dose validation/QA of the commercial ADC for patient plans. Using the independent MC, we can more efficiently commission ADC by reducing the amount of measured data required for low dose “halo” modeling, especially when a range shifter is employed.

Improvements in pencil beam scanning proton therapy dose calculation accuracy in brain tumor cases with a commercial Monte Carlo algorithm

A commercial Monte Carlo (MC) algorithm (RayStation version 6.0.024) for the treatment of brain tumors with pencil beam scanning (PBS) proton therapy is validated and compared via measurements and analytical calculations in clinically realistic scenarios. For the measurements a 2D ion chamber array detector (MatriXX PT) was placed underneath the following targets: (1) an anthropomorphic head phantom (with two different thicknesses) and (2) a biological sample (i.e. half a lamb's head). In addition, we compared the MC dose engine versus the RayStation pencil beam (PB) algorithm clinically implemented so far, in critical conditions such as superficial targets (i.e. in need of a range shifter (RS)), different air gaps, and gantry angles to simulate both orthogonal and tangential beam arrangements. For every plan the PB and MC dose calculations were compared to measurements using a gamma analysis metrics (3%, 3 mm). For the head phantom the gamma passing rate (GPR) was always  >96% and on average  >99% for the MC algorithm; the PB algorithm had a GPR of  ⩽90% for all the delivery configurations with a single slab (apart 95% GPR from the gantry of 0° and small air gap) and in the case of two slabs of the head phantom the GPR was  >95% only in the case of small air gaps for all three (0°, 45°, and 70°) simulated beam gantry angles. Overall the PB algorithm tends to overestimate the dose to the target (up to 25%) and underestimate the dose to the organ at risk (up to 30%). We found similar results (but a bit worse for the PB algorithm) for the two targets of the lamb's head where only two beam gantry angles were simulated. Our results suggest that in PBS proton therapy a range shifter (RS) needs to be used with caution when planning a treatment with an analytical algorithm due to potentially great discrepancies between the planned dose and the dose delivered to the patient, including in the case of brain tumors where this issue could be underestimated. Our results also suggest that a MC evaluation of the dose has to be performed every time the RS is used and, mostly, when it is used with large air gaps and beam directions tangential to the patient surface.

A study of lateral fall-off (penumbra) optimisation for pencil beam scanning (PBS) proton therapy

The lateral fall-off is crucial for sparing organs at risk in proton therapy. It is therefore of high importance to minimize the penumbra for pencil beam scanning (PBS). Three optimisation approaches are investigated: edge-collimated uniformly weighted spots (collimation), pencil beam optimisation of uncollimated pencil beams (edge-enhancement) and the optimisation of edge collimated pencil beams (collimated edge-enhancement). To deliver energies below 70 MeV, these strategies are evaluated in combination with the following pre-absorber methods: field specific fixed thickness pre-absorption (fixed), range specific, fixed thickness pre-absorption (automatic) and range specific, variable thickness pre-absorption (variable). All techniques are evaluated by Monte Carlo simulated square fields in a water tank. For a typical air gap of 10 cm, without pre-absorber collimation reduces the penumbra only for water equivalent ranges between 4-11 cm by up to 2.2 mm. The sharpest lateral fall-off is achieved through collimated edge-enhancement, which lowers the penumbra down to 2.8 mm. When using a pre-absorber, the sharpest fall-offs are obtained when combining collimated edge-enhancement with a variable pre-absorber. For edge-enhancement and large air gaps, it is crucial to minimize the amount of material in the beam. For small air gaps however, the superior phase space of higher energetic beams can be employed when more material is used. In conclusion, collimated edge-enhancement combined with the variable pre-absorber is the recommended setting to minimize the lateral penumbra for PBS. Without collimator, it would be favourable to use a variable pre-absorber for large air gaps and an automatic pre-absorber for small air gaps.

Evaluation of the water-equivalence of plastic materials in low- and high-energy clinical proton beams

The aim of this work was to evaluate the water-equivalence of new trial plastics designed specifically for light-ion beam dosimetry as well as commercially available plastics in clinical proton beams. The water-equivalence of materials was tested by computing a plastic-to-water conversion factor, [Formula: see text]. Trial materials were characterized experimentally in 60 MeV and 226 MeV un-modulated proton beams and the results were compared with Monte Carlo simulations using the FLUKA code. For the high-energy beam, a comparison between the trial plastics and various commercial plastics was also performed using FLUKA and Geant4 Monte Carlo codes. Experimental information was obtained from laterally integrated depth-dose ionization chamber measurements in water, with and without plastic slabs with variable thicknesses in front of the water phantom. Fluence correction factors, [Formula: see text], between water and various materials were also derived using the Monte Carlo method. For the 60 MeV proton beam, [Formula: see text] and [Formula: see text] factors were within 1% from unity for all trial plastics. For the 226 MeV proton beam, experimental [Formula: see text] values deviated from unity by a maximum of about 1% for the three trial plastics and experimental results showed no advantage regarding which of the plastics was the most equivalent to water. Different magnitudes of corrections were found between Geant4 and FLUKA for the various materials due mainly to the use of different nonelastic nuclear data. Nevertheless, for the 226 MeV proton beam, [Formula: see text] correction factors were within 2% from unity for all the materials. Considering the results from the two Monte Carlo codes, PMMA and trial plastic #3 had the smallest [Formula: see text] values, where maximum deviations from unity were 1%, however, PMMA range differed by 16% from that of water. Overall, [Formula: see text] factors were deviating more from unity than [Formula: see text] factors and could amount to a few percent for some materials.

Factors influencing the performance of patient specific quality assurance for pencil beam scanning IMPT fields

PURPOSE

A detailed analysis of 2728 intensity modulated proton therapy (IMPT) fields that were clinically delivered to patients between 2007 and 2013 at Paul Scherrer Institute (PSI) was performed. The aim of this study was to analyze the results of patient specific dosimetric verifications and to assess possible correlation between the quality assurance (QA) results and specific field metrics.

METHODS

Dosimetric verifications were performed for every IMPT field prior to patient treatment. For every field, a steering file was generated containing all the treatment unit information necessary for treatment delivery: beam energy, beam angle, dose, size of air gap, nuclear interaction (NI) correction factor, number of range shifter plates, number of Bragg peaks (BPs) with their position and weight. This information was extracted and correlated to the results of dosimetric verification of each field which was a measurement of two orthogonal profiles using an orthogonal ionization chamber array in a movable water column.

RESULTS

The data analysis has shown more than 94% of all verified plans were within defined clinical tolerances. The differences between measured and calculated dose depend critically on the number of BPs, total thickness of all range shifter plates inserted in the beam path, and maximal range. An increase of the dose difference was observed with smaller number of BPs (i.e., smaller tumor) and smaller ranges (i.e., superficial tumors). The results of the verification do not depend, however, on the prescribed dose, NI correction, or the size of the air gap. There is no dependency of the transversal and longitudinal spot position precision on the beam angle. The value of NI correction depends on the number of spots and number of range shifter plates.

CONCLUSIONS

The presented study has shown that the verification method used at Centre for Proton Therapy at Paul Scherrer Institute is accurate and reproducible for performing patient specific QA. The results confirmed that the dose discrepancy is dependent on the size and location of the tumor.

A benchmarking method to evaluate the accuracy of a commercial proton monte carlo pencil beam scanning treatment planning system

AcurosPT is a Monte Carlo algorithm in the Eclipse 13.7 treatment planning system, which is designed to provide rapid and accurate dose calculations for proton therapy. Computational run-time in minimized by simplifying or eliminating less significant physics processes. In this article, the accuracy of AcurosPT was benchmarked against both measurement and an independent MC calculation, TOPAS. Such a method can be applied to any new MC calculation for the detection of potential inaccuracies. To validate multiple Coulomb scattering (MCS) which affects primary beam broadening, single spot profiles in a Solidwater^® phantom were compared for beams of five selected proton energies between AcurosPT, measurement and TOPAS. The spot Gaussian sigma in AcurosPT was found to increase faster with depth than both measurement and TOPAS, suggesting that the MCS algorithm in AcurosPT overestimates the scattering effect. To validate AcurosPT modeling of the halo component beyond primary beam broadening, field size factors (FSF) were compared for multi-spot profiles measured in a water phantom. The FSF for small field sizes were found to disagree with measurement, with the disagreement increasing with depth. Conversely, TOPAS simulations of the same FSF consistently agreed with measurement to within 1.5%. The disagreement in absolute dose between AcurosPT and measurement was smaller than 2% at the mid-range depth of multi-energy beams. While AcurosPT calculates acceptable dose distributions for typical clinical beams, users are cautioned of potentially larger errors at distal depths due to overestimated MCS and halo implementation.

Reply to comment on 'Proton beam monitor chamber calibration'

This reply shows that the discrepancy of about 3% between Faraday cup dosimetry and reference dosimetry using a cylindrical ionization chamber found in Gomà (2014 Phys. Med. Biol. 59 4961-71) seems to be due to an overestimation of the beam quality correction factors tabulated in IAEA TRS-398 for the cylindrical chamber used, rather than to 'unresolved problems with Faraday cup dosimetry', as suggested by Palmans and Vatnitsky (2016 Phys. Med. Biol. 61 6585-93). Furthermore, this work shows that a good agreement between reference dosimetry and Faraday cup dosimetry is possible, provided accurate beam quality correction factors for proton beams are used. The review on W air values presented by Palmans and Vatnitsky is believed to be inaccurate, as it is based on the imprecise assumption of ionization chamber perturbation correction factors in proton beams being equal to unity.

Clinical Commissioning of a Pencil Beam Scanning Treatment Planning System for Proton Therapy

Purpose

In this report, we present the commissioning and validation results for a commercial proton pencil beam scanning RayStation treatment planning system.

Materials and Methods

The commissioning data requirements are (1) integrated depth dose curves, (2) spot profiles, (3) absolute dose/monitor unit calibration, and (4) virtual source position. An 8-cm parallel plate chamber was used to measure the integrated depth dose curves by scanning a beam composed of a single spot in a water phantom. The spot profiles were measured at 5 different planes using a 2-dimensional scintillation detector. The absolute dose/monitor unit calibration was based on dose measurements in single-layer fields of size 10 × 10 cm². The virtual-source position was calculated from the change in spot spacing with the distance from the isocenter. The beam model validation consisted of a comparison against commissioning data as well as a new set of verification measurements. For end-to-end testing, a series of phantom plans were created. These plans were measured at 1 to 3 depths using a 2-dimensional ion chamber array and evaluated for gamma index using the 3% and 3 mm criteria.

Results

The maximum deviation for spot sigma measured versus calculated was -0.2 mm. The point-dose measurements for single-layer beams were within ± 3%, except for the largest field size (29 × 29 cm²) and the highest energy (226 MeV). The point doses in the spread-out Bragg peak plans showed a trend in which differences > 3% were seen for ranges > 30 cm, field sizes > 15 × 15 cm², and depths > 25 cm. For end-to-end testing, 34 planes corresponding to 13 beams were analyzed for gamma index with a minimum pass rate of 92.8%.

Conclusion

The acceptable verification results and successful end-to-end testing ensured that all components of the treatment planning system were functional and the system was ready for clinical use.