Validation of a virtual source model for Monte Carlo dose calculations of a flattening filter free linac

An automated commissioning method based on virtual source models: Customizing Monte Carlo dose verification models for individual accelerators

BACKGROUND

In pursuit of precise dose calculation and verification, the importance of beam modelling cannot be overstated, as it ensures an accurate distribution of particles incident upon the human body. The virtual source model, as one of the beam modelling methods, offers the advantage of not requiring detailed accelerator information. Although various virtual source models exist, manual adjustment to these models demands a substantial investment of time and computational resources. There has long been a desire to develop an efficient and automated approach for model commissioning.

PURPOSE

To develop an automatic commissioning method for the virtual source model to customize the accelerator model for independent Monte Carlo dose verification.

METHODS

Initially, the accelerator model is established using the virtual source model and self-developed Jaw and MLC models. Then, a fully automated iteration process is employed to adjust the parameters of the virtual source model. Three types of objective functions are designed to represent differences from water tank measurements. Each objective function is paired with a specific parameter for adjustment, and their effectiveness is demonstrated through physical evidence. In each iteration, parameters with the highest objective function percentage are chosen for adjustment, and step length is determined based on current objective function values. Iteration is terminated when changes in any direction from the optimal solution no longer produce an improvement. Dose verification model for nine accelerators has been accomplished using this method. Additionally, under the same initial conditions, verification models for Versa HD accelerator (FF and FFF modes) are established using this method, Nelder-Mead Simplex optimization method, and the Bayesian optimization method to compare the efficiency and quality of these three iterative approaches.

RESULTS

Iterations for all nine accelerators are completed within 30 iterations. The relative dose differences in dose fall-off region compared to water tank measurements are all less than 2%, and the average gamma passing rates (3%/2 mm) for ArcCHECK measurements in QA plans are all higher than 97%. For Versa HD accelerator in FFF and FF modes, the proposed method achieves an average relative dose difference below 1% within 11 and 13 iterations, respectively. In contrast, the Simplex optimization reached 1% within 78 iterations in FFF mode. Furthermore, the Simplex optimization in FF mode and Bayesian optimization in both modes failed to achieve a 1% difference within 100 iterations.

CONCLUSIONS

The proposed iterative method achieves fast and automated commissioning of dose verification models, contributing to accurate and reliable clinical dose verification.

Acceptance, commissioning and quality assurance of the MRIdian®: Site experience and three years follow-up

PURPOSE

This study presents the methodology and results of the acceptance and periodical quality controls on the MRIdian®.

MATERIALS AND METHODS

The impact of the magnetic field on other machines was investigated by controlling nearby linacs dose profiles. The image quality of the 0.345T MR scanner was evaluated, also assessing the integrated linear accelerator influence. The photon beams lateral and depth dose profiles were measured in motorized water tanks, along dose rate and output factors, and compared to Monte Carlo (MC) calculations. The isocenter position, gantry angles and multi-leaf collimator (MLC) position were controlled using film dosimetry. Gating latency and dosimetric accuracy were controlled with a dynamic phantom.

RESULTS

The magnetic field had no significant impact on other nearby linacs. Image quality was within tolerances and did not vary over time. Dose profiles measured showed good agreement with MC data, with maximum differences of 1.3% in-field. Output factors were within 0.8% of calculated values. Imaging and radiative isocenter matched within 0.9±0.4mm over all monthly controls. Gantry rotation was precise within -0.1±0.2°, with an isocenter variation of 1.4±0.3mm diameter. The average MLC position was within 0.4±0.1mm of theoretical value. Finally, the gating latency was 0.14±0.07sec and the gated dose within 0.3% of base value.

CONCLUSION

All results are within the tolerances fixed by ViewRay® and show low variations over 2 years, comforting the use of small margins and gating for high-dose adaptive treatments.

Comparison of four commercial dose calculation algorithms in different evaluation tests

BACKGROUND

Accurate and fast dose calculation is crucial in modern radiation therapy. Four dose calculation algorithms (AAA, AXB, CCC, and MC) are available in Varian Eclipse and RaySearch Laboratories RayStation Treatment Planning Systems (TPSs).

OBJECTIVES

This study aims to evaluate and compare dosimetric accuracy of the four dose calculation algorithms applying to homogeneous and heterogeneous media, VMAT plans (based on AAPM TG-119 test cases), and the surface and buildup regions.

METHODS

The four algorithms are assessed in homogeneous (IAEA-TECDOCE 1540) and heterogeneous (IAEA-TECDOC 1583) media. Dosimetric evaluation accuracy for VMAT plans is then analyzed, along with the evaluation of the accuracy of algorithms applying to the surface and buildup regions.

RESULTS

Tests conducted in homogeneous media revealed that all algorithms exhibit dose deviations within 5% for various conditions, with pass rates exceeding 95% based on recommended tolerances. Additionally, the tests conducted in heterogeneous media demonstrate high pass rates for all algorithms, with a 100% pass rate observed for 6 MV and mostly 100% pass rate for 15 MV, except for CCC, which achieves a pass rate of 94%. The results of gamma index pass rate (GIPR) for dose calculation algorithms in IMRT fields show that GIPR (3% /3 mm) for all four algorithms in all evaluated tests based on TG119, are greater than 97%. The results of the algorithm testing for the accuracy of superficial dose reveal variations in dose differences, ranging from -11.9% to 7.03% for 15 MV and -9.5% to 3.3% for 6 MV, respectively. It is noteworthy that the AXB and MC algorithms demonstrate relatively lower discrepancies compared to the other algorithms.

CONCLUSIONS

This study shows that generally, two dose calculation algorithms (AXB and MC) that calculate dose in medium have better accuracy than other two dose calculation algorithms (CCC and AAA) that calculate dose to water.

Beam modeling and commissioning for Monte Carlo photon beam on an Elekta Versa HD LINAC

PURPOSE

This study aims at analyzing beam data commissioning along with verifying Monte Carlo-based treatment planning system on the basis of the manufacturer guidelines for Elekta Versa HD Linear Accelerator. Moreover, evaluating the beam match process in terms of quality index, photon profile and multi leaf collimator (MLC) offset is aimed as well.

MATERIALS AND METHODS

The process of collecting beam data for Monaco 5.51 Treatment Planning System commissioning was done based on the instructions provided by the manufacturer as well as AAPM TG-106. Monte Carlo analysis was done for output factors in water, percent depth dose and beam profiles. A set of eight static and intensity modulated radiation therapy fields were used to verify the MLC parameters.

RESULTS

The difference between the measured and modeled penetrative quality (D₁₀) was achieved to be 0.54%. The output factors for 6 MV photon energy were measured and the difference between the measured and Monte Carlo output results was smaller than 1% for all the fields. The average percentage of passing the gamma criteria for commissioning test fields was (95+-4)%, however, the minimum agreement was 87.5% belonging to "7SEGA". Additionally, the agreement between both LINAC is 96%, however, the second LINAC reveals a positive offset in the point of approximately -4 cm on the x-axis at the crossplane.

CONCLUSION

Test commissioning was successfully verified using a homogeneous phantom for point dose measurements, post modelling MLC parameters and patient QA plans. All plan parameters pass the gamma criteria. 6 MV photon beam was successfully commissioned for Elekta VersaHD LINAC and is ready for clinical implementation.

Implementation of a new virtual source model in Gate 9.0 package to simulate Elekta Synergy MLCi2 6 MV accelerator

Monte Carlo simulation is appreciated as an extraordinary technique to investigate particle physic processes in Radiation Therapy. This task offers a new Virtual Source Model (VSM) based on an innovative reconstruction method to extract energy and angular distribution from the Python phase space output data. Extensive comparisons of dose distributions are performed to evaluate VSM simulation precision. Four squared field configurations extending from 3 × 3 to 20 × 20 cm²are chosen for dose calculation to test field size and symmetry influences. To evaluate simulation accuracy, the beam quality parameters (such asD₁₀(%),d_max(cm),d₈₀(cm), andTPR(20/10)) also validation tests (gamma index formalism for 2%/2 mm criteria, Distance To Agreement DTA, and the estimator standard error (ϵ,ϵ_max)) are determined. Good agreement is achieved in terms of beam quality parameters and validation tests for each evaluated beam size, within a computation time of 58 hours and 17 hours on 20 nodes (presents 160 CPUs) of the full simulation and the VSM, respectively. This advanced VSM generated for the Elekta Synergy MLCi2 platform displays an uncomplicated approach. It is a great example of reconstructing different x-ray beams of various linac accelerators to facilitate its integration in cancer treatment.

Comparison of the RayStation photon Monte Carlo dose calculation algorithm against measured data under homogeneous and heterogeneous irradiation geometries

PURPOSE

This work compares Monte Carlo dose calculations performed using the RayStation treatment planning system against data measured on a Varian Truebeam linear accelerator with 6 MV and 10 MV FFF photon beams.

METHODS

The dosimetric performance of the RayStation Monte Carlo calculations was evaluated in a variety of irradiation geometries employing homogeneous and heterogeneous phantoms. Profile and depth dose comparisons against measurement were carried out in relative mode using the gamma index as a quantitative measure of similarity within the central high dose regions.

RESULTS

The results demonstrate that the treatment planning system dose calculation engine agrees with measurement to within 2%/1 mm for more than 95% of the data points in the high dose regions for all test cases. A systematic underestimation was observed at the tail of the profile penumbra and out of field, with mean differences generally <0.5 mm or 1% of curve dose maximum respectively. Out of field agreement varied between evaluated beam models.

CONCLUSIONS

The RayStation implementation of photon Monte Carlo dose calculations show good agreement with measured data for the range of scenarios considered in this work and is deemed sufficiently accurate for introduction into clinical use.

Enhancement of the Dose on 12 MV Linac with Free Flattening Filter Mode

PURPOSE

In the last years, some studies investigated dosimetric benefits of a free flattening filter for the photon mode in the radiotherapy field. This study aims to provide a theoretical study published and analysis of basic dosimetric properties for a Saturne 43 Linac head to implement free flattening filter beams clinically.

MATERIAL AND METHODS

This is the first Monte Carlo study for the head of Saturne 43 with replacement flattening filter mode investigating beam dosimetric characteristics, including central axis absorbed doses, beam profiles and photon energy spectra. The later ones were analyzed for flattening filter and replacement flattening filter beams using BEAMnrc and DOSXYZnrc Monte Carlo codes for 10 × 10 cm², 5 × 5 cm² and 2 × 2 cm² square field sizes.

RESULTS

A 3.94-fold increase of dose rate and electron contaminating increased by 246.4 % with the replacement flattening filter mode for field size of 10 × 10 cm². Reduction was made by replacement flattening filter beam in the peripheral dose up to 30%, and the time was reduced more than 50 %.

CONCLUSION

Results obtained from our study revealed that some characteristic dosimetries such as the maximum increase in depth dose rate, decrease in out-of-depth dose, and reducing time can be beneficial for the unflattened beam to be used in the radiotherapy for the advanced techniques.

Technical Note: An approach to building a Monte Carlo simulation model for a double scattering proton beam system

PURPOSE

The purpose of this study was to demonstrate and develop a Monte Carlo (MC) simulation model for a passive double scattering compact proton therapy system based on limited information of the mechanical components.

METHOD

We built a virtual machine source model (VMSM) which included a detailed definition of each beam-modifying component in the nozzle. Conceptually, it is similar to the conventional virtual analytical source model (VASM), except that the numerical machine nozzle or beamline is constructed in the VMSM, whereas in the VASM analytical parameters characterizing the energy spectrum and source fluence distribution are sought. All major beam shaping components were included in the VMSM and the model simulates interactions of the beam with a rotating range modulation wheel (RMW) combined with the beam current modulation. The RMWs, the first and second scatterer in the system were generated and tuned to reproduce measurement data as closely as possible. To validate the model, we compared the percent depth dose curves, spread out Bragg peaks (SOBPs) and lateral profiles against measured commissioning beam data.

RESULTS

The agreement of beam range between the MC calculation and measurement was within 1 mm for all beam options. The distal-falloff length was in good agreement as well (<1 mm for the large and deep groups, <1.5 mm for the small group). Agreement to within 2.5 mm of measured SOBP widths was obtained for all MC calculations. For lateral profiles, differences were found to be less than 2 mm.

CONCLUSIONS

We demonstrated that with limited geometrical information it is possible to build an acceptable source model for MC simulations of a passive double scattering compact proton therapy system. The agreement between the measurements and the MC model provides validation for use of the model for further studies of the dosimetric effects in patient treatments.

Efficient independent planar dose calculation for FFF IMRT QA with a bivariate Gaussian source model

The aim of this study is to perform a direct comparison of the source model for photon beams with and without flattening filter (FF) and to develop an efficient independent algorithm for planar dose calculation for FF‐free (FFF) intensity‐modulated radiotherapy (IMRT) quality assurance (QA). The source model consisted of a point source modeling the primary photons and extrafocal bivariate Gaussian functions modeling the head scatter, monitor chamber backscatter, and collimator exchange effect. The model parameters were obtained by minimizing the difference between the calculated and measured in‐air output factors (S_c). The fluence of IMRT beams was calculated from the source model using a backprojection and integration method. The off‐axis ratio in FFF beams were modeled with a fourth degree polynomial. An analytical kernel consisting of the sum of three Gaussian functions was used to describe the dose deposition process. A convolution‐based method was used to account for the ionization chamber volume averaging effect when commissioning the algorithm. The algorithm was validated by comparing the calculated planar dose distributions of FFF head‐and‐neck IMRT plans with measurements performed with a 2D diode array. Good agreement between the measured and calculated S_c was achieved for both FF beams (<0.25%) and FFF beams (<0.10%). The relative contribution of the head‐scattered photons reduced by 34.7% for 6 MV and 49.3% for 10 MV due to the removal of the FF. Superior agreement between the calculated and measured dose distribution was also achieved for FFF IMRT. In the gamma comparison with a 2%/2 mm criterion, the average passing rate was 96.2 ± 1.9% for 6 MV FFF and 95.5 ± 2.6% for 10 MV FFF. The efficient independent planar dose calculation algorithm is easy to implement and can be valuable in FFF IMRT QA.

Beam Characterization of 10-MV Photon Beam from Medical Linear Accelerator without Flattening Filter

Aim:

This work investigated the dosimetric properties of a 10-MV photon beam emitted from a medical linear accelerator (linac) with no flattening filter (FF). The aim of this study is to analyze the radiation fluence and energy emitted from the flattening filter free (FFF) linac using Monte Carlo (MC) simulations.

Materials and Methods:

The FFF linac was created by removing the FF from a linac in clinical use. Measurements of the depth dose (DD) and the off-axis profile were performed using a three-dimensional water phantom with an ionization chamber. A MC simulation for a 10-MV photon beam from this FFF linac was performed using the BEAMnrc code.

Results:

The off-axis profiles for the FFF linac exhibited a chevron-like distribution, and the dose outside the irradiation field was found to be lower for the FFF linac than for a linac with an FF (FF linac). The DD curves for the FFF linac included many contaminant electrons in the build-up region.

Conclusion:

Therefore, for clinical use, a metal filter is additionally required to reduce the effects of the electron contamination. The mean energy of the FFF linac was found to be lower than that of the FF linac owing to the absence of beam hardening caused by the FF.

Automatic commissioning of a GPU-based Monte Carlo radiation dose calculation code for photon radiotherapy

Monte Carlo (MC) simulation is commonly considered as the most accurate method for radiation dose calculations. Commissioning of a beam model in the MC code against a clinical linear accelerator beam is of crucial importance for its clinical implementation. In this paper, we propose an automatic commissioning method for our GPU-based MC dose engine, gDPM. gDPM utilizes a beam model based on a concept of phase-space-let (PSL). A PSL contains a group of particles that are of the same type and close in space and energy. A set of generic PSLs was generated by splitting a reference phase-space file. Each PSL was associated with a weighting factor, and in dose calculations the particle carried a weight corresponding to the PSL where it was from. Dose for each PSL in water was pre-computed, and hence the dose in water for a whole beam under a given set of PSL weighting factors was the weighted sum of the PSL doses. At the commissioning stage, an optimization problem was solved to adjust the PSL weights in order to minimize the difference between the calculated dose and measured one. Symmetry and smoothness regularizations were utilized to uniquely determine the solution. An augmented Lagrangian method was employed to solve the optimization problem. To validate our method, a phase-space file of a Varian TrueBeam 6 MV beam was used to generate the PSLs for 6 MV beams. In a simulation study, we commissioned a Siemens 6 MV beam on which a set of field-dependent phase-space files was available. The dose data of this desired beam for different open fields and a small off-axis open field were obtained by calculating doses using these phase-space files. The 3D γ-index test passing rate within the regions with dose above 10% of dmax dose for those open fields tested was improved averagely from 70.56 to 99.36% for 2%/2 mm criteria and from 32.22 to 89.65% for 1%/1 mm criteria. We also tested our commissioning method on a six-field head-and-neck cancer IMRT plan. The passing rate of the γ-index test within the 10% isodose line of the prescription dose was improved from 92.73 to 99.70% and from 82.16 to 96.73% for 2%/2 mm and 1%/1 mm criteria, respectively. Real clinical data measured from Varian, Siemens, and Elekta linear accelerators were also used to validate our commissioning method and a similar level of accuracy was achieved.

Evaluating IMRT and VMAT dose accuracy: practical examples of failure to detect systematic errors when applying a commonly used metric and action levels

PURPOSE

This study (1) examines a variety of real-world cases where systematic errors were not detected by widely accepted methods for IMRT/VMAT dosimetric accuracy evaluation, and (2) drills-down to identify failure modes and their corresponding means for detection, diagnosis, and mitigation. The primary goal of detailing these case studies is to explore different, more sensitive methods and metrics that could be used more effectively for evaluating accuracy of dose algorithms, delivery systems, and QA devices.

METHODS

The authors present seven real-world case studies representing a variety of combinations of the treatment planning system (TPS), linac, delivery modality, and systematic error type. These case studies are typical to what might be used as part of an IMRT or VMAT commissioning test suite, varying in complexity. Each case study is analyzed according to TG-119 instructions for gamma passing rates and action levels for per-beam and/or composite plan dosimetric QA. Then, each case study is analyzed in-depth with advanced diagnostic methods (dose profile examination, EPID-based measurements, dose difference pattern analysis, 3D measurement-guided dose reconstruction, and dose grid inspection) and more sensitive metrics (2% local normalization/2 mm DTA and estimated DVH comparisons).

RESULTS

For these case studies, the conventional 3%/3 mm gamma passing rates exceeded 99% for IMRT per-beam analyses and ranged from 93.9% to 100% for composite plan dose analysis, well above the TG-119 action levels of 90% and 88%, respectively. However, all cases had systematic errors that were detected only by using advanced diagnostic techniques and more sensitive metrics. The systematic errors caused variable but noteworthy impact, including estimated target dose coverage loss of up to 5.5% and local dose deviations up to 31.5%. Types of errors included TPS model settings, algorithm limitations, and modeling and alignment of QA phantoms in the TPS. Most of the errors were correctable after detection and diagnosis, and the uncorrectable errors provided useful information about system limitations, which is another key element of system commissioning.

CONCLUSIONS

Many forms of relevant systematic errors can go undetected when the currently prevalent metrics for IMRT∕VMAT commissioning are used. If alternative methods and metrics are used instead of (or in addition to) the conventional metrics, these errors are more likely to be detected, and only once they are detected can they be properly diagnosed and rooted out of the system. Removing systematic errors should be a goal not only of commissioning by the end users but also product validation by the manufacturers. For any systematic errors that cannot be removed, detecting and quantifying them is important as it will help the physicist understand the limits of the system and work with the manufacturer on improvements. In summary, IMRT and VMAT commissioning, along with product validation, would benefit from the retirement of the 3%/3 mm passing rates as a primary metric of performance, and the adoption instead of tighter tolerances, more diligent diagnostics, and more thorough analysis.

Commissioning and verification of the collapsed cone convolution superposition algorithm for SBRT delivery using flattening filter-free beams

Linacs equipped with flattening filter‐free (FFF) megavoltage photon beams are now commercially available. However, the commissioning of FFF beams poses challenges that are not shared with traditional flattened megavoltage X‐ray beams. The planning system must model a beam that is peaked in the center and has an energy spectrum that is softer than the flattened beam. Removing the flattening filter also increases the maximum possible dose rates from 600 MU/min up to 2400 MU/min in some cases; this increase in dose rate affects the recombination correction factor, Pion, used during absolute dose calibration with ionization chambers. We present the first‐reported experience of commissioning, verification, and clinical use of the collapsed cone convolution superposition (CCCS) dose calculation algorithm for commercially available flattening filter‐free beams. Our commissioning data are compared to previously reported measurements and Monte Carlo studies of FFF beams. Commissioning was verified by making point‐dose measurement of test plans, irradiating the RPC lung phantom, and performing patient‐specific QA. The average point‐dose difference between calculations and measurements of all test plans and all patient specific QA measurements is 0.80%, and the RPC phantom absolute dose differences for the two thermoluminescent dosimeters (TLDs) in the phantom planning target volume (PTV) were 1% and 2%, respectively. One hundred percent (100%) of points in the RPC phantom films passed the RPC gamma criteria of 5% and 5 mm. Our results show that the CCCS algorithm can accurately model FFF beams and calculate SBRT dose distributions using those beams.

PACS number: 87.55.kh

Monte Carlo investigation into feasibility and dosimetry of flat flattening filter free beams

Flattening filter free (FFF) beams due to their non-uniformity, are sub-optimal for larger field sizes. The purpose of this study was to investigate the incident electron beam distributions that would produce flat FFF (F4) beams without the use of a flattening filter (FF). Monte Carlo (MC) simulations with BEAMnrc and DOSXYZnrc codes have been performed to evaluate feasibility of this approach. The dose distributions in water for open 6 MV beams were simulated using the Varian 21EX linac head model, which will be called the FF model. The FF was then removed from the FF model, and MC simulations were performed using (1) 6 MeV electrons incident on the target and (2) a 6 MeV electron beam with electron angular distributions optimized to provide as flat dose profiles as possible. Configuration (1) represents FFF beam while configuration (2) allowed producing a F4 beam. Optimizations have also been performed to produce flattest profiles for a set of dose rates (DRs) in the range from 1.25 to 2.4 of the DR of FF beam. Profiles and percentage depth doses (PDDs) from 6 MV F4 beams have been calculated and compared to those from the FF beam. Calculated profiles demonstrated improved flatness of the FFF beams. In fact, up to field sizes within the circle of 35 cm diameter the flatness of F4 beam at dmax was better or comparable to that of FF beam. At 20 cm off-axis the dose increased from 52% for FFF to 92% for F4 beam. Also, profiles of F4 beams did not change considerably with depth. PDDs from F4 beams were similar to those of the FFF beam. The DR for the largest modeled (44 cm diameter) F4 beam was higher than the DR from FF beam by a factor of 1.25. It was shown that the DR can be increased while maintaining beam flatness, but at the cost of reduced field size.

A comprehensive procedure for characterizing arbitrary azimuthally symmetric photon beams

PURPOSE

A new Monte Carlo (MC) source model (SM) has been developed for azimuthally symmetric photon beams.

METHODS

The MC simulation tallied phase space file (PSF) is divided into two categories depending on the relationship of the particle track line to the beam central axis: multiple point source (MPS) and spatial mesh based surface source (SMBSS). To validate this SM, MCNPX2.6 was used to generate two PSFs for a 6 MV photon beam from a Varian 2100C/D linear accelerator.

RESULTS

PDDs and profiles were calculated using the SM and original PSF for different field sizes from 5 × 5 to 40 × 40 cm2. Agreement was within 2% of the maximum dose at 100 cm SSD and 2.5% of the maximum dose at 200 cm SSD for beam profiles at depths of 3.5 cm and 15 cm with respect to the original PSF. Differences between the source model and the PSF in the out-of-field regions were less than 0.5% of the profile maximum value at 100 cm SSD. Differences between measured and calculated points were less than 2% of the maximum dose or 2 mm distance to agreement (DTA) at 100 cm SSD.

CONCLUSIONS

This SM is unique in that it accounts for a higher level of energy dependence on the particle's direction and it is independent from accelerator components, unlike other published SMs. The model can be applied to any arbitrary azimuthally symmetric beam and has source biasing capabilities that significantly increase the simulation speed up to 3300 for certain field sizes.