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Purpose: A good Monte Carlo model with an accurate head shielding model is important in estimating the long-term risks of unwanted radiation exposure during radiation therapy. The aim of this paper was to validate the Monte Carlo simulation of a TrueBeam linear accelerator (linac) head shielding model. We approach this by evaluating the accuracy of out-of-field dose predictions at extended distances which are comprised of scatter from within the patient and treatment head leakage and thus reflect the accuracy of the head shielding model. We quantify the out-of-field dose of a TrueBeam linac for low-energy photons, 6X and 6X-FFF beams, and compare measurements to Monte Carlo simulations using Varian VirtuaLinac that include a realistic head shielding model, for a variety of jaw sizes and angles up to a distance of 100 cm from the isocenter, in both positive and negative directions. Given the high value and utility of the VirtuaLinac model, it is critical that this model is validated thoroughly and the results be available to the medical physics community., Materials and Method: Simulations were done using VirtuaLinac, the GEANT4-based Monte Carlo model of the TrueBeam treatment head from Varian Medical Systems, and an in-house GEANT4-based code. VirtuaLinac included a detailed model of the treatment head shielding and was run on the Amazon Web Services cloud to generate spherical phase space files surrounding the treatment head. These phase space files were imported into the in-house code, which modeled the measurement setup with a solid water buildup, the carbon fiber couch, and the gantry stand. For each jaw size (2 × 2 cm 2 , 4 × 4 cm 2 , 10 × 10 cm 2 , and 20 × 20 cm 2 ) and angular setting (0°, 90°, 45°, 135°), the dose was calculated at intervals of 5 cm along each measurement direction., Results: For the 10 × 10 cm 2 jaw size, both 6X and 6X-FFF showed very good agreement between simulation and measurement in both in-plane directions, with no apparent systematic bias. The percentage deviations for these settings were as follows: (mean, STDEV, maximum) (8.34, 6.44, 24.84) for 6X and (13.21, 8.93, 35.56) for 6X-FFF. For all jaw sizes, simulation agreed well in the in-plane direction going away from the gantry, but, some deviations were observed moving toward the gantry at larger distances. At larger distances, for the jaw sizes smaller than 10 × 10 cm 2 , the simulation underestimates the dose compared with measurement, while for jaw sizes larger than 10 × 10 cm 2 , it overestimates dose. For all comparisons between ±50 cm from isocenter, average absolute agreement between simulation and measurement was better than 28%., Conclusion: We have validated the Varian VirtuaLinac's head shielding model via out-of-field doses and quantified the differences between TrueBeam head shielding model created out-of-field doses and measurements for an extended distance of 100 cm., (© 2018 American Association of Physicists in Medicine.)

Purpose: Challenges in proton therapy include identifying patients most likely to benefit; ensuring consistent, high‐quality plans as its adoption becomes more widespread; and recognizing biological uncertainties that may be related to increased relative biologic effectiveness driven by linear energy transfer (LET). Knowledge‐based planning (KBP) is a domain that may help to address all three. Methods: Artificial neural networks were trained using 117 unique treatment plans and associated dose and dose‐weighted LET (LETD) distributions. The data set was split into training (n = 82), validation (n = 17), and test (n = 18) sets. Model performance was evaluated on the test set using dose‐ and LETD‐volume metrics in the clinical target volume (CTV) and nearby organs at risk and Dice similarity coefficients (DSC) comparing predicted and planned isodose lines at 50%, 75%, and 95% of the prescription dose. Results: Dose‐volume metrics significantly differed (α = 0.05) between predicted and planned dose distributions in only one dose‐volume metric, D2% to the CTV. The maximum observed root mean square (RMS) difference between corresponding metrics was 4.3 GyRBE (8% of prescription) for D1cc to optic chiasm. DSC were 0.90, 0.93, and 0.88 for the 50%, 75%, and 95% isodose lines, respectively. LETD‐volume metrics significantly differed in all but one metric, L0.1cc of the brainstem. The maximum observed difference in RMS differences for LETD metrics was 1.0 keV/μm for L0.1cc to brainstem. Conclusions: We have devised the first three‐dimensional dose and LETD‐prediction model for cranial proton radiation therapy has been developed. Dose accuracy compared favorably with that of previously published models in other treatment sites. The agreement in LETD supports future investigations with biological doses in mind to enable the full potential of KBP in proton therapy. [ABSTRACT FROM AUTHOR]

Purpose: To measure the out‐of‐field mean photon energy and dose imparted by the secondary radiation field generated by 6 MV and 6 MV FFF beams using TLD‐300 and TLD‐100 dosimeters and to use the technique to quantify the contributions from the different sources that generate out‐of‐field radiation. Methods: The mean photon energy and the dose were measured using the TLD‐300 glow curve properties and the TLD‐100 response, respectively. The TLD‐300 glow curve shape was energy‐calibrated with gamma rays from 99mTc, 18F, 137Cs, and 60Co sources, and its energy dependence was quantified by a parameter obtained from the curve deconvolution. The TLD‐100 signal was calibrated in absorbed dose‐to‐water inside the primary field. Dosimeters were placed on the linac head, and on the surface and at 4.5 cm depth in PMMA at 1–15 cm lateral distances from a 10 × 10 cm2 field edge at the isocenter plane. Three configurations of dosimeters around the linac were defined to identify and quantify the contributions from the different sources of out‐of‐field radiation. Results: Typical energies of head leakage were about 500 keV for both beams. The mean energy of collimator‐scattered radiation was equal to or larger than 1250 keV and, for phantom‐scattered radiation, mean photon energies were 400 keV for the 6 MV and 300 keV for the 6 MV FFF beam. Relative uncertainties to determine mean photon energy were better than 15% for energies below 700 keV, and 40% above 1000 keV. The technique lost its sensitivity to the incident photon energy above 1250 keV. On the phantom surface and at 1–15 cm from the field edge, 80%–90% of out‐of‐field dose came from scattering in the secondary collimator. At 4.5 cm deep in the phantom and 1–5 cm from the field edge, 50%–60% of the out‐of‐field dose originated in the phantom. At the points of measurement, the head leakage imparted less than 0.1% of the dose at the isocenter. The 6 MV FFF beam imparted 8–36% less out‐of‐field dose than the 6 MV beam. These energy results are consistent with general Monte Carlo simulation predictions and show excellent agreement with simulations for a similar linac. The measured out‐of‐field doses showed good agreement with independent evaluations. Conclusions: The out‐of‐field mean photon energy and dose imparted by the secondary radiation field were quantified by the applied TLD‐300/TLD‐100 method. The main sources of out‐of‐field dose were identified and quantified using three configurations of dosimeters around the linac. This technique could be of value to validate Monte Carlo simulations where the linac head design, configuration, or material composition are unavailable. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Purpose: To develop and validate a Monte Carlo model of the Varian TrueBeam to study electron collimation using the existing photon multi‐leaf collimators (pMLC), instead of conventional electron applicators and apertures. Materials and Methods: A complete Monte Carlo model of the Varian TrueBeam was developed using Tool for particle simulation (TOPAS) (version 3.1.p3). Vendor‐supplied information was used to model the treatment head components and the source parameters. A phase space plane was setup above the collimating jaws and captured particles were reused until a statistical uncertainty of 1% was achieved in the central axis. Electron energies 6, 9, 12, 16, and 20 MeV with a jaw‐defined field of 20 × 20 cm2 at iso‐center, pMLC‐defined fields of 6.8 × 6.8 cm2 and 11.4 × 11.4 cm2 at 80 cm source‐to‐surface distance (SSD) and an applicator‐defined field of 10 × 10 cm2 at iso‐center were evaluated. All the measurements except the applicator‐defined fields were measured using an ionization chamber in a water tank using 80 cm SSD. The dose difference, distance‐to‐agreement and gamma index were used to evaluate the agreement between the Monte Carlo calculations and measurements. Contributions of electron scattering off pMLC leaves and inter‐leaf leakage on dose profiles were evaluated and compared with Monte Carlo calculations. Electron transport through a heterogeneous phantom was simulated and the resulting dose distributions were compared with film measurements. The validated Monte Carlo model was used to simulate several clinically motivated cases to demonstrate the benefit of pMLC‐based electron delivery compared to applicator‐based electron delivery. Results: Calculated and measured percentage depth‐dose (PDD) curves agree within 2% after normalization. The agreement between normalized percentage depth dose curves were evaluated using one‐dimensional gamma analysis with a local tolerance of 2%/1 mm and the %points passing gamma criteria was 100% for all energies. For jaw‐defined fields, calculated profiles agree with measurements with pass rates of >97% for 2%/2 mm gamma criteria. Calculated FWHM and penumbra width agree with measurements within 0.4 cm. For fields with tertiary collimation using an pMLC or applicator, the average gamma pass rate of compared profiles was 98% with 2%/2 mm gamma criteria. The profiles measured to evaluate the pMLC leaf scattering agreed with Monte Carlo calculations with an average gamma pass rate of 96.5% with 3%/2 mm gamma criteria. Measured dose profiles below the heterogenous phantom agreed well with calculated profiles and matched within 2.5% for most points. The calculated clinically applicable cases using TOPAS MC and Eclipse TPS for single enface electron beam, electron‐photon mixed beam and a matched electron‐electron beam exhibited a reasonable agreement in PDDs, profiles and dose volume histograms. Conclusion: We present a validation of a Monte Carlo model of Varian TrueBeam for pMLC‐based electron delivery. Monte Carlo calculations agreed with measurements satisfying gamma criterion of 1%/1 mm for depth dose curves and 2%/1 mm for dose profiles. The simulation of clinically applicable cases demonstrated the clinical utility of pMLC‐based electrons and the use of MC simulations for development of advanced radiation therapy techniques. [ABSTRACT FROM AUTHOR]

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., (© 2022 The Authors. Medical Physics published by Wiley Periodicals LLC on behalf of American Association of Physicists in Medicine.)

Purpose: With increasing cancer treatment success rates, many patients go on to live long, productive lives following recovery. Therefore, minimizing potential side effects due to dose outside the treated field is becoming a significant consideration in radiation therapy. With many potential treatment configurations available, it is important to quantify how out‐of‐field dose varies with common variables such as distance from isocenter, couch angle, jaw size, and flattening‐filter setting. The accurate quantification of out‐of‐field dose at extended distances could also benefit researchers and detector developers. While data exist for out‐of‐field dose from older linear accelerator (Linac) models, the phenomenon has not been described for the latest generation of machines, such as the Varian TrueBeam. The purpose of this study was to comprehensively quantify out‐of‐field dose for the Varian TrueBeam Linac low energy photons in a wide range of positions and treatment geometries. Method and Materials: Out‐of‐field doses were measured using two phantom setups: (a) A large volume ion chamber with a buildup sleeve to quantify head leakage and collimator scatter background dose; and (b) A farmer ion chamber in solid water to incorporate phantom scatter in addition to collimator scatter, and head leakage background dose. In both cases, the ion chamber was positioned with its length along the slowly varying transverse direction (perpendicular to the radial from isocenter). Doses were measured for four symmetric jaw settings (2 × 2 cm2, 4 × 4 cm2, 10 × 10 cm2, and 20 × 20 cm2) for a range of distances from the isocenter (0–100 cm). The angular dependence of the out‐of‐field dose was measured using four different angles: 0°, 45°, 90°, and 135° with respect to the in‐plane direction. All measurements were performed for both 6X and 6X‐flattening‐filter‐free (FFF) beams. Results: The lowest out‐of‐field doses were observed at 60 cm away from isocenter in both in‐plane and cross‐plane directions for fields smaller than 10 × 10 cm2. Out‐of‐field dose decreased with decreasing jaw size (a factor of 4.7 for 6X‐FFF and a factor of 3.1 for 6X going from 20 × 20 cm2 to 2 × 2 cm2 at 60 cm from isocenter in the in‐plane direction). The 6X‐FFF beam produced out‐of‐field doses as low as 64% of the 6X beam. Conclusion: This study presents a comprehensive description of 6X and 6X‐FFF out‐of‐field doses on a Varian TrueBeam Linac including measurements at a range of positions, angles, and jaw settings and with and without phantom scatter. [ABSTRACT FROM AUTHOR]