Validation of Varian TrueBeam electron phase–spaces for Monte Carlo simulation of MLC‐shaped fields

Lloyd, Samantha A. M.; Gagne, Isabelle; Bazalova‐Carter, Magdalena; Zavgorodni, Sergei

doi:10.1118/1.4949000

Cited by 11 publications

(16 citation statements)

References 55 publications

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“…29,30,50 Gamma pass rates of 2%/2 mm and 3%/3 mm have been reported for pMLC-shaped beams with high-and low-energy beams respectively. 33,34 In our study, Monte Carlo calculations agreed with measurements satisfying gamma criterion of 1%/1 mm for PDD calculations and 2%/ 1 mm for profiles. However, we were only able to achieve FWHM and penumbra within 0.4 cm of the measurements.…”

Section: B Monte Carlo Model Validationsupporting

confidence: 81%

“…Monte Carlo simulations have been used to demonstrate the feasibility and to further advance the use of existing photon MLC for electron collimation to produce clinically acceptable dose distributions. 1,5,9,20,[30][31][32][33][34][35][36][37] In a more recent study, Mueller et al have performed a comprehensive study of pMLC collimated electron beams including MC simulations of patient cases. 38 It was concluded that treatment plans using cut-outs can be replaced by plans of similar quality using pMLC collimation.…”

Section: Introductionmentioning

confidence: 99%

“…They validated the vendor provided phase space files only for conventional electron therapy, therefore the pMLCs were fully retracted. Monte Carlo simulations have been used to demonstrate the feasibility and to further advance the use of existing photon MLC for electron collimation to produce clinically acceptable dose distributions 1,5,9,20,30–37 . In a more recent study, Mueller et al have performed a comprehensive study of pMLC collimated electron beams including MC simulations of patient cases 38 .…”

Section: Introductionmentioning

confidence: 99%

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Validation of a Monte Carlo model for multi leaf collimator based electron delivery

et al. 2020

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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 ga...

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Section: B Monte Carlo Model Validationsupporting

confidence: 81%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Validation of a Monte Carlo model for multi leaf collimator based electron delivery

et al. 2020

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“…These issues were addressed in recent studies by demonstrating the viability of delivering modulated electron radiation therapy (MERT) with a photon multileaf collimator (MLC) to eliminate the accessory requirements (applicators, cut‐outs, electron‐specific MLC) of traditional electron RT . By delivering MERT plans with a standard photon MLC, the potential arises for seamless mixed electron–photon delivery without interrupting treatment delivery between modality changes.…”

Section: Introductionmentioning

confidence: 99%

Robust mixed electron–photon radiation therapy optimization

2019

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Purpose Mixed beam electron–photon radiation therapy (MBRT) is an emerging technique that has the potential to reduce dose to normal tissue while improving target coverage for cancer sites with superficial tumors. Advances in optimization algorithms and robotic linear accelerators have made the creation and delivery of complex MBRT plans realistic without the need for special additional collimators, devices, or resetup of the patient. However, no study has been performed on the robustness of MBRT dose distributions to patient setup errors. Intensity‐modulated delivery of other charged particles such as protons have been shown to require robust planning techniques to maintain adequate target coverage under positioning errors. We therefore assess the sensitivity of MBRT treatment plans to positioning uncertainties when created under the traditional planning target volume (PTV)‐based planning paradigm and present a novel optimization model for the creation of robust MBRT plans. Methods The column generation method was applied to robust MBRT treatment planning by deriving the pricing problem for stochastic and “worst case” minimax optimization models, two common formulations of robustness. Robust treatment plans were created for two patient cases representative of the cancer sites which stand to benefit from MBRT: soft tissue sarcoma (STS) irradiation and chest wall irradiation with deep‐seated internal mammary, axillary, and supraclavicular nodes (CW‐N). For both patient cases, beamlet dose distributions for electrons and photons were generated for positioning shifts in six directions, ±5mm(xfalse^,yfalse^,zfalse^) in addition to a nominal unshifted scenario, for a total of seven sets of beamlets. Robust plans were created by specifying dose coverage constraints to the clinical target volume (CTV), as opposed to the PTV. Comparisons were performed against traditional PTV‐based plans created with a single set of unshifted beamlets. Results The dose distributions of traditional PTV‐based MBRT plans showed significant degradation in target coverage homogeneity when patient positioning errors were considered. For both cancer sites, cold spots below 95% and hot spots above 108% of the prescription dose appeared within the CTV when shifting the patient by 5 mm, corresponding to the margin added to the CTV to form the PTV. In contrast, CTV‐based robust plans created with the new optimization model maintained target coverage within the 95%–108% limits, for all positioning errors. Conclusion The quality of MBRT treatment plans created using a traditional PTV‐based optimization model was highly sensitive to patient positioning errors. For both patient cases, positioning errors resulted in perturbations to the nominal dose distributions which would have rendered PTV‐based plans clinically unacceptable. In contrast, CTV‐based robust plans were able to maintain adequate target coverage under all positioning error scenarios considered. We therefore conclude that to ensure the fidelity of the dose distribution delivered to the pat...

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“…The geometrical description of the Millennium 120 MLC is done according to the blueprints of the collimator. By following this approach and conducting an accurate transport of radiation through all the collimating and beam-modifying elements of the gantry, it is possible to reproduce the dose distributions conformed by the linac without resorting to non-physical parameters such as the dynamic leaf gap ( 26 , 30 , 31 ). The geometrical description of the MLC is a faithful model of the actual collimator.…”

Section: Methodsmentioning

confidence: 99%

Experimental Validation of an Analytical Program and a Monte Carlo Simulation for the Computation of the Far Out-of-Field Dose in External Beam Photon Therapy Applied to Pediatric Patients

et al. 2022

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BackgroundThe out-of-the-field absorbed dose affects the probability of primary second radiation-induced cancers. This is particularly relevant in the case of pediatric treatments. There are currently no methods employed in the clinical routine for the computation of dose distributions from stray radiation in radiotherapy. To overcome this limitation in the framework of conventional teletherapy with photon beams, two computational tools have been developed—one based on an analytical approach and another depending on a fast Monte Carlo algorithm. The purpose of this work is to evaluate the accuracy of these approaches by comparison with experimental data obtained from anthropomorphic phantom irradiations.Materials and MethodsAn anthropomorphic phantom representing a 5-year-old child (ATOM, CIRS) was irradiated considering a brain tumor using a Varian TrueBeam linac. Two treatments for the same planned target volume (PTV) were considered, namely, intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). In all cases, the irradiation was conducted with a 6-MV energy beam using the flattening filter for a prescribed dose of 3.6 Gy to the PTV. The phantom had natLiF : Mg, Cu, P (MCP-N) thermoluminescent dosimeters (TLDs) in its 180 holes. The uncertainty of the experimental data was around 20%, which was mostly attributed to the MCP-N energy dependence. To calculate the out-of-field dose, an analytical algorithm was implemented to be run from a Varian Eclipse TPS. This algorithm considers that all anatomical structures are filled with water, with the exception of the lungs which are made of air. The fast Monte Carlo code dose planning method was also used for computing the out-of-field dose. It was executed from the dose verification system PRIMO using a phase-space file containing 3x109 histories, reaching an average standard statistical uncertainty of less than 0.2% (coverage factor k = 1 ) on all voxels scoring more than 50% of the maximum dose. The standard statistical uncertainty of out-of-field voxels in the Monte Carlo simulation did not exceed 5%. For the Monte Carlo simulation the actual chemical composition of the materials used in ATOM, as provided by the manufacturer, was employed.ResultsIn the out-of-the-field region, the absorbed dose was on average four orders of magnitude lower than the dose at the PTV. For the two modalities employed, the discrepancy between the central values of the TLDs located in the out-of-the-field region and the corresponding positions in the analytic model were in general less than 40%. The discrepancy in the lung doses was more pronounced for IMRT. The same comparison between the experimental and the Monte Carlo data yielded differences which are, in general, smaller than 20%. It was observed that the VMAT irradiation produces the smallest out-of-the-field dose when compared to IMRT.ConclusionsThe proposed computational methods for the routine calculation of the out-of-the-field dose produce results that are similar, in most cases, with the experimental data. It has been experimentally found that the VMAT irradiation produces the smallest out-of-the-field dose when compared to IMRT for a given PTV.

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Validation of Varian TrueBeam electron phase–spaces for Monte Carlo simulation of MLC‐shaped fields

Cited by 11 publications

References 55 publications

Validation of a Monte Carlo model for multi leaf collimator based electron delivery

Validation of a Monte Carlo model for multi leaf collimator based electron delivery

Robust mixed electron–photon radiation therapy optimization

Experimental Validation of an Analytical Program and a Monte Carlo Simulation for the Computation of the Far Out-of-Field Dose in External Beam Photon Therapy Applied to Pediatric Patients

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