The influence of nuclear models and Monte Carlo radiation transport codes on stray neutron dose estimations in proton therapy

Saint-Hubert, Marijke De; Farah, Jad; Kłodowska, Magdalena; Romero-Expósito, Maite; Tymińska, K; Mares, V.; Olko, P.; Stolarczyk, Liliana; Trinkl, Sebastian

doi:10.1016/j.radmeas.2021.106693

Cited by 14 publications

(12 citation statements)

References 47 publications

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“…The development of a validated Monte Carlo (MC) framework forms an important aspect in the assessment and characterization of out-of-field doses in PT. Nevertheless, the use of general-purpose MC simulations in out-of-field dosimetry is often restricted to detector calibration, and it has highlighted important differences between MC codes and models ( 3 ). Moreover, the coupling of MC to advanced measurement and proper benchmarking of the MC codes and models are still unknown today.…”

Section: Introductionmentioning

confidence: 99%

Validation of a Monte Carlo Framework for Out-of-Field Dose Calculations in Proton Therapy

et al. 2022

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Proton therapy enables to deliver highly conformed dose distributions owing to the characteristic Bragg peak and the finite range of protons. However, during proton therapy, secondary neutrons are created, which can travel long distances and deposit dose in out-of-field volumes. This out-of-field absorbed dose needs to be considered for radiation-induced secondary cancers, which are particularly relevant in the case of pediatric treatments. Unfortunately, no method exists in clinics for the computation of the out-of-field dose distributions in proton therapy. To help overcome this limitation, a computational tool has been developed based on the Monte Carlo code TOPAS. The purpose of this work is to evaluate the accuracy of this tool in comparison to experimental data obtained from an anthropomorphic phantom irradiation. An anthropomorphic phantom of a 5-year-old child (ATOM, CIRS) was irradiated for a brain tumor treatment in an IBA Proteus Plus facility using a pencil beam dedicated nozzle. The treatment consisted of three pencil beam scanning fields employing a lucite range shifter. Proton energies ranged from 100 to 165 MeV. A median dose of 50.4 Gy(RBE) with 1.8 Gy(RBE) per fraction was prescribed to the initial planning target volume (PTV), which was located in the cerebellum. Thermoluminescent detectors (TLDs), namely, Li-7-enriched LiF : Mg, Ti (MTS-7) type, were used to detect gamma radiation, which is produced by nuclear reactions, and secondary as well as recoil protons created out-of-field by secondary neutrons. Li-6-enriched LiF : Mg,Cu,P (MCP-6) was combined with Li-7-enriched MCP-7 to measure thermal neutrons. TLDs were calibrated in Co-60 and reported on absorbed dose in water per target dose (μGy/Gy) as well as thermal neutron dose equivalent per target dose (μSv/Gy). Additionally, bubble detectors for personal neutron dosimetry (BD-PND) were used for measuring neutrons (>50 keV), which were calibrated in a Cf-252 neutron beam to report on neutron dose equivalent dose data. The Monte Carlo code TOPAS (version 3.6) was run using a phase-space file containing 1010 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 primary beam was modeled following a Fermi–Eyges description of the spot envelope fitted to measurements. For the Monte Carlo simulation, the chemical composition of the tissues represented in ATOM was employed. The dose was tallied as dose-to-water, and data were normalized to the target dose (physical dose) to report on absorbed doses per target dose (mSv/Gy) or neutron dose equivalent per target dose (μSv/Gy), while also an estimate of the total organ dose was provided for a target dose of 50.4 Gy(RBE). Out-of-field doses showed absorbed doses that were 5 to 6 orders of magnitude lower than the target dose. The discrepancy between TLD data and the corresponding scored values in the Monte Carlo calculations involving proton and gamma contributions was on average 18%. The comparison between the neutron equivalent doses between the Monte Carlo simulation and the measured neutron doses was on average 8%. Organ dose calculations revealed the highest dose for the thyroid, which was 120 mSv, while other organ doses ranged from 18 mSv in the lungs to 0.6 mSv in the testes. The proposed computational method for routine calculation of the out-of-the-field dose in proton therapy produces results that are compatible with the experimental data and allow to calculate out-of-field organ doses during proton therapy.

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Section: Introductionmentioning

confidence: 99%

Validation of a Monte Carlo Framework for Out-of-Field Dose Calculations in Proton Therapy

et al. 2022

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“…A standard uncertainty from simulation has a statistical component associated to the number of source particles. However, simulations in this energy range rely on nuclear models and different results can be obtained when using different models or even codes ( 27 ). This variability could be used as a measure of the accuracy of simulations, and, based on results from De-Saint Hubert et al.…”

Section: Methodsmentioning

confidence: 99%

“…This variability could be used as a measure of the accuracy of simulations, and, based on results from De-Saint Hubert et al. ( 27 ), we estimate a value of 20% to combine to statistical uncertainty.…”

Section: Methodsmentioning

confidence: 99%

Peripheral Organ Equivalent Dose Estimation Procedure in Proton Therapy

et al. 2022

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The aim of this work is to present a reproducible methodology for the evaluation of total equivalent doses in organs during proton therapy facilities. The methodology is based on measuring the dose equivalent in representative locations inside an anthropomorphic phantom where photon and neutron dosimeters were inserted. The Monte Carlo simulation was needed for obtaining neutron energy distribution inside the phantom. The methodology was implemented for a head irradiation case in the passive proton beam of iThemba Labs (South Africa). Thermoluminescent dosimeter (TLD)-600 and TLD-700 pairs were used as dosimeters inside the phantom and GEANT code for simulations. In addition, Bonner sphere spectrometry was performed inside the treatment room to obtain the neutron spectra, some relevant neutron dosimetric quantities per treatment Gy, and a percentual distribution of neutron fluence and ambient dose equivalent in four energy groups, at two locations. The neutron spectrum at one of those locations was also simulated so that a reasonable agreement between simulation and measurement allowed a validation of the simulation. Results showed that the total out-of-field dose equivalent inside the phantom ranged from 1.4 to 0.28 mSv/Gy, mainly due to the neutron contribution and with a small contribution from photons, 10% on average. The order of magnitude of the equivalent dose in organs was similar, displaying a slow reduction in values as the organ is farther from the target volume. These values were in agreement with those found by other authors in other passive beam facilities under similar irradiation and measurement conditions.

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“…Both protons and neutrons were tracked with a high-energy cutoff at 240 MeV, well above the maximum proton energy of 212 MeV. The Bertini Dresner intranuclear cascade and evaporation model was selected because it gave reliable results in previous simulations of neutron doses in proton therapy facilities ( 4 , 22 ). Furthermore, a limited sensitivity study showed less than 10% difference in simulated neutron H *(10) values when using the default cross sections and physics models in MCNP6.2.…”

Section: Methodsmentioning

confidence: 99%

“…Several studies have already characterized the out-of-field neutron doses in proton therapy for some specific cases using Monte Carlo (MC) radiation transport simulations, measurements, or analytical models ( 3 , 4 ). However, it is not straightforward to compare neutron doses in the literature and to estimate the neutron dose for a specific patient based on these studies due to the strong dependence of the neutron dose on the treatment plan parameters.…”

Section: Introductionmentioning

confidence: 99%

Simulation and experimental verification of ambient neutron doses in a pencil beam scanning proton therapy room as a function of treatment plan parameters

Hoey

Stolarczyk²,

Lillhök³

et al. 2022

Front. Oncol.

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Out-of-field patient doses in proton therapy are dominated by neutrons. Currently, they are not taken into account by treatment planning systems. There is an increasing need to include out-of-field doses in the dose calculation, especially when treating children, pregnant patients, and patients with implants. In response to this demand, this work presents the first steps towards a tool for the prediction of out-of-field neutron doses in pencil beam scanning proton therapy facilities. As a first step, a general Monte Carlo radiation transport model for simulation of out-of-field neutron doses was set up and successfully verified by comparison of simulated and measured ambient neutron dose equivalent and neutron fluence energy spectra around a solid water phantom irradiated with a variation of different treatment plan parameters. Simulations with the verified model enabled a detailed study of the variation of the neutron ambient dose equivalent with field size, range, modulation width, use of a range shifter, and position inside the treatment room. For future work, it is planned to use this verified model to simulate out-of-field neutron doses inside the phantom and to verify the simulation results by comparison with previous in-phantom measurement campaigns. Eventually, these verified simulations will be used to build a library and a corresponding tool to allow assessment of out-of-field neutron doses at pencil beam scanning proton therapy facilities.

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The influence of nuclear models and Monte Carlo radiation transport codes on stray neutron dose estimations in proton therapy

Cited by 14 publications

References 47 publications

Validation of a Monte Carlo Framework for Out-of-Field Dose Calculations in Proton Therapy

Validation of a Monte Carlo Framework for Out-of-Field Dose Calculations in Proton Therapy

Peripheral Organ Equivalent Dose Estimation Procedure in Proton Therapy

Simulation and experimental verification of ambient neutron doses in a pencil beam scanning proton therapy room as a function of treatment plan parameters

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