The insignificant change in the CLR with delivered dose indicates that a single CLR value over the investigated dose range is sufficient for accurate Čerenkov subtraction. Variations in the stem-effect spectrum shape can occur with changes in irradiation geometry. The magnitude of the changes are governed by the fiber construction and the optical properties of the fiber. The observed spectral shape changes can be explained by a combination of variations in optical path length through the fiber and the fiber fluorescent signal contribution to the stem-effect. These spectral shape variations directly influence the calculated CLR values. This work confirms that careful characterization of scintillation detectors is important as changes in the stem-effect spectrum can cause changes in the CLR. If the CLR changes between the reference and measurement conditions, this could result in an incorrect stem-effect subtraction and reduced measurement accuracy.
The aim of this work was to develop and benchmark a magnetic resonance (MR)-guided linear accelerator head model using the GEANT4 Monte Carlo (MC) code. The validated model was compared to the treatment planning system (TPS) and was also used to quantify the electron return effect (ERE) at a lung-water interface. Methods: The average energy, including the spread in the energy distribution, and the radial intensity distribution of the incident electron beam were iteratively optimized in order to match the simulated beam profiles and percent depth dose (PDD) data to measured data. The GEANT4 MC model was then compared to the TPS model using several photon beam tests including oblique beams, an off-axis aperture, and heterogeneous phantoms. The benchmarked MC model was utilized to compute output factors (OFs) with the 0.35 T magnetic field turned on and off. The ERE was quantified at a lung-water interface by simulating PDD curves with and without the magnetic field for 6.6 × 6.6 cm 2 and 2.5 × 2.5 cm 2 field sizes. A 2%/2 mm gamma criterion was used to compare the MC model with the TPS data throughout this study. Results: The final incident electron beam parameters were 6.0 MeV average energy with a 1.5 MeV full width at half maximum (FWHM) Gaussian energy spread and a 1.0 mm FWHM Gaussian radial intensity distribution. The MC-simulated OFs were found to be in agreement with the TPS-calculated and measured OFs, and no statistical difference was observed between the 0.35 T and 0.0 T OFs. Good agreement was observed between the TPS-calculated and MC-simulated data for the photon beam tests with gamma pass rates ranging from 96% to 100%. An increase of 4.3% in the ERE was observed for the 6.6 × 6.6 cm 2 field size relative to the 2.5 × 2.5 cm 2 field size. The ratio of the 0.35 T PDD to the 0.0 T PDD was found to be up to 1.098 near lung-water interfaces for the 6.6 × 6.6 cm 2 field size using the MC model. Conclusions: A vendor-independent Monte Carlo model has been developed and benchmarked for a 0.35 T/6 MV MR-linac. Good agreement was obtained between the GEANT4 and TPS models except near heterogeneity interfaces.
The purpose of this work was to characterize intensity and spectral response changes in a plastic scintillation detector (PSD) as a function of magnetic field strength. Spectra measurements as a function of magnetic field strength were performed using an optical spectrometer. The response of both a PSD and PMMA fiber were investigated to isolate the changes in response from the scintillator and the noise signal as a function of magnetic field strength. All irradiations were performed in water at a photon beam energy of 6 MV. Magnetic field strengths of (0, ±0.35, ±0.70, ±1.05, and ±1.40) T were investigated. Four noise subtraction techniques were investigated to evaluate the impact on the resulting noise-subtracted scintillator response with magnetic field strength. The noise subtraction methods included direct spectral subtraction, the spectral method, and variants thereof. The PMMA fiber exhibited changes in response of up to 50% with magnetic field strength due to the directional light emission from [Formula: see text]erenkov radiation. The PSD showed increases in response of up to 10% when not corrected for the noise signal, which agrees with previous investigations of scintillator response in magnetic fields. Decreases in the [Formula: see text]erenkov light ratio with negative field strength were observed with a maximum change at -1.40 T of 3.2% compared to 0 T. The change in the noise-subtracted PSD response as a function of magnetic field strength varied with the noise subtraction technique used. Even after noise subtraction, the PSD exhibited changes in response of up to 5.5% over the four noise subtraction methods investigated.
The purpose of this work was to investigate the accuracy and efficiency of electron transport in GEANT4 with and without a magnetic field present. Fano cavity simulations were performed in GEANT4 version 10.02 and 10.04.p01 using two multiple scattering (MSC) algorithms for two slab and one pseudo-ion chamber geometries. An iterative approach was used to optimize the transport parameters to obtain agreement with theory. Similar to previous works, the step lengths had to be severely restricted to obtain agreement with theory when using the Urban MSC model in GEANT4 v10.02. Using the Goudsmit–Saunderson MSC model with the UseSafetyPlus MSC step limitation in GEANT4 v10.04.p01 limited the maximum discrepancy from theory to 0.5%. Minor adjustments to the transport parameters were needed to obtain agreement within 0.16% of theory for all simulation configurations without a magnetic field present. The maximum deviation from theory was within 2% for all simulation configurations in the presence of a magnetic field except for two setups that exhibited discrepancies of up to 10.8%. This anomalous behavior was mitigated by forcing single scattering within the detector gas volume. Further adjustments to the transport parameters resulted in agreement with theory at the 0.21% level. Agreement with theory in the absence of a magnetic field can be obtained without significantly restricting the step size if the Goudsmit–Saunderson MSC model is used with the UseSafetyPlus MSC step limitation in GEANT4 v10.04.p01. The large discrepancies from theory observed for two simulation setups with a magnetic field present were attributed to an issue with energy loss sampling over a step when strict magnetic field transport parameters are used. This problem can be corrected by forcing single scattering within the detector gas volume; however, more work is needed to identify the cause of this anomalous behavior. This work has shown that GEANT4 can perform accurate electron transport with and without a magnetic field present without applying significant step-size reductions.
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