Validation of Monte Carlo-based calculations for megavolt electron beams for IORT and FLASH-IORT.

In Intra-Operative Radiation Therapy (IORT) the tumour site is surgically exposed and normal tissue located around the tumour may be avoided. Electron applicators would require large surgical incisions; therefore, the preferred mechanism for beam collimation is the IORT cone system. FLASH radiotherapy (FLASH-RT) involves the treatment of tumours at ultra-high dose rates and the IORT cone system can also be used. This study validates the Monte Carlo-based calculations for these small electron beams to accurately determine the dose characteristics of each possible cone-energy combination as well as custom-built alloy cutouts attached to the end of the IORT cone. This will contribute to accurate dose distribution and output factor calculations that are essential to all radiation therapy treatments. A Monte Carlo (MC) model was modelled for electron beams produced by a Siemens Primus LINAC and the IORT cones. The accelerator was built with the component modules available in the BEAMnrc code. The phase-space file generated by the BEAM simulation was used as the source input for the subsequent DOSXYZnrc simulations. Percentage Depth Dose (PDD) data and profiles were extracted from the dose distributions obtained with the DOSXYZnrc simulations. These beam characteristics were compared with measured data for 6, 12, and 18 MeV electron beams for the IORT open cones of diameters 19, 45, and 64 mm and irregularly shaped cutouts. The MC simulations could replicate electron beams within a criterion of 3%/3 mm. Applicator factors were within 0.7%, and cone factors showed good agreement, except for the 9 mm cone size. Based on the successful comparisons between measurement and MC-calculated dose distributions, output factors for the open cones and for small irregularly shaped IORT beams, it may be concluded that the Monte Carlo based dose calculation could replicate electron beams used for IORT and FLASH-IORT.

Measured, calculated, and egs_brachy I-125 dose distributions in a gold plaque for retinoblastoma treatment.

BACKGROUND: This study measured and calculated dose distributions around a unique gold plaque for whole-eye radiotherapy (to treat retinoblastoma). The applicator consists of a pericorneal ring attached to the four extraocular muscles and four legs, each loaded with I-125 seeds. They are inserted beneath the conjunctiva in-between each pair of muscles and attached anteriorly to the ring. The applicator was designed in such a way that the dose is directed toward the middle of the eye while sparing surrounding tissues. PURPOSE: (I) To compare the measured and calculated data obtained by thermoluminescent dosimeters (TLDs) in a solid-water phantom, a Gafchromic film in a solid-water phantom, the treatment planning systems, and Monte Carlo simulations; (II) to use Monte Carlo simulations for the determination of the dose to the organs at risk by taking the gold shielding and the anisotropy into account. METHODS: The dose around the applicator was measured using TLDs and Gafchromic EBT2 film in eye-shaped solid-water phantoms. Dose calculations were performed with the TheraPlan Plus and BrachyVision planning system and Monte Carlo simulations with egs_brachy code. A computer-aided design drawing of the applicator was created and used to create the input file for the Monte Carlo simulations. RESULTS: Monte Carlo calculated dose to the optic nerve is 64.8% of the central dose in the eye, whereas the planned dose is 93.7%. The Monte Carlo lens dose varies from 72.0% to 86.1%, whereas the planned dose varies from 73.0% to 84.3%. Monte Carlo-calculated dose to the bony orbit is 11.3%, whereas the planned dose is as high as 54.7% compared to the dose in the center region of the eye. CONCLUSIONS: The measured and Monte Carlo-simulated dose distributions matched well, whereas planned dose distributions showed discrepancies in some areas of the eye and outside of the eye due to their ignorance of the shielding effects of the plaque.

The relation between XR-QA2 and RT-QA2 GafchromicTMfilm optical density and absorbed dose in water produced by radionuclides.

Purpose. In this study, Monte Carlo (MC) simulations were done to relate the dose-response of the film to that in water. The effect of backscattering materials (PMMA, lead, polystyrene, and air) was investigated on its influence on film density for radionuclides including Am-241, Tc-99m, I-131, Cs-137.Methods. A BEAMnrc MC simulation was designed to score a phase-space file (PSF) below the container of the radionuclide under consideration to use as an input file for the subsequent DOSXYZnrc MC simulation. The geometry of the container holding the radionuclide was built using the component modules available in BEAMnrc. BEAMDP was used to investigate the container effect on the radionuclide spectrum as well as the fluence. The DOSXYZnrc simulation produced the absorbed dose in XR-QA2 and RT-QA2 GafchromicTMfilms. The DOSXYZnrc simulations were repeated for the GafchromicTMfilm now replaced with water to get the absorbed dose in water. From these results, conversion factors for the dose in water to the film dose for the different radionuclides, Am-241, Tc-99m, I-131, and Cs-137 were obtained. The actual film dose was calculated using the specific gamma exposure constant (Γ) at a distance of 50 cm for a point source approximation. From the BEAMnrc simulations, the particle fluence was extracted from PSFs to correct for the fluence at 0.1 cm below the sources from the fluence 50 cm away since the inverse square law will not apply to finite-size sources. The absorbed dose profiles in the film were compared to the absorbed dose profiles from the MC simulations.Results. A fitting function based on the neutron depletion model fits the optical density versus absorbed film dose data well and can be used as a calibration tool to obtain the film dose from its optical density. Lead as a backscatter material results in a higher optical density change but a lower absorbed dose. The XR-QA2 GafchromicTMfilm is more sensitive than the RT-QA2 GafchromicTMfilm, showing a more responsive optical density (OD) change in the energy range of radionuclides used in this study. Conversion factors were determined to convert the dose in water to the dose in GafchromicTMfilm. The Am-241 and I-131 simulated absorbed dose in the film to dose in water does not fluctuate as much as the simulated absorbed dose in film and water when using Tc-99m and Cs-137. Validation was shown for the comparison of the film and MC simulation absorbed dose profiles.Conclusions. MC BEAMnrc simulations are useful to simulate radionuclides and their containers. BEAMDP extracted energy spectra showed that the radionuclide containers produced a Compton effect on the energy spectra and added filtration on the lower spectral photon components. Extracted fluence ratios from PSFs were used to calculate the absorbed dose value at 0.1 cm distance from the source. By using the fit function, the dose in the film can be determined for known optical density values. The effect of the backscatter materials showed that the XR-QA2 GafchromicTMfilm results in higher optical density values than the RT-QA2 GafchromicTMfilm. The absorbed dose in both the films is comparable but not for a radionuclide such as Am-241 with an activity of 74MBq. The lead backscatter material showed to be the most prominent in optical density enhancement, and the air equivalent material was the least prominent. The XR-QA2 GafchromicTMfilm is the most sensitive and will be the best option if working with low energies. The absorbed dose in the XR-QA2 GafchromicTMfilm also showed a good comparison to the absorbed dose in water for the Am-241 radionuclide with an activity of 74MBq. The absorbed dose in the films compares well to the MC simulated doses.

Validation of a Lévy electron energy straggling model for an Elekta Synergy® linear accelerator.

AIM: In this study, an EGSnrc based Monte Carlo electron model was validated for an Elekta Synergy® 160-leaf Agility™ linear accelerator. A previously reported electron energy straggling model based on a Lévy distribution was tested against water tank measurements and a specially designed heterogeneous multi-layered phantom. This included PDD, beam profile, and relative output factor (ROF) comparison. All data passed a 2%/2 mm gamma criterion with the exception of some ROF data, which showed discrepancies of up to 2.7%. METHODS: BEAMnrc was used to accurately model the linac that included the improved exit electron energy spectrum based on a Lévy distribution. The resulting BEAMnrc phase space files were used as sources in DOSXYZnrc for water tank dose distribution simulations consisting of 6 electron beam energies, 11 field sizes, and source-to-surface distances (SSDs) of 95 and 100 cm. Evaluation parameters included PDD, dose profiles, and relative output factors, as well as phantom PDD and dose profile measurements with EBT3 gafchromic film. RESULTS: The improved exit electron beam energy spectrum caused simulated data to comply with measured data (PPD's and dose profiles) with a 100% pass rate using a 2%/2 mm criterion except for some relative output factors that deviated by 2.7% from measured ones in water. This was observed for both 95 and 100 cm SSD data. Good agreement was obtained between film and simulation data within 2% in more than 90% of PDD and profile measurements. CONCLUSIONS: The Lévy based energy straggling model for electron beams allowed for accurate electron beam characterization in water tank and phantom measurements.

Multi-Energy Computed Tomography Breast Imaging with Monte Carlo Simulations: Contrast-to-Noise-Based Image Weighting.

CONTEXT: Photon-counting detectors and breast computed tomography imaging have been an active area of research. With these detectors, photons are assigned an equal weight and weighting schemes can be enabled. More weight can be assigned to lower energies, resulting in an increase in the contrast-to-noise ratio (CNR). AIMS: The aim of this study is to develop and evaluate an energy weighting imaging technique to improve the CNR of simulated breast phantoms and to improve tumour detection. MATERIALS AND METHODS: Breast phantoms consisting of adipose, glandular, malignant tissues and iodine contrast were constructed with BreastSimulator software. The phantoms were used in egs_cbct simulations for energies ranging between 20 and 65 keV from which multiple images were reconstructed. A new CNR-based image weighting method was proposed based on the CNR values obtained from the images. This method improves on previous methods and can be applied to complicated phantoms since no structural information is needed. RESULTS: An increase in the CNR can be seen for lower energies. A sharp increase in the CNR is seen just above the K-edge for the phantoms with the iodine contrast. The CNR-based image weighting leads to a 68.47% (1.68-fold) increase in the CNR for the malignant tissue without iodine. For the malignant tissue with iodine contrast, the increase in the CNR was 96.14% (1.96-fold). CONCLUSIONS: The new proposed CNR-based image weighting scheme is easy to implement and can be used for complicated phantoms with varying structures. A large increase in the CNR is seen with or without the use of iodine contrast.

EGS_cbct: Simulation of a fan beam CT and RMI phantom for measured HU verification.

INTRODUCTION: A mathematical 3D model of an existing computed tomography (CT) scanner was created and used in the EGSnrc-based BEAMnrc and egs_cbct Monte Carlo codes. Simulated transmission dose profiles of a RMI-465 phantom were analysed to verify Hounsfield numbers against measured data obtained from the CT scanner. METHODS AND MATERIALS: The modelled CT unit is based on the design of a Toshiba Aquilion 16 LB CT scanner. As a first step, BEAMnrc simulated the X-ray tube, filters, and secondary collimation to obtain phase space data of the X-ray beam. A bowtie filter was included to create a more uniform beam intensity and to remove the beam hardening effects. In a second step the Interactive Data Language (IDL) code was used to build an EGSPHANT file that contained the RMI phantom which was used in egs_cbct simulations. After simulation a series of profiles were sampled from the detector model and the Feldkamp-Davis-Kress (FDK) algorithm was used to reconstruct transversal images. The results were tested against measured data obtained from CT scans. RESULTS: The egs_cbct code can be used for the simulation of a fan beam CT unit. The calculated bowtie filter ensured a uniform flux on the detectors. Good correlation between measured and simulated CT numbers was obtained. CONCLUSIONS: In principle, Monte Carlo codes such as egs_cbct can model a fan beam CT unit. After reconstruction, the images contained Hounsfield values comparable to measured data.