Sensitivity of the IQM and MatriXX detectors in megavolt photon beams.

AIM: This study focused on evaluating the sensitivity of integral quality monitoring (IQM®) system and MatriXX detectors. These two detectors are recommended for radiotherapy pre-treatment quality assurance (QA). BACKGROUND: IQM is a large wedged-shaped ionisation chamber mounted to the linear accelerator (linac) head in practice. MatriXX consists of an array of ionisation chambers also attached to the linac head. MATERIALS AND METHODS: In this study, the dosimetric performance and sensitivity of MatriXX and IQM detectors were evaluated using the following characteristics: reproducibility, linearity, error detection capability and three-dimensional conformal radiotherapy (3D-CRT) plans of the head and neck, thorax and pelvic regions. RESULTS: This study indicates that the signal responses of the large ionisation chamber device (IQM) and the small pixel array of ionisation chambers device (MatriXX) are reproducible, linear and sensitive to MLC positional errors, backup jaw positional errors and dose errors. The local percentage differences for dose errors of 1%, 2%, and 3% were, respectively, within 0.35-8.23%, 0.78-16.21%, and 1.10-24.41% for the IQM device. While for the MatriXX detector, the ranges were between 0.24-3.19, 0.57-6.43 and 0.81-12.95, respectively. Since IQM is essentially a double wedge-shaped large ionisation chamber, its reproducibility and detection capability are competitive to that of MatriXX. In addition, the sensitivity of the two QA systems increases with an increase in escalation percentage, and the signal responses are patient plan specific. CONCLUSIONS: The two detectors response signals have good correlations and they are accurate for pre-treatment QA. Statistically, (P < 0.05) there is a significant difference between the IQM and MatriXX response to dose errors.

Practical Dosimetry Considerations for Small MLC-Shaped Electron Fields at 60 cm SSD.

Superficial tumours can be treated with megavoltage electron beams. The underlying tissue can be spared through the steep dose fall-off gradients over a range of a few centimetres. An accurate Monte Carlo model for an Elekta Precise was determined and dose distribution was simulated. Dosimetric parameters were calculated to set guidelines for tumour irradiation. Elekta Precise multi-leaf collimators (MLC), which shaped electron fields were investigated using a benchmarked Monte Carlo model. BEAMnrc modelled the Elekta Precise and results were benchmarked against measurements. Percentage depth dose and beam profile data were simulated within 2% / 2 mm accuracy of the measured data. The DOSXYZnrc code simulated the 3-D dose data in water between 4 and 15 MeV. The relative (P80-20) penumbra, percentage depth dose (PDD), range to 90% of dose maximum (R90), dose fall-off range R80-20 (DFR), and the percentage bremsstrahlung dose (BSD), were extracted from the simulated data. The relative penumbra ranged from 90% to 10% at 6 MeV and 15 MeV, respectively. R90 values ranged between 0.8 cm at 4 MeV and 4.5 cm at 15 MeV. The DFR ranged between 0.8 cm at 4 MeV and 3.5 cm at 15 MeV. The BSD was the highest for low beam energies and small fields. Developed guidelines indicated that intermediate-sized MLC fields are most suited for therapy since they have lower BSD, longer R90, shorter DFR but larger P80-20. The DFR increases and R90 decreases for small fields at higher beam energies and more distal tissue will receive doses > 20%.

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.

Extraction of electron beam dose parameters from EBT2 film data scored in a mini phantom.

Quality assurance of medical linear accelerators includes dosimetric parameter measurement of therapeutic electron beams e.g. relative dose at a depth of 80% (R80). This parameter must be within a tolerance of 0.2 cm of the declared value. Cumbersome water tank measurements can be regarded as a benchmark to measure electron depth dose curves. A mini-phantom was designed and built, in which a strip of GAFCHROMIC® EBT2 film could be encased tightly for electron beam depth dose measurement. Depth dose data were measured for an ELEKTA Sl25 MLC, ELEKTA Precise, and ELEKTA Synergy (Elekta Oncology Systems, Crawley, UK) machines. The electron beam energy range was between 4 and 22 MeV among the machines. A 10 × 10 cm² electron applicator with 95 cm source-surface-distance was used on all the machines. 24 h after irradiation, the EBT2 film strips were scanned on Canon CanoScan N670U scanner. Afterwards, the data were analysed with in-house developed software that entailed optical density to dose conversion, and optimal fitting of the PDD data to de-noise the raw data. From the PDD data R80 values were solved for and compared with acceptance values. A series of tests were also carried out to validate the use of the scanner for film Dosimetry. These tests are presented in this study. It was found that this method of R80 evaluation was reliable with good agreement with benchmark water tank measurements using a commercial parallel plate ionization chamber as the radiation detector. The EBT2 film data yielded R80 values that were on average 0.06 cm different from benchmark water tank measured R80 values.