A CAD interface for GEANT4.

Often CAD models already exist for parts of a geometry being simulated using GEANT4. Direct import of these CAD models into GEANT4 however, may not be possible and complex components may be difficult to define via other means. Solutions that allow for users to work around the limited support in the GEANT4 toolkit for loading predefined CAD geometries have been presented by others, however these solutions require intermediate file format conversion using commercial software. Here within we describe a technique that allows for CAD models to be directly loaded as geometry without the need for commercial software and intermediate file format conversion. Robustness of the interface was tested using a set of CAD models of various complexity; for the models used in testing, no import errors were reported and all geometry was found to be navigable by GEANT4.

Radiotherapy Monte Carlo simulation using cloud computing technology.

Cloud computing allows for vast computational resources to be leveraged quickly and easily in bursts as and when required. Here we describe a technique that allows for Monte Carlo radiotherapy dose calculations to be performed using GEANT4 and executed in the cloud, with relative simulation cost and completion time evaluated as a function of machine count. As expected, simulation completion time decreases as 1/n for n parallel machines, and relative simulation cost is found to be optimal where n is a factor of the total simulation time in hours. Using the technique, we demonstrate the potential usefulness of cloud computing as a solution for rapid Monte Carlo simulation for radiotherapy dose calculation without the need for dedicated local computer hardware as a proof of principal.

Improved image quality for X-ray CT imaging of gel dosimeters.

PURPOSE: This study provides a simple method for improving precision of X-ray computed tomography (CT) scans of irradiated polymer gel dosimetry. The noise affecting CT scans of irradiated gels has been an impediment to the use of clinical CT scanners for gel dosimetry studies. METHODS: In this study, it is shown that multiple scans of a single PAGAT gel dosimeter can be used to extrapolate a "zero-scan" image which displays a similar level of precision to an image obtained by averaging multiple CT images, without the compromised dose measurement resulting from the exposure of the gel to radiation from the CT scanner. RESULTS: When extrapolating the zero-scan image, it is shown that exponential and simple linear fits to the relationship between Hounsfield unit and scan number, for each pixel in the image, provide an accurate indication of gel density. CONCLUSIONS: It is expected that this work will be utilized in the analysis of three-dimensional gel volumes irradiated using complex radiotherapy treatments.

A hybrid radiation detector for simultaneous spatial and temporal dosimetry.

In this feasibility study an organic plastic scintillator is calibrated against ionisation chamber measurements and then embedded in a polymer gel dosimeter to obtain a quasi-4D radiation detector. This hybrid dosimeter was irradiated with megavoltage x-rays from a linear accelerator, with temporal measurements of the dose rate being acquired by the scintillator and spatial measurements acquired with the gel dosimeter. The detectors employed in this study are radiologically equivalent; and we show that neither detector perturbs the intensity of the radiation field of the other. By employing these detectors in concert, spatial and temporal variations in the radiation intensity can now be detected and gel dosimeters can be calibrated for absolute dose from a single irradiation.

Validation and automation of the DYNJAWS component module of the BEAMnrc Monte Carlo code.

The purpose of this work is to validate and automate the use of DYNJAWS; a new component module (CM) in the BEAMnrc Monte Carlo (MC) user code. The DYNJAWS CM simulates dynamic wedges and can be used in three modes; dynamic, step-and-shoot and static. The step-and-shoot and dynamic modes require an additional input file defining the positions of the jaw that constitutes the dynamic wedge, at regular intervals during its motion. A method for automating the generation of the input file is presented which will allow for the more efficient use of the DYNJAWS CM. Wedged profiles have been measured and simulated for 6 and 10 MV photons at three field sizes (5 cm × 5 cm, 10 cm × 10 cm and 20 cm × 20 cm), four wedge angles (15°, 30°, 45° and 60°), at d (max) and at 10 cm depth. Results of this study show agreement between the measured and the MC profiles to within 3% of absolute dose or 3 mm distance to agreement for all wedge angles at both energies and depths. The gamma analysis suggests that dynamic mode is more accurate than the step-and-shoot mode. The DYNJAWS CM is an important addition to the BEAMnrc code and will enable the MC verification of patient treatments involving dynamic wedges.

ASSESSMENT OF INTEGRITY AND LEAD-EQUIVALENCE OF SHIELDED GARMENTS USING TWO-DIMENSIONAL X-RAY IMAGES FROM A COMPUTED TOMOGRAPHY SCANNER.

Shielded garments are widely recommended for occupational radiation protection in diagnostic and interventional radiology. This study investigated a novel method for efficiently verifying shielded garment integrity while simultaneously acquiring data for lead-equivalence measurements, using two-dimensional topogram images from computed tomography (CT) scanners. This method was tested against more-conventional measurements with superficial and orthovoltage radiotherapy treatment beams, for 12 shielded garments containing 3 different lead-free shielding materials. Despite some energy-dependent results, all shielded garments approximately achieved their specified lead-equivalence for the energy range expected during clinical use for fluoroscopy procedures, except for three shielded skirts that required two layers of material to be overlapped at the front. All lead-equivalence measurements from CT topograms agreed with or conservatively underestimated the kV narrow-beam results. This method is potentially useful for independently assessing the shielding properties of new shielded garments and performing annual checks for damage or degradation of existing shielded garments.

Patient-specific quality assurance on a Varian Halcyon linear accelerator using a PTW Octavius 4D device.

This study investigates and validates the use of the Octavius 4D system for patient specific quality assurance on Halcyon, which is capable of rotating at 4 revolutions per minute (RPM). A commercially available PTW Octavius 4D system was used for this study which had a maximum rotation speed of 3 RPM. Initial validation included testing the accuracy of the inclinometer, percent depth doses (PDD), output factors, and dose profiles for selected static square fields. The same static fields were also subject to a gamma comparison with the TPS. This was followed by an evaluation of twelve clinical treatment plans and seven non-clinical plans with varying gantry rotation speeds. All testing was completed using detector array measurement times of 200 ms and 100 ms. Inclinometer accuracy was within 0.3° of actual gantry angle. Output factors varied less than 0.6%, PDD differences were no greater than 1.4%, and dose profile differences were less than 2.2%. Gamma pass rates for the static fields were 96.7% (2%/2mm) and 99.7% (3%/3mm). A prototype control unit, which had a maximum rotation speed of 4 RPM was also used to test the clinical and non-clinical plans. For the clinical plans, the mean gamma pass rates (2%/2mm) were 86.1% and 88.1% for the commercial unit and prototype unit respectively. Results using a measurement time of 200 ms were superior to those using 100 ms. For Halcyon deliveries greater than 3 RPM, worst case gamma results for the commercial unit were 28.6% compared to 98.5% using the prototype unit. Accurate patient specific quality assurance results can be obtained using the Octavius 4D system with a Halcyon linac, provided that the system measurement time is kept at 200 ms and the rotation speed of Halycon does not exceed 3 RPM. For higher RPM deliveries, an Octavius 4D unit with 4 RPM rotation capability is recommended.

Modeling a complex micro-multileaf collimator using the standard BEAMnrc distribution.

PURPOSE: The component modules in the standard BEAMnrc istribution may appear to be insufficient to model micro-multileaf collimators that have trifaceted leaf ends and complex leaf profiles. This note indicates, however, that accurate Monte Carlo simulations of radiotherapy beams defined by a complex collimation device can be completed using BEAMnrc's standard VARMLC component module. METHODS: That this simple collimator model can produce spatially and dosimetrically accurate microcollimated fields is illustrated using comparisons with ion chamber and film measurements of the dose deposited by square and irregular fields incident on planar, homogeneous water phantoms. RESULTS: Monte Carlo dose calculations for on-axis and off-axis fields are shown to produce good agreement with experimental values, even on close examination of the penumbrae. CONCLUSIONS: The use of a VARMLC model of the micro-multileaf collimator, along with a commissioned model of the associated linear accelerator, is therefore recommended as an alternative to the development or use of in-house or third-party component modules for simulating stereotactic radiotherapy and radiosurgery treatments. Simulation parameters for the VARMLC model are provided which should allow other researchers to adapt and use this model to study clinical stereotactic radiotherapy treatments.

Monte Carlo calculated output correction factors for Gafchromic EBT3 film for relative dosimetry in small stereotactic radiosurgery fields.

To calculate small field output correction factors, [Formula: see text], for Gafchromic EBT3 film using Monte Carlo simulations. These factors were determined for a Novalis Trilogy linear accelerator equipped with Brainlab circular cones with diameters of 4.0 to 30.0 mm. The BEAMnrc Monte Carlo code was used to simulate the Novalis Trilogy linear accelerator and the Brainlab cones with diameters 4.0 to 30 mm. The DOSXYZnrc code was used to simulate Gafchromic EBT3 film with the atomic composition specified by the manufacturer. Small field correction factors were calculated according to new IAEA TRS-483 Code of Practice for small field dosimetry. The depth of calculation was 10 cm and a source to surface distance of 100 cm. The X-ray beam used in the simulations was a 6 MV SRS. The correction factors were then used to determine field output factors with Gafchromic EBT3 film. These field output factors were validated using three solid state detectors and applying correction factors from the TRS-483 Code of Practice. The solid state detectors were IBA SFD diode, PTW 60018 SRS diode and PTW 60019 microDiamond. The Monte Carlo calculated output correction factors, [Formula: see text], for Gafchromic EBT3 film ranged between 0.998 to 1.004 for Brainlab circular cones with diameters between 4.0 and 30.0 mm. The uncertainty for these factors was 2.0%. The field output factors obtained with Gafchromic EBT3 film were within 2% of the mean results obtained with the three solid state detectors. For field sizes 4 mm diameter and above, Gafchromic EBT3 film has field output correction factors within 1% of unity. Therefore, Gafchromic EBT3 film can be considered to be correction less and supports the assumption made about this film in the TRS-483 Code of Practice.

Electron interaction with gel dosimeters: effective atomic numbers for collisional, radiative and total interaction processes.

The effective atomic number is widely employed in radiation studies, particularly for the characterization of interaction processes in dosimeters, biological tissues and substitute materials. Gel dosimeters are unique in that they comprise both the phantom and dosimeter material. In this work, effective atomic numbers for total and partial electron interaction processes have been calculated for the first time for a Fricke gel dosimeter, five hypoxic and nine normally oxygenated polymer gel dosimeters. A range of biological materials are also presented for comparison. The spectrum of energies studied spans 10 keV to 100 MeV, over which the effective atomic number varies by 30%. The effective atomic numbers of gels match those of soft tissue closely over the full energy range studied; greater disparities exist at higher energies but are typically within 4%.

A very low diffusion Fricke gel dosimeter with functionalised xylenol orange-PVA (XOPVA).

A gel dosimeter has been developed utilising a recently reported system for reducing Fe3+ diffusion in a Fricke gel dosimeter which chelates xylenol orange to the gelling agent poly(vinyl alcohol) (PVA). Formulations were investigated using both gelatin and PVA as the gelling agent, along with the inclusion of glyoxal. The resulting gel had an optical density dose response of 0.0031 Gy-1, an auto-oxidation rate of 0.000 23 h-1, and a diffusion rate of 0.132 mm2 h-1 which is a significant improvement over previously reported gelatin based Fricke gel dosimeters. The gel was also shown to be energy and dose-rate independent and could be reused after irradiation. Thus, this gel dosimeter has the potential to provide a safe and practical solution to three dimensional radiation dosimetry in the medical environment.

The effective atomic number of dosimetric gels.

Radiological properties of gel dosimeters and phantom materials are often compared against each other and against water or tissue by consideration parameters including their effective atomic number, Zeff. Effective atomic numbers have been calculated for a range of ferrous-sulphate and polymeric gel dosimeters using mass attenuation coefficient data over the energy range 10 keV to 10 MeV. Data is presented relative to water to allow direct comparison over a range of energies. These data provide energy specific values of Zeff which improves on the practice of applying a power-law based formula to estimate an energy independent value. For applications that require a single value of Zeff, the data presented here allows the choice of a value appropriate to the energy of the photon source or a spectrum-weighted average. Studying the variation of Zeff, which is equivalent to taking into account the variation of mass attenuation coefficients with photon energy, it is found that gels typically match water better than water matches human tissues. As such, the subtle differences in effective atomic number between water and gels are small and may be considered negligible. Consideration of the mean disparity over a large energy range shows, broadly, BANG-1 to be the most water equivalent gel.

The clinical application of PET/CT: a contemporary review.

The combination of positron emission tomography (PET) scanners and x-ray computed tomography (CT) scanners into a single PET/CT scanner has resulted in significant improvements in the diagnosis and staging of disease, particularly in the field of oncology. A decade on from the publication of the details of the first PET/CT scanner, we review the technology and applications of the modality. We examine the design aspects of combining two different imaging types into a single scanner, and the artefacts produced such as attenuation correction, motion and CT truncation artefacts. The article also provides a discussion and literature review of the applications of PET/CT to date, covering detection of tumours, radiotherapy treatment planning, patient management, and applications external to the field of oncology.

Systematic variations in polymer gel dosimeter calibration due to container influence and deviations from water equivalence.

There are a number of gel dosimeter calibration methods in contemporary usage. The present study is a detailed Monte Carlo investigation into the accuracy of several calibration techniques. Results show that for most arrangements the dose to gel accurately reflects the dose to water, with the most accurate method involving the use of a large diameter flask of gel into which multiple small fields of varying dose are directed. The least accurate method was found to be that of a long test tube in a water phantom, coaxial with the beam. The large flask method is also the most straightforward and least likely to introduce errors during the set-up, though, to its detriment, the volume of gel required is much more than other methods.

Improving the quality of reconstructed X-ray CT images of polymer gel dosimeters: zero-scan coupled with adaptive mean filtering.

This study evaluated the feasibility of combining the 'zero-scan' (ZS) X-ray computed tomography (CT) based polymer gel dosimeter (PGD) readout with adaptive mean (AM) filtering for improving the signal to noise ratio (SNR), and to compare these results with available average scan (AS) X-ray CT readout techniques. NIPAM PGD were manufactured, irradiated with 6 MV photons, CT imaged and processed in Matlab. AM filter for two iterations, with 3 × 3 and 5 × 5 pixels (kernel size), was used in two scenarios (a) the CT images were subjected to AM filtering (pre-processing) and these were further employed to generate AS and ZS gel images, and (b) the AS and ZS images were first reconstructed from the CT images and then AM filtering was carried out (post-processing). SNR was computed in an ROI of 30 × 30 for different pre and post processing cases. Results showed that the ZS technique combined with AM filtering resulted in improved SNR. Using the previously-recommended 25 images for reconstruction the ZS pre-processed protocol can give an increase of 44% and 80% in SNR for 3 × 3 and 5 × 5 kernel sizes respectively. However, post processing using both techniques and filter sizes introduced blur and a reduction in the spatial resolution. Based on this work, it is possible to recommend that the ZS method may be combined with pre-processed AM filtering using appropriate kernel size, to produce a large increase in the SNR of the reconstructed PGD images.

Radiological properties of 3D printed materials in kilovoltage and megavoltage photon beams.

PURPOSE: This study evaluates the radiological properties of different 3D printing materials for a range of photon energies, including kV and MV CT imaging and MV radiotherapy beams. METHODS: The CT values of a number of materials were measured on an Aquilion One CT scanner at 80kVp, 120kVp and a Tomotherapy Hi Art MVCT imaging beam. Attenuation of the materials in a 6MV radiotherapy beam was investigated. RESULTS: Plastic filaments printed with various infill densities have CT values of -743±4, -580±1 and -113±3 in 120kVp CT images which approximate the CT values of low-density lung, high-density lung and soft tissue respectively. Metal-infused plastic filaments printed with a 90% infill density have CT values of 658±1 and 739±6 in MVCT images which approximate the attenuation of cortical bone. The effective relative electron density REDeff is used to describe the attenuation of a megavoltage treatment beam, taking into account effects relating to the atomic number and mass density of the material. Plastic filaments printed with a 90% infill density have REDeff values of 1.02±0.03 and 0.94±0.02 which approximate the relative electron density RED of soft tissue. Printed resins have REDeff values of 1.11±0.03 and 1.09±0.03 which approximate the RED of bone mineral. CONCLUSIONS: 3D printers can model a variety of body tissues which can be used to create phantoms useful for both imaging and dosimetric studies.

An investigation of dose calculation accuracy in intensity-modulated radiotherapy of sites in the head & neck.

Knowledge of the accuracy of dose calculations in intensity-modulated radiotherapy of the head and neck is essential for clinical confidence in these highly conformal treatments. High dose gradients are frequently placed very close to critical structures, such as the spinal cord, and good coverage of complex shaped nodal target volumes is important for long term-local control. A phantom study is presented comparing the performance of standard clinical pencil-beam and collapsed-cone dose algorithms to Monte Carlo calculation and three-dimensional gel dosimetry measurement. Calculations and measurements are individually normalized to the median dose in the primary planning target volume, making this a purely relative study. The phantom simulates tissue, air and bone for a typical neck section and is treated using an inverse-planned 5-field IMRT treatment, similar in character to clinically used class solutions. Results indicate that the pencil-beam algorithm fails to correctly model the relative dose distribution surrounding the air cavity, leading to an cverestimate of the target coverage. The collapsed-cone and Monte Carlo results are very similar, indicating that the clinical collapsed-cone algorithm is perfectly sufficient for routine clinical use. The gel measurement shows generally good agreement with the collapsed-cone and Monte Carlo calculated dose, particularly in the spinal cord dose and nodal target coverage, thus giving greater confidence in the use of this class solution.

Effects of inaccurate small field dose measurements on calculated treatment doses.

Given the difficulty and potential time- or financial-costs associated with accurate small field dosimetry, this study aimed to establish the clinical necessity of obtaining accurate small field output factor measurements and to evaluate the effects on planned doses that could arise if accurate measurements are not used in treatment planning dose calculations. Isocentre doses, in heterogeneous patient anatomy, were calculated and compared for 571 beams from 48 clinical radiotherapy treatments, using a clinical radiotherapy treatment planning system, with reference to two different sets of beam configuration data. One set of beam configuration data included field output factors (total scatter factors) from precisely positioned and response-corrected diode measurements and the other included field output factors measured using a conventional technique that would have been better suited to larger field measurements. Differences between the field output factor measurements made with the two different techniques equated to 14.2 % for the 6 [Formula: see text] 6 mm[Formula: see text] field, 1.8 % for the 12 [Formula: see text] 12 mm[Formula: see text] field, and less than 0.5 % for the larger fields. This led to isocentre dose differences of up to 3.3 % in routine clinical fields smaller than 9 mm across and and up to 11 % in convoluted fields smaller than 15 mm across. If field widths smaller than 15 mm are used clinically, then accurate measurement (or-remeasurement) of small field output factors in the treatment planning system's beam data is required in order to achieve dose calculation accuracy within 3 %. If such measurements are not completed, then errors in excess of 10 % may occur if very small, narrow, concave or convoluted treatment fields are used.

Bulk evaluation and comparison of radiotherapy treatment plans for breast cancer.

This study provides a bulk, retrospective analysis of 151 breast and chest wall radiotherapy treatment plans, as a small-scale demonstration of the potential breadth and value of the information that may be obtained from clinical data mining. The treatments were planned at three centres belonging to one organisation over a period of 3 months. All 151 plans were used to evaluate inter-centre consistency and compliance with a local planning protocol. A subset of 79 plans, from one centre, were used in a more detailed evaluation of the effects of anatomical asymmetry on heart and lung dose, the effects of a metallic temporary tissue expander port on dose homogeneity and the overall conformity and homogeneity achieved in routine breast treatment planning. Differences in anatomical structure contouring and nomenclature were identified between the three centres, with all centres showing some non-compliance with the local planning protocol. When evaluated against standard conformity indices, these breast plans performed relatively poorly. However, when evaluated against recommended organ-at-risk tolerances, all evaluated plans performed sufficiently well that tighter planning tolerances could be recommended for future planning. Heart doses calculated in left breast and chest wall treatments were significantly higher than heart doses calculated in right sided breast and chest wall treatments (p < 0.001). In the treatment involving a temporary tissue expander, the inflated implant effectively pushed the targeted breast tissue away from the healthy tissues, leading to a dose distribution that was relatively conformal, although attenuation through the tissue expander's metallic port may have been underestimated by the treatment planning system. The results of this study exemplify the use of bulk treatment planning data to evaluate clinical workloads and inform ongoing treatment planning.

Use of electronic portal imaging devices for electron treatment verification.

This study aims to help broaden the use of electronic portal imaging devices (EPIDs) for pre-treatment patient positioning verification, from photon-beam radiotherapy to photon- and electron-beam radiotherapy, by proposing and testing a method for acquiring clinically-useful EPID images of patient anatomy using electron beams, with a view to enabling and encouraging further research in this area. EPID images used in this study were acquired using all available beams from a linac configured to deliver electron beams with nominal energies of 6, 9, 12, 16 and 20 MeV, as well as photon beams with nominal energies of 6 and 10 MV. A widely-available heterogeneous, approximately-humanoid, thorax phantom was used, to provide an indication of the contrast and noise produced when imaging different types of tissue with comparatively realistic thicknesses. The acquired images were automatically calibrated, corrected for the effects of variations in the sensitivity of individual photodiodes, using a flood field image. For electron beam imaging, flood field EPID calibration images were acquired with and without the placement of blocks of water-equivalent plastic (with thicknesses approximately equal to the practical range of electrons in the plastic) placed upstream of the EPID, to filter out the primary electron beam, leaving only the bremsstrahlung photon signal. While the electron beam images acquired using a standard (unfiltered) flood field calibration were observed to be noisy and difficult to interpret, the electron beam images acquired using the filtered flood field calibration showed tissues and bony anatomy with levels of contrast and noise that were similar to the contrast and noise levels seen in the clinically acceptable photon beam EPID images. The best electron beam imaging results (highest contrast, signal-to-noise and contrast-to-noise ratios) were achieved when the images were acquired using the higher energy electron beams (16 and 20 MeV) when the EPID was calibrated using an intermediate (12 MeV) electron beam energy. These results demonstrate the feasibility of acquiring clinically-useful EPID images of patient anatomy using electron beams and suggest important avenues for future investigation, thus enabling and encouraging further research in this area. There is manifest potential for the EPID imaging method proposed in this work to lead to the clinical use of electron beam imaging for geometric verification of electron treatments in the future.