[en] Purpose: Dose distributions around low energy (< 60 keV) brachytherapy sources, such as ¹²⁵I, are known to be very sensitive to changes in tissue composition. Available ¹²⁵I dosimetry data describe the effects of replacing the entire water medium by heterogeneous material. This work extends our knowledge of tissue heterogeneity effects to the domain of bounded tissue heterogeneities, simulating clinical situations. Our goals are three-fold: (a) to experimentally characterized the variation of dose rate as a function of location and dimensions of the heterogeneity, (b) to confirm the accuracy of Monte Carlo dose calculation methods in the presence of bounded tissue heterogeneities, and (c) to use the Monte Carlo method to characterize the dependence of heterogeneity correction factors (HCF) on the irradiation geometry. Methods and Materials: Thermoluminescent dosimeters (TLD) were used to measure the deviations from the homogeneous dose distribution of an ¹²⁵I seed due to cylindrical tissue heterogeneities. A solid water phantom was machined accurately to accommodate the long axis of the heterogeneous cylinder in the transverse plane of a ¹²⁵I source. Profiles were obtained perpendicular to and along the cylinder axis, in the region downstream of the heterogeneity. Measurements were repeated at the corresponding points in homogeneous solid water. The measured heterogeneity correction factor (HCF) was defined as the ratio of the detector reading in the heterogeneous medium to that in the homogeneous medium at that point. The same ratio was simulated by a Monte Carlo photon transport (MCPT) code, using accurate modeling of the source, phantom, and detector geometry. In addition, Monte Carlo-based parametric studies were performed to identify the dependence of HCF on heterogeneity dimensions and distance from the source. Results: Measured and calculated HCFs reveal excellent agreement (≤ 5% average) over a wide range of materials and geometries. HCFs downstream of 20 mm diameter by 10 mm thick hard bone cylinders vary from 0.12 to 0.30 with respect to distance, while for an inner bone cylinder of the same dimension, it varies from 0.72 to 0.83. For 6 mm diameter by 10 mm thick hard bone and inner bone cylinders, HCF varies 0.27-0.58 and 0.77-0.88, respectively. For lucite, fat, and air, the dependence of HCF on the 3D irradiation geometry was much less pronounced. Conclusion: Monte Carlo simulation is a powerful, convenient, and accurate tool for investigating the long neglected area of tissue composition heterogeneity corrections. Simple one dimensional dose calculation models that depend only on the heterogeneity thickness cannot accurately characterize ¹²⁵I dose distributions in the presence of bone-like heterogeneities

[en] The advantage of radiographic film is that it allows two-dimensional, high-resolution dose measurement. While there is concern over its photon energy dependence, these problems are considered acceptable within small fields, where the scatter component is small. The application of film dosimetry to intensity modulated radiotherapy (IMRT) raises additional concern since the primary fluence may vary significantly within the field. The varying primary fluence in combination with a large scatter fraction, present for large fields and large depths, causes the spectrum at various points within the IMRT field to differ from the spectrum in the uniform fields typically used for calibrating the film. As a result, significant artifacts are introduced in the measured dose distribution. The purpose of this work is to quantify and develop a method to correct for these artifacts. Two approaches based on Monte Carlo (MC) simulations are examined. In the first method, the film artifact, as quantified by film and ion chamber output measurements in uniform square fields, is derived from the MC calculated ratio of absorbed doses to film and to water. In the second method, the measured film artifact is correlated with MC calculated photon spectra, revealing a strong correlation between the measured artifact and the 'scatter'-to-'primary' ratio, defined by the ratio of the number of photons below to the number of photons above 0.1 MeV, independent of field size and depth. These methods are evaluated in high- and low-dose regions of a large intensity-modulated field created with a central block. The spectral approach is also tested with a clinical IMRT field. The absorbed dose method accurately corrects the measured film dose in the open part of the field and in points under the block and outside the field. The dose error is reduced from as much as 16% of the open field dose to less than 1%, as verified with an ion chamber. The spectral method accurately corrects the measured film dose in the open region of the centrally blocked field, but does not fully correct for the film artifact for points under the block and outside the field, where the spectrum is substantially different. Applied to the clinical field, the corrected film measurement shows good agreement with data obtained with a two-dimensional diode array

[en] In brachytherapy clinical practice, applicator shielding and tissue heterogeneities are usually not explicitly taken into account. None of the existing dose computational methods are able to reconcile accurate dose calculation in complex three-dimensional (3D) geometries with high efficiency and simplicity. We propose a new model that performs two-dimensional integration of the scattered dose component. The model calculates the effective primary dose at the point of interest and estimates the scatter dose as a superposition of the scatter contributions from pyramid-shaped minibeams. The approach generalizes a previous scatter subtraction model designed to calculate the dose for axial points in simple cylindrically symmetric geometry by dividing the scattering volume into spatial regions coaxial with the source-to-measurement point direction. To allow for azimuthal variation of the primary dose, these minibeams were divided into equally spaced azimuthally distributed pyramidal volumes. The model uses precalculated scatter-to-primary ratios (SPRs) for collimated isotropic sources. Effective primary dose, which includes the radiation scattered in the source capsule, is used to achieve independence from the source structure. For realistic models of the ¹⁹²Ir HDR and PDR sources, the algorithm agrees with Monte Carlo within 2.5% and for the ¹²⁵I type 6702 seed within 6%. The 2D scatter integration (2DSI) model has the potential to estimate the dose behind high-density heterogeneities both accurately and efficiently. The algorithm is much faster than Monte Carlo methods and predicts the dose around sources with different γ-ray energies and differently shaped capsules with high accuracy. (author)

[en] The higher sensitivity to low-energy scattered photons of radiographic film compared to water can lead to significant dosimetric error when the beam quality varies significantly within a field. Correcting for this artifact will provide greater accuracy for intensity modulated radiation therapy (IMRT) verification dosimetry. A procedure is developed for correction of the film energy-dependent response by creating a pencil beam kernel within our treatment planning system to model the film response specifically. Film kernels are obtained from EGSnrc Monte Carlo simulations of the dose distribution from a 1 mm diameter narrow beam in a model of the film placed at six depths from 1.5 to 40 cm in polystyrene and solid water phantoms. Kernels for different area phantoms (50x50 cm² and 25x25 cm² polystyrene and 30x30 cm² solid water) are produced. The Monte Carlo calculated kernel is experimentally verified with film, ion chamber and thermoluminescent dosimetry (TLD) measurements in polystyrene irradiated by a narrow beam. The kernel is then used in convolution calculations to predict the film response in open and IMRT fields. A 6 MV photon beam and Kodak XV2 film in a polystyrene phantom are selected to test the method as they are often used in practice and can result in large energy-dependent artifacts. The difference in dose distributions calculated with the film kernel and the water kernel is subtracted from film measurements to obtain a practically film artifact free IMRT dose distribution for the Kodak XV2 film. For the points with dose exceeding 5 cGy (11% of the peak dose) in a large modulated field and a film measurement inside a large polystyrene phantom at depth of 10 cm, the correction reduces the fraction of pixels for which the film dose deviates from dose to water by more than 5% of the mean film dose from 44% to 6%

[en] We have used Monte Carlo photon transport simulations to calculate the dosimetric parameters of a new ¹²⁵I seed, the Source Tech Medical Model STM1251 source for interstitial brachytherapy. We followed the recommendations of the AAPM Task Group 43 and determined the following parameters: dose-rate constant, radial dose function, anisotropy function, anisotropy factor, and anisotropy constant. The recently (January 1999) revised National Institute of Standards and Technology I-125 standard for air-kerma strength calibration was taken into account as well as updated interaction cross-section data. The calculated dose-rate constant, when normalized to the simulated wide-angle, free-air chamber measurement of air-kerma strength, is 0.980 cGy h-1 U-1. The calculated radial dose function for the Model STM1251 source is more penetrating than that of the model 6711 seed (by 18% at 5 cm distance), but agrees closely (within statistical errors) with that of the model 6702 seed up to distances of 10 cm. The STM1251 source anisotropy functions indicate that its dose distribution is somewhat more anisotropic than that of the model 6702 and 6711 seeds at 1 cm distance but is comparable at larger distances. The Model STM1251 anisotropy constant is very similar to that of the model 6711, 6702, and MED3631 A/M seeds

[en] In this study, a measurement protocol is presented that improves the precision of dose measurements using a flat-bed document scanner in conjunction with two new GafChromic registered film models, HS and Prototype A EBT exposed to 6 MV photon beams. We established two sources of uncertainties in dose measurements, governed by measurement and calibration curve fit parameters contributions. We have quantitatively assessed the influence of different steps in the protocol on the overall dose measurement uncertainty. Applying the protocol described in this paper on the Agfa Arcus II flat-bed document scanner, the overall one-sigma dose measurement uncertainty for an uniform field amounts to 2% or less for doses above around 0.4 Gy in the case of the EBT (Prototype A), and for doses above 5 Gy in the case of the HS model GafChromic registered film using a region of interest 2x2 mm² in size

[en] Two recently introduced GafChromic film models, HS and XR-T, have been developed as more sensitive and uniform alternatives to GafChromic MD-55-2 film. The HS model has been specifically designed for measurement of absorbed dose in high-energy photon beams (above 1 MeV), while the XR-T model has been introduced for dose measurements of low energy (0.1 MeV) photons. The goal of this study is to compare the sensitometric curves and estimated dosimetric uncertainties associated with seven different GafChromic film dosimetry systems for the two new film models. The densitometers tested are: LKB Pharmacia UltroScan XL, Molecular Dynamics Personal Densitometer, Nuclear Associates Radiochromic Densitometer Model 37-443, Photoelectron Corporation CMR-604, Laser Pro 16, Vidar VXR-16, and AGFA Arcus II document scanner. Pieces of film were exposed to different doses in a dose range from 0.5 to 50 Gy using 6 MV photon beam. Functional forms for dose vs net optical density have been determined for each of the GafChromic film-dosimetry systems used in this comparison. Two sources of uncertainties in dose measurements, governed by the experimental measurement and calibration curve fit procedure, have been compared for the densitometers used. Among the densitometers tested, it is found that for the HS film type the uncertainty caused by the experimental measurement varies from 1% to 3% while the calibration fit uncertainty ranges from 2% to 4% for doses above 5 Gy. Corresponding uncertainties for XR-T film model are somewhat higher and range from 1% to 5% for experimental and from 2% to 7% for the fit uncertainty estimates. Notwithstanding the significant variations in sensitivity, the studied densitometers exhibit very similar precision for GafChromic film based dose measurements above 5 Gy

[en] The recently developed GATE (GEANT4 application for tomographic emission) Monte Carlo package, designed to simulate positron emission tomography (PET) and single photon emission computed tomography (SPECT) scanners, provides the ability to model and account for the effects of photon noncollinearity, off-axis detector penetration, detector size and response, positron range, photon scatter, and patient motion on the resolution and quality of PET images. The objective of this study is to validate a model within GATE of the General Electric (GE) Advance/Discovery Light Speed (LS) PET scanner. Our three-dimensional PET simulation model of the scanner consists of 12 096 detectors grouped into blocks, which are grouped into modules as per the vendor's specifications. The GATE results are compared to experimental data obtained in accordance with the National Electrical Manufactures Association/Society of Nuclear Medicine (NEMA/SNM), NEMA NU 2-1994, and NEMA NU 2-2001 protocols. The respective phantoms are also accurately modeled thus allowing us to simulate the sensitivity, scatter fraction, count rate performance, and spatial resolution. In-house software was developed to produce and analyze sinograms from the simulated data. With our model of the GE Advance/Discovery LS PET scanner, the ratio of the sensitivities with sources radially offset 0 and 10 cm from the scanner's main axis are reproduced to within 1% of measurements. Similarly, the simulated scatter fraction for the NEMA NU 2-2001 phantom agrees to within less than 3% of measured values (the measured scatter fractions are 44.8% and 40.9±1.4% and the simulated scatter fraction is 43.5±0.3%). The simulated count rate curves were made to match the experimental curves by using deadtimes as fit parameters. This resulted in deadtime values of 625 and 332 ns at the Block and Coincidence levels, respectively. The experimental peak true count rate of 139.0 kcps and the peak activity concentration of 21.5 kBq/cc were matched by the simulated results to within 0.5% and 0.1% respectively. The simulated count rate curves also resulted in a peak NECR of 35.2 kcps at 10.8 kBq/cc compared to 37.6 kcps at 10.0 kBq/cc from averaged experimental values. The spatial resolution of the simulated scanner matched the experimental results to within 0.2 mm

[en] Purpose: Positron emission-tomography (PET) using 2-[¹⁸F]fluoro-2-deoxyglucose (FDG-PET) increases sensitivity and specificity of disease detection in lymphoma and thus is standard in lymphoma management. This study examines the effects of coregistering FDG-PET and computed tomography (CT) (PET/CT) scans on treatment planning for lymphoma patients. Methods and Materials: Twenty-nine patients (30 positive PET scans) underwent PET/CT treatment planning from July 2004 to February 2007 and were retrospectively studied. For each patient, gross tumor volume was blindly contoured on the CT-only and PET/CT studies by a radiation oncologist. Treatment plans were generated for both the CT-only and PET/CT planning target volumes (PTVs) for all patients. Normal tissue doses and PTV coverage were evaluated using dose--volume histograms for all sites. Results: Thirty-two treatment sites were evaluated. Twenty-one patients had non-Hodgkin lymphoma, 5 patients had Hodgkin lymphoma, and 3 patients had plasma cell neoplasms. Previously undetected FDG-avid sites were identified in 3 patients during PET/CT simulation, resulting in one additional treatment field. Due to unexpected PET/CT simulation findings, 2 patients did not proceed with radiation treatment. The addition of PET changed the volume of 23 sites (72%). The PTV was increased in 15 sites (47%) by a median of 11% (range, 6-40%) and reduced in 8 sites (25%) by a median of 20% (range, 6%-75%). In six (19%) replanned sites, the CT-based treatment plan would not have adequately covered the PTV defined by PET/CT. Conclusions: Incorporation of FDG-PET into CT-based treatment planning for lymphoma patients resulted in considerable changes in management, volume definition, and normal tissue dosimetry for a significant number of patients.

[en] Built on top of the Geant4 toolkit, GATE is collaboratively developed for more than 15 years to design Monte Carlo simulations of nuclear-based imaging systems. It is, in particular, used by researchers and industrials to design, optimize, understand and create innovative emission tomography systems. In this paper, we reviewed the recent developments that have been proposed to simulate modern detectors and provide a comprehensive report on imaging systems that have been simulated and evaluated in GATE. Additionally, some methodological developments that are not specific for imaging but that can improve detector modeling and provide computation time gains, such as Variance Reduction Techniques and Artificial Intelligence integration, are described and discussed. (topical review)