Emily Poon

Accuracy of the photon and electron physics in GEANT4 for radiotherapy applications

This work involves a validation of the photon and electron transport of the GEANT4 particle simulation toolkit for radiotherapy physics applications. We examine the cross sections and sampling algorithms of the three electromagnetic physics models in version 4.6.1 of the toolkit: Standard, Low-energy, and Penelope. The depth dose distributions in water for incident monoenergetic and clinical beams are compared to the EGSNRC results. In photon beam simulations, all three models agree with EGSNRC to within 2%, except for the buildup region. Larger deviations are found for incident electron beams, and the differences are affected by user-imposed electron step limitations. Particle distributions through thin layers of clinical target materials, and perturbation effects near high-Z and low-Z interfaces are also investigated. The electron step size artifacts observed in our studies indicate potential problems with the condensed history algorithm. A careful selection of physics processes and transport parameters is needed for optimum efficiency and accuracy.

Spectroscopic characterization of a novel electronic brachytherapy system

The Axxent developed by Xoft Inc. is a novel electronic brachytherapy system capable of generating x-rays up to 50 keV. These low energy photon-emitting sources merit attention not only because of their ability to vary the dosimetric properties of the radiation, but also because of the radiobiological effects of low energy x-rays. The objective of this study is to characterize the x-ray source and to model it using the Geant4 Monte Carlo code. Spectral and attenuation curve measurements are performed at various peak voltages and angles and the source is characterized in terms of spectrum and half-value layers (HVLs). Also, the effects of source variation and source aging are quantified. Bremsstrahlung splitting, phase-space scoring and particle-tagging features are implemented in the Geant4 code, which is bench-marked against BEAMnrc simulations. HVLs from spectral measurements, attenuation curve measurements and Geant4 simulations mostly agree within uncertainty. However, there are discrepancies between measurements and simulations for photons emitted on the source transverse plane (90 degrees).

Consistency test of the electron transport algorithm in the GEANT4 Monte Carlo code

In this work, the condensed history algorithm in GEANT4 (version 4.6.2.p01) is examined. We performed simulations of an ionization chamber composed of water for 1.25 MeV incident photon beams under Fano conditions, and evaluated the consistency of the cavity response for several combinations of electron transport parameters. GEANT4 permits electrons to reach geometric boundaries in large steps, and underestimates lateral displacement near interfaces. Step size artefacts due to distortions in electron fluence and angular distributions reduce the cavity dose by up to 39%. Accurate cavity response can be achieved using severe user-imposed step size restrictions. We suggest that improvements in the electron transport algorithm in GEANT4 should address the handling of boundary crossing.

ALGEBRA: ALgorithm for the heterogeneous dosimetry based on GEANT4 for BRAchytherapy

Task group 43 (TG43)-based dosimetry algorithms are efficient for brachytherapy dose calculation in water. However, human tissues have chemical compositions and densities different than water. Moreover, the mutual shielding effect of seeds on each other (interseed attenuation) is neglected in the TG43-based dosimetry platforms. The scientific community has expressed the need for an accurate dosimetry platform in brachytherapy. The purpose of this paper is to present ALGEBRA, a Monte Carlo platform for dosimetry in brachytherapy which is sufficiently fast and accurate for clinical and research purposes. ALGEBRA is based on the GEANT4 Monte Carlo code and is capable of handling the DICOM RT standard to recreate a virtual model of the treated site. Here, the performance of ALGEBRA is presented for the special case of LDR brachytherapy in permanent prostate and breast seed implants. However, the algorithm is also capable of handling other treatments such as HDR brachytherapy.

Monte Carlo study of LDR seed dosimetry with an application in a clinical brachytherapy breast implant

A Monte Carlo (MC) study was carried out to evaluate the effects of the interseed attenuation and the tissue composition for two models of 125I low dose rate (LDR) brachytherapy seeds (Medi-Physics 6711, IBt InterSource) in a permanent breast implant. The effect of the tissue composition was investigated because the breast localization presents heterogeneities such as glandular and adipose tissue surrounded by air, lungs, and ribs. The absolute MC dose calculations were benchmarked by comparison to the absolute dose obtained from experimental results. Before modeling a clinical case of an implant in heterogeneous breast, the effects of the tissue composition and the interseed attenuation were studied in homogeneous phantoms. To investigate the tissue composition effect, the dose along the transverse axis of the two seed models were calculated and compared in different materials. For each seed model, three seeds sharing the same transverse axis were simulated to evaluate the interseed effect in water as a function of the distance from the seed. A clinical study of a permanent breast 125I implant for a single patient was carried out using four dose calculation techniques: (1) A TG-43 based calculation, (2) a full MC simulation with realistic tissues and seed models, (3) a MC simulation in water and modeled seeds, and (4) a MC simulation without modeling the seed geometry but with realistic tissues. In the latter, a phase space file corresponding to the particles emitted from the external surface of the seed is used at each seed location. The results were compared by calculating the relevant clinical metrics V85, V100, and V200 for this kind of treatment in the target. D90 and D50 were also determined to evaluate the differences in dose and compare the results to the studies published for permanent prostate seed implants in literature. The experimental results are in agreement with the MC absolute doses (within 5% for EBT Gafchromic film and within 7% for TLD-100). Important differences between the dose along the transverse axis of the seed in water and in adipose tissue are obtained (10% at 3.5 cm). The comparisons between the full MC and the TG-43 calculations show that there are no significant differences for V85 and V100. For V200, 8.4% difference is found coming mainly from the tissue composition effect. Larger differences (about 10.5% for the model 6711 seed and about 13% for the InterSource125) are determined for D90 and D50. These differences depend on the composition of the breast tissue modeled in the simulation. A variation in percentage by mass of the mammary gland and adipose tissue can cause important differences in the clinical dose metrics V200, D90, and D50. Even if the authors can conclude that clinically, the differences in V85, V100, and V200 are acceptable in comparison to the large variation in dose in the treated volume, this work demonstrates that the development of a MC treatment planning system for LDR brachytherapy will improve the dose determination in the treated region and consequently the dose-outcome relationship, especially for the skin toxicity.

Dosimetric characterization of a novel intracavitary mold applicator for 192Ir high dose rate endorectal brachytherapy treatment

The dosimetric properties of a novel intracavitary mold applicator for Ir192 high dose rate (HDR) endorectal cancer treatment have been investigated using Monte Carlo (MC) simulations and experimental methods. The 28cm long applicator has a flexible structure made of silicone rubber for easy passage into cavities with deep-seated tumors. It consists of eight source catheters arranged around a central cavity for shielding insertion, and is compatible for use with an endocavitary balloon. A phase space model of the HDR source has been validated for dose calculations using the GEANT4 MC code. GAFCHROMIC™ EBT model film was used to measure dose distributions in water around shielded and unshielded applicators with two loading configurations, and to quantify the shielding effect of a balloon injected with an iodine solution (300mgI∕mL). The film calibration procedure was performed in water using an Ir192 HDR source. Ionization chamber measurements in a Lucite phantom show that placing a tungsten rod in the applicator attenuates the dose in the shielded region by up to 85%. Inserting the shielded applicator into a water-filled balloon pushes the neighboring tissues away from the radiation source, and the resulting geometric displacement reduces the dose by up to 53%; another 8% dose reduction can be achieved when the balloon is injected with an iodine solution. All experimental results agree with the GEANT4 calculations within measurement uncertainties.

Development of a scatter correction technique and its application to HDR 192Ir multicatheter breast brachytherapy

This article introduces a scatter correction (SC) technique for high-dose-rate (HDR) I192r brachytherapy dose calculations in the absence of a full scatter environment near the skin. The technique uses dosimetry data derived by Monte Carlo (MC) simulations for the Nucletron microSelectron v2 HDR I192r source. The data include the primary and scatter components of the radial dose function and the anisotropy function in addition to a SC table. The dose to a point of interest for each dwell position is estimated by first calculating the primary and scatter doses in an infinite water phantom. The scatter dose is then scaled by a SC factor that depends on the distances between the point of interest, the dwell positions, and the body contour of the patient. SC calculations in water phantoms of three different shapes, as well as computed tomography-based geometries of 18 multicatheter breast patients, are compared with Task Group 43 (TG-43) and PTRAN_CT MC calculations. The SC calculations show improvement over TG-43 for all test cases while taking 50% longer to run. The target and skin doses for the breast patient plans are unaffected by tissue inhomogeneities, as indicated by an agreement better than 1% between the SC and MC results. On average, TG-43 overestimates the target coverage by 2% and the dose to the hottest 0.1cm3 (D0.1cc) of the skin by 5%. The low-density lung causes the lung and heart D0.1cc to differ by up to 3% for the SC method and by 2%-5% for TG-43 compared with MC calculations. The SC technique is suitable for HDR I192r dose calculations near the skin provided that the dose is nearly unperturbed by internal inhomogeneities. It has been validated for multicatheter breast brachytherapy.

Patient-specific Monte Carlo dose calculations for high-dose-rate endorectal brachytherapy with shielded intracavitary applicator.

PURPOSEnAn integrated software platform was developed to perform a patient-specific dosimetric study on high-dose-rate (192)Ir endorectal brachytherapy. Monte Carlo techniques were used to examine the perturbation effects of an eight-channel intracavitary applicator with shielding and a liquid-inflatable balloon. Such effects are ignored in conventional treatment planning systems that assume water-equivalent geometries.nnnMETHODS AND MATERIALSnA total of 40 Task Group 43-based rectal patient plans were calculated using the PTRAN_CT Monte Carlo photon transport code. The silicone applicator, tungsten or lead shielding, contrast solution-filled balloon, and patient anatomy were included in the simulations. The dose to water and dose to medium were scored separately. The effects of heterogeneities and uncertainties in source positioning were examined. A superposition calculation method using pregenerated Monte Carlo dose distributions about the shielded applicator in water was developed and validated for efficient treatment planning purposes.nnnRESULTSnOn average, metal shielding decreases the mean dose to the contralateral normal tissues by 24% and reduces the target volume covered by the prescribed dose from 97% to 94%. Tissue heterogeneities contribute to dose differences of <1% relative to the prescribed dose. The differences in the dose volume indices between dose to water and dose to medium-based calculations were <1% for soft tissues, <2% for bone marrow, and >20% for cortical bone. A longitudinal shift of +/-2.5 mm and a rotational shift of +/-15 degrees in applicator insertion reduced the target volume receiving the prescribed dose by </=4%.nnnCONCLUSIONnThe shielded applicator improved dose conformity and normal tissue sparing; however, Task Group 43-based treatment planning might compromise target coverage by not accounting for shielding.

BrachyGUI: an adjunct to an accelerated Monte Carlo photon transport code for patient-specific brachytherapy dose calculations and analysis

A number of accelerated Monte Carlo (MC) codes have been developed in recent years for brachytherapy applications, one of which is PTRAN_CT. Developed as an extension to the well-benchmarked PTRAN code, PTRAN_CT can be used to perform efficient patient-specific dose calculations. The code can explicitly account for the patient geometry converted from computed-tomography (CT) images, as well as perturbations due to the brachytherapy applicator and seeds. We have developed a software tool called BrachyGUI that provides an integrated environment for preparing patient and treatment plan-specific input data files for PTRAN_CT. It also comes with dose calculation, analysis, and treatment planning capabilities. In this article, we will describe the interface of BrachyGUI with PTRAN_CT for CT-based calculations, and examine the calculation efficiency of PTRAN_CT. We conclude that it is now feasible to use PTRAN_CT for high dose rate brachytherapy treatment planning on a routine clinical basis.

A CT-based analytical dose calculation method for HDR 192Ir brachytherapy

PURPOSEnThis article presents an analytical dose calculation method for high-dose-rate 192Ir brachytherapy, taking into account the effects of inhomogeneities and reduced photon backscatter near the skin. The adequacy of the Task Group 43 (TG-43) two-dimensional formalism for treatment planning is also assessed.nnnMETHODSnThe proposed method uses material composition and density data derived from computed tomography images. The primary and scatter dose distributions for each dwell position are calculated first as if the patient is an infinite water phantom. This is done using either TG-43 or a database of Monte Carlo (MC) dose distributions. The latter can be used to account for the effects of shielding in water. Subsequently, corrections for photon attenuation, scatter, and spectral variations along medium- or low-Z inhomogeneities are made according to the radiological paths determined by ray tracing. The scatter dose is then scaled by a correction factor that depends on the distances between the point of interest, the body contour, and the source position. Dose calculations are done for phantoms with tissue and lead inserts, as well as patient plans for head-and-neck, esophagus, and MammoSite balloon breast brachytherapy treatments. Gamma indices are evaluated using a dose-difference criterion of 3% and a distance-to-agreement criterion of 2 mm. PTRAN_CT MC calculations are used as the reference dose distributions.nnnRESULTSnFor the phantom with tissue and lead inserts, the percentages of the voxels of interest passing the gamma criteria (Pgamma > or = 1) are 100% for the analytical calculation and 91% for TG-43. For the breast patient plan, TG-43 overestimates the target volume receiving the prescribed dose by 4% and the dose to the hottest 0.1 cm3 of the skin by 9%, whereas the analytical and MC results agree within 0.4%. Pgamma > or = 1 are 100% and 48% for the analytical and TG-43 calculations, respectively. For the head-and-neck and esophagus patient plans, Pgamma > or = 1 are > or = 99% for both calculation methods.nnnCONCLUSIONSnA correction-based dose calculation method has been validated for HDR 192Ir brachytherapy. Its high calculation efficiency makes it feasible for use in treatment planning. Because tissue inhomogeneity effects are small and primary dose predominates in the near-source region, TG-43 is adequate for target dose estimation provided shielding and contrast solution are not used.