John L. Horton

Comparison of a finite-element multigroup discrete-ordinates code with Monte Carlo for radiotherapy calculations

Radiotherapy calculations often involve complex geometries such as interfaces between materials of vastly differing atomic number, such as lung, bone and/or air interfaces. Monte Carlo methods have been used to calculate accurately the perturbation effects of the interfaces. However, these methods can be computationally expensive for routine clinical calculations. An alternative approach is to solve the Boltzmann equation deterministically. We present one such deterministic code, Attila. Further, we computed a brachytherapy example and an external beam benchmark to compare the results with data previously calculated by MCNPX and EGS4. Our data suggest that the presented deterministic code is as accurate as EGS4 and MCNPX for the transport geometries examined in this study.

Contrast effects on dosimetry of a partial breast irradiation system

MammoSite is a high-dose rate brachytherapy procedure for partial breast irradiation, which uses a balloon filled with radiopaque iodine-based contrast solution and catheter for insertion of 192Ir high-dose-rate source. The radiopaque material helps visualizing the balloon contour, catheter, and source position within the balloon, which is essential for computerized tomography-based treatment planning and for daily QA using x-ray radiographs. Because of the high content of iodine in contrast media, increased absorption and attenuation of photons may take place within the balloon, which would affect the resultant dose rates outside the balloon. The impact of the concentration of the radiopaque solution on the physical dosimetry of this brachytherapy procedure is investigated in this study using MCNPX (version 2.4) Monte Carlo simulation. Calculations were based on a 30 cm diameter water sphere phantom. The source geometry was that of the Nucletron microSelectron HDR v2 192Ir source. Concentration of the iodine-based radiopaque solution was varied from 5% to 25% by volume, a range recommended by the balloons manufacturer. Balloon diameters of 4, 5, and 6 cm were simulated. Dose rate per unit air-kerma strength was calculated in 1 mm scoring bin steps. The dose rate reduction at the typical prescription line of 1 cm away from the balloon surface ranged from - 0.8% for the smallest balloon diameter and contrast concentration to a maximum of - 5.7% for the largest balloon diameter and contrast concentration, relative to a water-filled balloon. Limiting the contrast concentration to 10% would insure less than 3% reduction in the prescription dose, regardless of balloon diameter.

Optimization of deterministic transport parameters for the calculation of the dose distribution around a high dose-rate 192Ir brachytherapy source

The goal of this work was to calculate the dose distribution around a high dose-rate 192Ir brachytherapy source using a multi-group discrete ordinates code and then to compare the results with a Monte Carlo calculated dose distribution. The unstructured tetrahedral mesh discrete ordinates code Attila version 6.1.1 was used to calculate the photon kerma rate distribution in water around the Nucletron microSelectron mHDRv2 source. MCNPX 2.5.c was used to compute the Monte Carlo water photon kerma rate distribution. Two hundred million histories were simulated, resulting in standard errors of the mean of less than 3% overall. The number of energy groups, S(n) (angular order), P(n) (scattering order), and mesh elements were varied in addition to the method of analytic ray tracing to assess their effects on the deterministic solution. Water photon kerma rate matrices were exported from both codes into an in-house data analysis software. This software quantified the percent dose difference distribution, the number of points within +/- 3% and +/- 5%, and the mean percent difference between the two codes. The data demonstrated that a 5 energy-group cross-section set calculated results to within 0.5% of a 15 group cross-section set. S12 was sufficient to resolve the solution in angle. P2 expansion of the scattering cross-section was necessary to compute accurate distributions. A computational mesh with 55 064 tetrahedral elements in a 30 cm diameter phantom resolved the solution spatially. An efficiency factor of 110 with the above parameters was realized in comparison to MC methods. The Attila code provided an accurate and efficient solution of the Boltzmann transport equation for the mHDRv2 source.

FEASIBILITY OF A MULTIGROUP DETERMINISTIC SOLUTION METHOD FOR THREE-DIMENSIONAL RADIOTHERAPY DOSE CALCULATIONS

PURPOSE To investigate the potential of a novel deterministic solver, Attila, for external photon beam radiotherapy dose calculations. METHODS AND MATERIALS Two hypothetical cases for prostate and head-and-neck cancer photon beam treatment plans were calculated using Attila and EGSnrc Monte Carlo simulations. Open beams were modeled as isotropic photon point sources collimated to specified field sizes. The sources had a realistic energy spectrum calculated by Monte Carlo for a Varian Clinac 2100 operated in a 6-MV photon mode. The Attila computational grids consisted of 106,000 elements, or 424,000 spatial degrees of freedom, for the prostate case, and 123,000 tetrahedral elements, or 492,000 spatial degrees of freedom, for the head-and-neck cases. RESULTS For both cases, results demonstrate excellent agreement between Attila and EGSnrc in all areas, including the build-up regions, near heterogeneities, and at the beam penumbra. Dose agreement for 99% of the voxels was within the 3% (relative point-wise difference) or 3-mm distance-to-agreement criterion. Localized differences between the Attila and EGSnrc results were observed at bone and soft-tissue interfaces and are attributable to the effect of voxel material homogenization in calculating dose-to-medium in EGSnrc. For both cases, Attila calculation times were <20 central processing unit minutes on a single 2.2-GHz AMD Opteron processor. CONCLUSIONS The methods in Attila have the potential to be the basis for an efficient dose engine for patient-specific treatment planning, providing accuracy similar to that obtained by Monte Carlo.

Gadolinium neutron capture therapy for brain tumors: A computer study

A Monte Carlo computer study of the total dose distribution from neutrons and prompt gamma emissions (but excluding the contribution from conversion and Auger electrons) for gadolinium neutron capture therapy of brain tumors has been carried out in order to test the theoretic feasibility of this modality using commercially available magnetic resonance contrast media. The three-dimensional dose distribution calculations were performed in a spherical head phantom with a spherical tumor at the center. Potentially achievable gadolinium concentrations of 150 micrograms/g of tissue in tumor and 3 micrograms/g in normal tissue were assumed with enrichment to 79.9% gadolinium-157, as supplied by Oak Ridge National Laboratory. Irradiation was assumed to be with a 2-keV monoenergetic cylindrical epithermal neutron beam having a radius of 4 cm. The three-dimensional thermal neutron fluence resulting from the 2-keV beam propagation through the tissue was modeled. For a single neutron beam, the maximum dose is delivered within the tumor but the dose is very inhomogeneous across the tumor volume due to rapid decrease of thermal neutron fluence with depth. Two parallel opposed neutron beams deliver to the interface of normal and malignant tissue 70%-80% of the maximum dose received at the center of the tumor. To deliver an average tumor dose of 500 cGy in 10 min would require a 2-keV source neutrons number of 8.0 x 10(11) per s within the geometry of the beam.

Lessons learned from investigations of therapy misadministration events

PURPOSE Investigation teams composed of Idaho National Engineering Laboratory (INEL), United States Nuclear Regulatory Commission (NRC), and subcontractor personnel performed detailed investigations and analyses of seven misadministration events that were specifically selected on the basis of particular characteristics. These events were analyzed to identify the direct causes, contributing factors, actions to mitigate the event, and the consequences of these events. The INEL also sought to determine the role played by the recent Quality Management Rule. METHODS AND MATERIALS The investigation teams were multidisciplinary and, depending on the nature of the event, included three or more team members with appropriate expertise in the areas of radiation oncology, medical physics, nuclear medicine technology, risk analysis, and human factors. The investigations focused on the general areas of causes of the event, mitigating actions, and corrective actions. Seven misadministration events were investigated by the teams during 1991 and 1992. RESULTS Results from the events investigated indicated that (a) the institutional traditions of some licensees contributed to the potential for misadministrations, (b) many misadministrations occurred primarily due to lack of procedures or procedures that were not clearly written, (c) some licensees in this study had not effectively implemented their Quality Management programs, and (d) limited involvement on the part of the Radiation Safety Officer and Authorized Users and changes in routine and unique conditions contribute to the potential for misadministrations. CONCLUSIONS The project shows that licensees that have experienced misadministration events appear to lack comprehensive safety cultures, where all aspects of daily operations are shaped with patient and staff safety being the primary objective of all activities.

Gadolinium neutron capture therapy

A computer study of the dose distribution for gadolinium neutron capture therapy is carried out to determine its feasibility. Gadolinium is a potential neutron capture therapy (NCT) agent that produces gamma radiation, conversion electrons, and Auger electrons. The dose distribution from neutrons, neutron-induced gammas, and the reaction products from neutron capture in gadolinium were modeled using the Los Alamos National Laboratory Monte Carlo neutron photon computer code. The results of these calculations are that gadolinium has promise as an NCT agent. Using two parallel opposed epithermal neutron beams for a tumor at an 8.0-cm depth with a gadolinium loading of 100[mu]g/g, the tumor to peak normal tissue dose was determined to be 1.48.

COMPARISON OF MONTE CARLO CALCULATIONS AROUND A FLETCHER SUIT DELCLOS OVOID WITH RADIOCHROMIC FILM AND NORMOXIC POLYMER GEL DOSIMETRY

The Fletcher Suit Delclos (FSD) ovoids employed in intracavitary brachytherapy (ICB) for cervical cancer contain shields to reduce dose to the bladder and rectum. Many treatment planning systems (TPS) do not include the shields and other ovoid structures in the dose calculation. Instead, TPSs calculate dose by summing the dose contributions from the individual sources and ignoring ovoid structures such as the shields. The goal of this work was to calculate the dose distribution with Monte Carlo around a Selectron FSD ovoid and compare these calculations with radiochromic film (RCF) and normoxic polymer gel dosimetry. Monte Carlo calculations were performed with MCNPX 2.5.c for a single Selectron FSD ovoid with and without shields. RCF measurements were performed in a plane parallel to and displaced laterally 1.25 cm from the long axis of the ovoid. MAGIC gel measurements were performed in a polymethylmethacrylate phantom. RCF and MAGIC gel were irradiated with four 33μGym2h-1 Cs-137 pellets for a period of 24 h. Results indicated that MCNPX calculated dose to within ±2% or 2 mm for 98% of points compared with RCF measurements and to within ±3% or 3 mm for 98% of points compared with MAGIC gel measurements. It is concluded that MCNPX 2.5.c can calculate dose accurately in the presence of the ovoid shields, that RCF and MAGIC gel can demonstrate the effect of ovoid shields on the dose distribution and the ovoid shields reduce the dose by as much as 50%.

The effect of titanium stabilization rods on spinal cord radiation dose

The purpose of this study was to investigate the dosimetric effect of a titanium-rod spinal stabilization system on surrounding tissue, especially the spinal cord. Ion chamber dosimetry was performed for 6- and 18-MV photon beams in a water phantom containing a titanium-rod spinal stabilization system. Isodose curves were obtained in the phantom with and without rods. To assess the ability of a treatment planning system to reproduce the effects of the stabilization system on the radiation dose delivered to surrounding tissue, dose distributions were calculated after appropriate modifications were made in the computed tomography number-to-density conversion table to account for the increased density of the titanium rods. The resultant heterogeneity-corrected plans were compared with uncorrected plans. At a 7-cm depth in the water phantom, corresponding to the depth of the spinal cord, the beam was attenuated by 4% under the rods alone and by 13% rods under the rods with screws for the 6-MV photon beam as compared with curves generated in the absence of rods. The beam was attenuated by 3% and 11%, respectively, for the 18-MV beam. Using anteroposterior (18-MV) and posteroanterior (6-MV) photon beams, with and without heterogeneity correction for the rods, the corrected isodose plan showed an approximately 2% beam attenuation 4 cm anterior to the rods as compared with the uncorrected plan. No significant difference in the spinal cord dose was observed between the 2 plans, however. The titanium-rod spinal stabilization system tested in this study caused a decrease in the dose delivered distal to the rods but did not significantly affect the dose delivered to the spinal cord.

A method for verifying treatment times for simple high‐dose‐rate endobronchial brachytherapy procedures

An empirical method for verifying the total treatment time for either a one- or a two-catheter high-dose-rate procedure has been developed. The method can be performed quickly and allows for easy verification of the accuracy of the treatment time arrived at by a computerized planning system. The method is designed to confirm the treatment time to within 10%.