Monica Ghita

Characterization of a commercially-available, optically-stimulated luminescent dosimetry system for use in computed tomography.

Optically-stimulated luminescent (OSL) nanoDot dosimeters, commercially available from Landauer, Inc. (Glenwood, IL), were assessed for use in computed tomography (CT) for erasure and reusability, linearity and reproducibility of response, and angular and energy response in different scattering conditions. Following overnight exposure to fluorescent room light, the residual signal on the dosimeters was 2%. The response of the dosimeters to identical exposures was consistent, and reported doses were within 4% of each other. The dosimeters responded linearly with dose up to 1 Gy. The dosimeter response to the CT beams decreased with increased tube voltage, showing up to a −16% difference when compared to a 0.6-cm3 NIST-traceable calibrated ionization chamber for a 135 kVp CT beam. The largest range in percent difference in dosimeter response to scatter at central and peripheral positions inside CTDI phantoms was 14% at 80 kVp CT tube voltage, when compared to the ionization chamber. The dosimeters responded uniformly to x-ray tube angle over the ranges of increments of 0° to 75° and 105° to 180° when exposed in air, and from 0° to 360° when exposed inside a CTDI phantom. While energy and scatter correction factors should be applied to dosimeter readings for the purpose of determining absolute doses, these corrections are straightforward but depend on the accuracy of the ionization chamber used for cross-calibration. The linearity and angular responses, combined with the ability to reuse the dosimeters, make this OSL system an excellent choice for clinical CT dose measurements.

Electron Dose Kernels to Account for Secondary Particle Transport in Deterministic Simulations

Abstract For low-energy photons, charged-particle equilibrium usually exists within the patient treatment volume, in which case the photon absorbed dose D is equal to the collisional kerma Kc; however, this is not true for the dose buildup region near the surface of the patient or at interfaces of dissimilar materials, such as tissue/lung, where corrections for secondary electron transport may be significant. This is readily treated in Monte Carlo codes, yet difficult to treat explicitly in deterministic codes due to the large optical thicknesses and added numerical complexities in reaching convergence in photon-electron transport problems. To properly treat three-dimensional electron transport physics deterministically, yet still achieve reasonably fast and accurate whole-body computation times using high-energy photons, angular-energy-dependent transport “electron dose kernels” (EDK-SN) have been developed. These kernels were derived via full physics Monte Carlo electron transport simulations and are applied using scaling based on rapid deterministic photon solutions over the problem phase-space, thereby accounting for the dose from charged-particle electron transport. As a result, accurate whole-body doses may be rapidly achieved for high-energy photon sources by performing a single deterministic SN multigroup photon calculation on a parallel cluster with PENTRAN, then linking the SN-derived photon fluxes and net currents to Monte Carlo–based EDKs to account for a full physics dose. Water phantom results using a uniform 0- to 8-MeV step uniform beam indicate that the dose can be accurately obtained within the uncertainty of a full physics Monte Carlo simulation. Followup work will implement this method on phantoms.

CRITICAL DISCRETIZATION ISSUES IN 3-D SN SIMULATIONS RELEVANT TO DOSIMETRY AND MEDICAL PHYSICS

Abstract Dosimetry problems inherently involve dose determinations among widely varying materials and densities, and may require complex, detailed investigations of the angular, spatial, and energy behavior of the applied radiation transporting throughout the simulation geometry. Traditionally, Monte Carlo codes have been implemented in solving these types of problems using voxelized geometries and phantoms. The motivation of this work is to investigate the discretization requirements for deterministic radiation transport simulations for these problems via direct solutions of the linear Boltzmann transport equation, focusing on the discrete ordinates (SN) method. The SN method can yield accurate global solutions, provided the inherent discretizations among the angular, spatial, and energy domains properly represent problem physics. In this paper, the SN approach is implemented using a three-dimensional (3-D) 60Co photon transport simulation to highlight the critical issues encountered in performing deterministic photon simulations in dosimetry problems. Calculations were performed using the PENTRAN parallel SN code to obtain a 3-D distribution of flux and dose computed using a collisional kerma approximation. For an acceptable result, we determined that a minimum angular Legendre-Chebychev quadrature of S32 with P3 anisotropy is required, with block-adaptive meshes on the order of 1 cm, even in air regions, implemented with an adaptive differencing scheme (implemented in the PENTRAN code) to yield optimal solution convergence. Also, photon cross-section libraries should be carefully evaluated for the problem studied; for our test problem, the BUGLE-96 photon library yielded the closest results to Monte Carlo (MCNP5) among those tested. Overall, this work details the levels of discretization involved in performing deterministic computations in dosimetry problems and will be useful in enabling future efforts to perform rapid deterministic computations of phantom doses.

MO-E-332-01: DXS a Diagnostic X-Ray Spectra Generator

Purpose: To numerically generate radiographic x‐ray spectra that can be conveniently employed in radiation transport simulations or other radiation detection applications. Method and Materials: Based initially on the Tucker, et al model, we developed and evaluated a new code, DXS (Diagnostic X‐Ray Spectra), to numerically generate spectra for tungsten‐target x‐ray tubes spanning the radiographic energy range. The model parameters in our code were adjusted by comparison with corresponding MCNP5 simulated spectra; we modified the semi‐analytical formulation for the characteristic x‐ray production, a caveat of Tuckers model, by incorporating a factor that better accounts for the dependence of the K‐peaks on the tube potential. Parametric fitting functions are used to model the self‐attenuation in the target and attenuation due to inherent and added filtration (aluminum,beryllium,copper,tantalum are the options implemented in the code), as well as for the tungsten mass stopping power and the Thomson‐Whiddington constant. Comparison with Monte Carlo simulated and published measured spectra were used to validate the new code. Results: Normalized to unit area DXS code‐generated spectra for several tube potentials from 50 to 140 kVp agree well, less than 2% relative difference in nearly all energy bins (2 keV), with corresponding MCNP5 simulated spectra for similar tube parameters. Few exceptions are noted and may be attributed to either poorer statistics in the low and high energy tails of the spectrum, or to insufficient accuracy of the numerical computations for the steepest part of the spectra at high accelerating potentials. Good agreement is seen between the DXS and Bhat et al measured spectra. Conclusion: The DXS code generates the spectra, according to user specified input parameters (tube potential, anode angle, filtratation) and energy intervals, and augments them into any discretized energy group structure. Hence, the code can be of great benefit in radiation transport simulations.

SU‐FF‐I‐50: Whole Body and Distal Organ‐Specific Dosimetry Using Parallel SN Methods

Purpose: To create a new methodology for dose computations applicable to general medical physics applications, based upon a direct deterministic solution of the (3‐D) Boltzmann transport equation (BTE). Method and Materials: Using the multigroup discrete ordinates (SN) deterministic method in the PENTRAN‐MP (Parallel Environment Neutral‐particle TRANsport‐Medical Physics) code system, fluxes and corresponding doses were determined for a clinical test case using a UF Series B whole‐body voxelized pediatric patient phantom. With a 90 keV planar x‐ray source, we performed two sets of transport calculations based on the same phantom model, employing both the Monte Carlo(MC) and SN methods. SN calculations were performed using PENTRAN, and the MCNP5 code was used for MC calculations. Post processing in the PENTRAN‐MP code system includes seamless parallel data extraction using the PENDATA code, followed by application of the 3D‐DOSE code to compute dose in each phantom voxel, with dose‐volume histograms for critical organs of interest. Results: We demonstrate that the deterministic SN and MC solutions are, in the vicinity of the source region, completely in agreement; the largest statistical error of the MC simulation was 2000 hrs) to produce meaningful results with an acceptable stochastic error to enable comparison with the well‐converged SN results. Conclusions: A new methodology for 3D dose calculations has been developed based on a parallel discrete ordinates (SN) solution of the BTE. With the proper discretization applied, the SN method presents an accurate, fast solution yielding a complete 3‐D dose distribution without stochastic error.