Ahmad Al-Basheer

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.

SU‐GG‐T‐408: Validation of a Novel Dose Calculation Approach for Heterogeneous Voxelized Phantoms in a Parallel Computation Environment Using Electron Dose Kernels for Radiotherapy

Purpose Here we present a rapid whole bodydose estimation approach applied to a clinical water phantom and whole body CT‐voxelized phantom, using electron dose kernels coupled with deterministic photon transport. This method yields fast, accurate whole body doses.Method and Materials A novel dose calculation methodology called EDK‐SN, or “Electron Dose Kernel‐Discrete Ordinates” rapidly estimates organdoses in a voxelized human phantom, accounts for in‐ and out‐of‐field doses using external photon beam therapy. We begin by solving the complete photon transport problem using parallel computing with the 3‐D discrete ordinates (SN) photon transport code PENTRAN. We then project pre‐computed (via Monte Carlo) voxel‐based Electron Dose Kernels (EDKs), mapping them to surrounding voxels via quaternion rotation, scaled by the magnitude of photon fluence from the SN calculation. An 8 MV flat‐weighted beam is incident on an 11×11×11 cm3 water phantom, and on a 15 year old human phantom, down‐sampled to 1×1×1 cm3 (60×27×167 voxels); a 6 MV Philips Elekta Linacphoton spectrum has also been simulated. The percent depth dose was compared to clinical CC04 chamber measurement results; comparison of doses using the EDK‐Sn method and clinical treatment planning system (in field dose) will also be presented. Results The EDK‐SN technique has demonstrated independent agreement with Monte Carlo photon‐electron transport calculations for whole body dose. The EDK‐SN method yields a speedup of ∼8 (30 minutes versus 4+ hours) over the traditional parallel Monte Carlo, with <7% difference in different organs (smaller given stochastic uncertainties). The Monte Carlo simulated percent depth dose and clinical chamber PDD measurement agree within 10% among different field sizes.Conclusion The EDK‐SN method for high energy photon external beam dose calculations has been validated based on clinical external therapy beam calculations. This method will help to determine both in‐field and out‐of‐field radiationdose for radiotherapy.

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.