Low-energy Photon Transport Research Articles

Modeling parameterized geometry in GPU-based Monte Carlo particle transport simulation for radiotherapy

Monte Carlo (MC) particle transport simulation on a graphics-processing unit (GPU) platform has been extensively studied recently due to the efficiency advantage achieved via massive parallelization. Almost all of the existing GPU-based MC packages were developed for voxelized geometry. This limited application scope of these packages. The purpose of this paper is to develop a module to model parametric geometry and integrate it in GPU-based MC simulations. In our module, each continuous region was defined by its bounding surfaces that were parameterized by quadratic functions. Particle navigation functions in this geometry were developed. The module was incorporated to two previously developed GPU-based MC packages and was tested in two example problems: (1) low energy photon transport simulation in a brachytherapy case with a shielded cylinder applicator and (2) MeV coupled photon/electron transport simulation in a phantom containing several inserts of different shapes. In both cases, the calculated dose distributions agreed well with those calculated in the corresponding voxelized geometry. The averaged dose differences were 1.03% and 0.29%, respectively. We also used the developed package to perform simulations of a Varian VS 2000 brachytherapy source and generated a phase-space file. The computation time under the parameterized geometry depended on the memory location storing the geometry data. When the data was stored in GPU’s shared memory, the highest computational speed was achieved. Incorporation of parameterized geometry yielded a computation time that was ~3 times of that in the corresponding voxelized geometry. We also developed a strategy to use an auxiliary index array to reduce frequency of geometry calculations and hence improve efficiency. With this strategy, the computational time ranged in 1.75–2.03 times of the voxelized geometry for coupled photon/electron transport depending on the voxel dimension of the auxiliary index array, and in 0.69–1.23 times for photon only transport.

SU‐FF‐T‐450: Verification of the GEANT4 Monte Carlo Code in Low Energy Photon Transport

Purpose: GEANT4 is a powerful and versatile general‐purpose Monte Carlo code. The reliability of GEANT4 in high energy photon transport has been largely confirmed. On the other hand, the discrepancy to other Monte Carlo codes in low energy range has also been reported in the past years. The purpose of this investigation is to explore the performance of GEANT4 at low photon energies (< 50 KeV), and to verify the recent improvements of GEANT4 in photon physics. Method and Materials: The MCNP Monte Carlo code will be used as the reference in this study. The physics of dominant photon interactions in low energy range (Photoelectric effect, Compton scattering, and Rayleigh scattering) implemented in the two codes and the corresponding cross section/attenuation coefficient data will be compared. The production of fluorescent photons, which play an important role in dose deposition of KV photons, will be especially addressed. Simplified cases involving 125I/103Pd or KV X‐ray beam will be modeled and simulated with both MCNP and the latest version of GEANT4 to exemplify the extent of influence under various situations. Results: Detailed physics descriptions are required for modeling low energy photon transport in Monte Carlo codes. Differences in physics implementation between MCNP and GEANT4 are known. These differences could induce noticeable differences in radiation dose and particle flux. Conclusion: GEANT4 has been gradually accepted to be a very useful Monte Carlo simulation tool to tackle many problems in radiation therapy. A comprehensive study of KV photon transport provides verification of the reliability of the code in the low energy range, and will potentially boost the applications of GEANT4 in radiotherapy modalities such as eye plaque therapy, prostate seed implant brachytherapy, and intraoperative radiotherapy (IORT).

Calibration of a phoswich type partial body counter by Monte Carlo simulation of low-energy photon transport

The Forschungszentrum Karlsruhe operates a partial body counter, which is designed for the in vivo measurement of low-energy photon emitters in the human body. Recently, a numerical procedure has been developed which allows for the calculation of individual calibration factors for this partial body counter. The procedure is based on a Monte Carlo simulation of the radiation transport from the contaminated organ or tissue within the body to the detectors using the MCNP5 code. For simulation of the human body, the MEET Man dataset of the Institute of Biomedical Techniques of the University Karlsruhe has been applied. The derived calibration factors were compared with the respective values measured using some physical phantoms such as the Lawrence Livermore National Laboratory torso phantom and the bone phantoms of the New York University Medical Center and the US Transuranium and Uranium Registry.

Benchmark of PENELOPE code for low-energy photon transport: dose comparisons with MCNP4 and EGS4

The expanding clinical use of low-energy photon emitting 125I and 103Pd seeds in recent years has led to renewed interest in their dosimetric properties. Numerous papers pointed out that higher accuracy could be obtained in Monte Carlo simulations by utilizing newer libraries for the low-energy photon cross-sections, such as XCOM and EPDL97. The recently developed PENELOPE 2001 Monte Carlo code is user friendly and incorporates photon cross-section data from the EPDL97. The code has been verified for clinical dosimetry of high-energy electron and photon beams, but has not yet been tested at low energies. In the present work, we have benchmarked the PENELOPE code for 10–150 keV photons. We computed radial dose distributions from 0 to 10 cm in water at photon energies of 10–150 keV using both PENELOPE and MCNP4C with either DLC-146 or DLC-200 cross-section libraries, assuming a point source located at the centre of a 30 cm diameter and 20 cm length cylinder. Throughout the energy range of simulated photons (except for 10 keV), PENELOPE agreed within statistical uncertainties (at worst ±5%) with MCNP/DLC-146 in the entire region of 1–10 cm and with published EGS4 data up to 5 cm. The dose at 1 cm (or dose rate constant) of PENELOPE agreed with MCNP/DLC-146 and EGS4 data within approximately ±2% in the range of 20–150 keV, while MCNP/DLC-200 produced values up to 9% lower in the range of 20–100 keV than PENELOPE or the other codes. However, the differences among the four datasets became negligible above 100 keV.

Systematic Comparisons of Measurements and EGS4 Regarding 20-40 keV Photons

To investigate the validity of the EGS4 code regarding photon transport in the keV region, we performed systematic comparisons of measurements and EGS4. The scattered-energy spectra of monochronized synchrotron-radiation photons toward 90° by samples (C, Cu, Pb) were measured using high-purity Ge detectors. The incident photons were in a linearly polarized beam of 20, 30 and 40 keV. EGS4 calculations were performed in default status and with improvements of low-energy photon transport (linearly polarized photon scattering, Doppler broadening, L-X and K-X). The measurement and calculations were compared in absolute values. Both the intensity and the shape of the peaks were compared. EGS4 calculations with improvements in low-energy photon transport agreed well with the measurements.

Improvements of low-energy photon transport in EGS4

The treatment of low-energy (≤1 MeV) photon transport in the EGS4 code [Nelson, W.R., Hirayama, H. and Rogers, D.W.O., 1985. The EGS4 Code System SLAC-265 Stanford Linear Accelerator Center] has been improved. This also includes an improvement of the treatment of the electrons generated from low-energy photons and photons generated from low-energy electrons. The phenomena treated are: (i) linearly polarized photon scattering, (ii) Doppler broadening of Compton-scattered photons, (iii) L-X-ray and L-photoelectron production and (iv) electron impact ionization. We made measurements to verify the improved EGS4 code using monochromatized synchrotron radiation.

Absorbed dose measurements and calculations in phantoms for 1.5 to 50 keV photons.

A Monte Carlo code EGS4 expanded for low energy photon transport was validated by measuring absorbed doses in a phantom for 30 and 10 keV monoenergetic photons from synchrotron radiation. Using the EGS4 code, depth doses at 0.07 mm, 0.02 to 0.1 mm, and 10 mm in the ICRU slab phantoms were calculated for 1.5 to 50 keV photons using the updated photon cross section data PHOTX. The results show that the doses at 0.02 to 0.1 mm below 10 keV are practical indices of effective dose as calculated by others, based on the 1990 ICRP recommendations (1991).