NASA Human Research Program Investigators' Workshop (2012)
4218.pdf
IMPLEMENTING CT-BASED MONTE CARLO SIMULATION TOOLS FOR SOLAR PARTICLE
EVENT DOSIMETRY
E. S. Diffenderfer, A. R. Kennedy, and K. A. Cengel
Department of Radiation Oncology, Perelman School of Medicine at the University of Pennsylvania, Philadelphia,
PA 19104, Electronic mail: Eric.Diffenderfer@uphs.upenn.edu
ABSTRACT
Dose absorbed from exposure to solar particle event (SPE) radiation is a serious concern for astronauts spending
extended time in a space environment without the additional radiation shielding protection afforded by Earth’s
atmosphere. SPE radiation is primarily composed of protons with kinetic energies ranging from 10 MeV to 500
MeV and beyond, with the majority of proton energies below 100 MeV. Protons have a limited range in tissue and
exhibit a depth profile that peaks with a high dose gradient as the end of the track is approached. The dose
distribution produced by protons from an SPE is highly inhomogeneous and sensitive to numerous physical
parameters such as energy spectrum, shielding, and the orientation of the radiation field trajectory. The acute
radiobiological effects of low dose-rate proton radiation fields are not well understood and are further complicated
by inhomogeneous distribution of dose to sensitive organs. Furthermore, due to dose inhomogeneities, experimental
observation and interpretation of acute effects in animal models present unique difficulties compared to photon field
radiation which exhibits comparably shallow dose gradients.
The Center for Acute Radiation Research (CARR) seeks to relate the acute effects of SPE radiation to those induced
by the more widely studied photon radiation. Part of the challenge presented by this task is to characterize the spatial
distribution of dose absorbed in sensitive organs. In support of this, we have developed software tools based on the
Geant4 Monte Carlo (MC) simulation framework. MC simulation of radiation transport in patient geometries that
are modeled from computed tomography (CT) image sets is the gold standard of dosimetry calculation techniques in
external beam radiation therapy. Commercial MC software packages are primarily available only for simulation of
photon radiation transport in CT based geometries. In this case, the photon attenuation in the simulated geometry is
directly related to the Hounsfield number (HU) of the individual CT voxels, which itself is a measure of the electron
density of the material. Proton interaction is, instead, a complicated function of kinetic energy and atomic
composition of the material.
Our tools construct detailed simulation geometries from whole-body CT data sets by converting each CT image
voxel into a material with specific atomic composition within the Geant4 C++ programming framework. With the
voxels in a CT image set spanning thousands of HU, our chosen method of defining materials is to assign atomic
compositions to a range of HU and vary the material density within that range. Since there is not a one-to-one
correspondence of electron density or HU to atomic composition, some organs/tissues may be mis-assigned in this
manner. By using organ volumes defined by drawing contours on individual CT image slices, we also have the
ability to override the material definitions with atomic compositions that represent the contoured organ.
The radiation field characteristics and environment surrounding the CT-based geometry are also implemented within
the Geant4 simulation. Animal model based experiments have used electron or proton beams produced by a clinical
radiotherapy accelerator to simulate exposure to SPE radiation. The beam characteristics are modeled within the
Geant4 simulation and validated through ion chamber and film dosimetry. The enclosure containing the animal
during irradiation is included in the simulation, and the movement of the animal during irradiation can be crudely
accounted for by varying the orientation of the animal CT geometry within the enclosure. Human exposure to SPE
radiation in a space environment is simulated by using published SPE particle energy spectra and assuming an
isotropic beam incidence. The shielding afforded by an astronaut's space suit can be approximated by including a
layer of material outside of the CT image body contour.
Dose is accumulated in the CT geometry voxel-by-voxel during the radiation transport simulation and written to a
file in the industry standard DICOM format at the conclusion of the simulation. The dose files from a number of
simulations at varying beam incidence angles or beam energies can be combined in a weighted sum to approximate
the irradiation conditions of any experiment. The dose files are then imported into a commercial radiotherapy
dosimetry software package where dose-volume histograms are produced for evaluation of organ dose distributions.
This research was supported by the NSBRI Center of Acute Radiation Research (CARR) grant. The NSBRI is
funded through NASA NCC 9-58.