Using electron beam radiation to simulate the dose distribution for whole body solar particle event proton exposure

Abstract

As a part of the near solar system exploration program, astronauts may receive significant total body proton radiation exposures during a solar particle event (SPE). In the Center for Acute Radiation Research (CARR), symptoms of the acute radiation sickness syndrome induced by conventional radiation are being compared to those induced by SPE-like proton radiation, to determine the relative biological effectiveness (RBE) of SPE protons. In an SPE, the astronaut's whole body will be exposed to radiation consisting mainly of protons with energies below 50 MeV. In addition to providing for a potentially higher RBE than conventional radiation, the energy distribution for an SPE will produce a relatively inhomogeneous total body dose distribution, with a significantly higher dose delivered to the skin and subcutaneous tissues than to the internal organs. These factors make it difficult to use a (60)Co standard for RBE comparisons in our experiments. Here, the novel concept of using megavoltage electron beam radiation to more accurately reproduce both the total dose and the dose distribution of SPE protons and make meaningful RBE comparisons between protons and conventional radiation is described. In these studies, Monte Carlo simulation was used to determine the dose distribution of electron beam radiation in small mammals such as mice and ferrets as well as large mammals such as pigs. These studies will help to better define the topography of the time-dose-fractionation versus biological response landscape for astronaut exposure to an SPE.

Fig. 1

50-MeV proton depth dose distribution. a This plot demonstrates the classic plateau and Bragg peak regions of the proton depth dose distribution. b This plot demonstrates the energy deposited per depth as unrestricted collisional stopping power versus incident proton energy and was created using the online PSTAR program from the National Institute of Standards and Technology (http://physics.nist.gov/PhysRefData/Star/Text/PSTAR.html)

Fig. 2

Dose response for OneDose MOSFET dosimeters. Three One-Dose MOSFET dosimeters were exposed to the indicated electron radiation dose and read using a OneDose reader. The results are presented as average ± standard deviation and are compared to the delivered dose as measured using an ionization chamber

Fig. 3

Dose distributions for ⁶⁰Co when compared to SPE protons and electrons. Depth dose distributions for electrons or the September 1989 SPE were produced using the GEANT4 Monte Carlo simulation tool-kit (Agostinelli et al. 2003). Simulated radiation field geometry is consistent with the total body irradiation treatment mode used during the animal studies (see “Materials and methods”). However, the simulations themselves were performed using a rectangular water phantom

Fig. 4

Cross-section of dose color map for 6 MeV and 6 + 12 MeV electrons in pigs. a The dose distribution of 6-MeV protons for a surface dose of 2.5 Gy per side approximates the dose distribution of SPE protons. b The same dose is given using mixed 6 MeV (80% of total fluence) and 12 MeV (20% to total fluence) electrons

Fig. 5

Comparison of proton range and proton energy. The proton energy for a 30-cm human or 20-cm pig is similar, which would give similar LET and RBE values for SPE protons, even with geometrical/energy scaling to account for the different animal sizes. However, for mice, to scale the energy of the protons to match the significantly smaller geometrical size would entail the use of higher LET/RBE protons. The proton range versus energy plot was created using the online PSTAR program from the National Institute of Standards and Technology (http://physics.nist.gov/PhysRefData/Star/Text/PSTAR.html)