Radiation Protection Dosimetry Advance Access originally published online on February 26, 2009
Radiation Protection Dosimetry 2009 133(2):65-72; doi:10.1093/rpd/ncp020
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Foetal dose conversion coefficients for ICRP-compliant pregnant models from idealised proton exposures
1 UCSF Comprehensive Cancer Center, University of California, 1600 Divisadero St, Suite H-1031, Box 1708, San Francisco, CA 94115, USA
2 Nuclear Engineering and Engineering Physics Program, Rensselaer Polytechnic Institute, 110 Eighth St, Troy, NY 12180, USA
* Corresponding author: xug2{at}rpi.edu
Received November 10, 2008, amended January 23, 2009, accepted February 5, 2009
Protection of pregnant women and their foetus against external proton irradiations poses a unique challenge. Assessment of foetal dose due to external protons in galactic cosmic rays and as secondaries generated in aircraft walls is especially important during high-altitude flights. This paper reports a set of fluence to absorbed dose conversion coefficients for the foetus and its brain for external monoenergetic proton beams of six standard configurations (the antero-posterior, the postero-anterior, the right lateral, the left lateral, the rotational and the isotropic). The pregnant female anatomical definitions at each of the three gestational periods (3, 6 and 9 months) are based on newly developed RPI-P series of models whose organ masses were matched within 1% with the International Commission on Radiological Protection reference values. Proton interactions and the transport of secondary particles were carefully simulated using the Monte Carlo N-Particle eXtended code (MCNPX) and the phantoms consisting of several million voxels at 3 mm resolution. When choosing the physics models in the MCNPX, it was found that the advanced Cascade-Exciton intranuclear cascade model showed a maximum of 9% foetal dose increase compared with the default model combination at intermediate energies below 5 GeV. Foetal dose results from this study are tabulated and compared with previously published data that were based on simplified anatomy. The comparison showed a strong dependence upon the source geometry, energy and gestation period: the dose differences are typically less than 20% for all sources except ISO where systematically 40–80% of higher doses were observed. Below 200 MeV, a larger discrepancy in dose was found due to the Bragg peak shift caused by different anatomy. The tabulated foetal doses represent the latest and most detailed study to date offering a useful set of data to improve radiation protection dosimetry against external protons.