Foetal dose conversion coefficients for ICRP-compliant pregnant models from idealised proton exposures

Abstract

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.

Figure 1.

Dominant generation of secondary electrons, photons and protons. The number of secondary particles per source proton, N, as a function of initial proton energy is shown for the RPI-P9 model and the AP beam. Kaons, which give negligible number of tracks, were excluded from the graph.

Figure 2.

Prevailing contribution of protons to the absorbed organ dose. The absorbed dose to the foetus (normalised per unit fluence) is shown as a function of source proton energy for the RPI-P9 model and the AP beam. Partial contributions of charged particles are also shown. Kaons and muons contribute a negligible fraction of dose.

Figure 3.

Energy dependence of dose conversion coefficients for the whole foetus. Monte Carlo results for five source configurations (AP, PA, LAT, ROT and ISO) are shown in comparison to the RPI-P3, -P6 and -P9 models (A, B and C, respectively).

Figure 4.

Ratio of RPI-P foetal doses to those published by Chen (2006) for the whole foetus (A, C and E for RPI-P3, -P6 and -P9, respectively) and its brain (B, D and F for RPI-P3, -P6 and -P9, respectively) as a function of source proton energy. Five standard irradiation geometries (AP, PA, LAT, ROT and ISO) and 3, 6 and 9 months models are shown.