### Abstract

New deterministic three-dimensional radiation transport code MultiTrans SP3 has been developed at VTT [1]. It has been applied e.g. to coupled neutron-photon transport problems in boron neutron capture therapy (BNCT) [2]. Objective of this study has been to apply MultiTrans SP3 also to coupled photon-electron radiation transport problems. Interesting applications for fast deterministic solution technique such as MultiTrans might be found in intensity modulated radiotherapy (IMRT) and brachytherapy. Electron transport is in general described by the Boltzmann-Fokker-Plank (BFP) equation. One form of the BFP-equation is the Boltzmann-CSD (continuous-slowing-down) approximation. However, the CSD-term can be included into electron cross sections and the pure Boltzmann equation - and simplified spherical harmonics (SP3) approximation used by MultiTrans - applies with pseudo cross sections [3]. From the pseudo cross section files formed by CEPXS code [4], MultiTrans SP3 can create macroscopic multigroup cross sections for arbitrary compounds in 35 photon and 35 electron groups, and is able to solve coupled photon-electron radiation transport problems. First collision source method is used to treat the uncollided flux, as the collided flux can be better approximated by a low-order spherical harmonics expansion. Two simplistic internal and external photon therapy dose planning problems have been defined as test cases. Radial dose distribution to water has been calculated for monoenergetic 25 keV - 1.75 MeV point sources situated in centre of a water sphere with 1 m diameter. Comparison results have been obtained by Monte Carlo code EGS4 [5]. With source energies less than 250 keV results agree within 6% in 10 cm distance from the source; but with higher energies results start to disagree more up to 18% in 10 cm distance with 1.75 MeV source energy, probably mainly due to the first collision source method used. For external monoenergetic 250 keV - 17.5 MeV monodirectional beams the first collision source method seems to apply better. Even with a square 17.5 MeV photon beam 125 mm each side, the dose to water calculated with MultiTrans SP3 at 50 cm depth seems to agree rather well with EGS4, taking notable stochastic fluctuations in the EGS4 calculation into account. [1] P. Kotiluoto, Nucl. Sci. Eng. 138 (2001) 269. [2] P. Kotiluoto, P. Hiismäki and S. Savolainen, Med. Phys. 28 (2001) 1905. [3] J. A. Josef and J. E. Morel, Phys. Rev. E 57 (1998) 6161. [4] L. J. Lorence, J. E. Morel and G. D. Valdez, SAND89-1685 (1989). [5] W. R. Nelson, H. Hirayama and D. W. O. Rogers, SLAC-265 (1985).

Original language | English |
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Title of host publication | Proceedings of the XXXVI Annual Conference of the Finnish Physical Society |

Place of Publication | Joensuu |

Publication status | Published - 2002 |

MoE publication type | Not Eligible |

Event | XXXVI Annual Conference of the Finnish Physical Society - Joensuu, Finland Duration: 13 Mar 2002 → 16 Mar 2002 Conference number: 36 |

### Publication series

Series | Selected papers. University of Joensuu, Department of Physics |
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Number | 7 |

ISSN | 1236-0376 |

### Conference

Conference | XXXVI Annual Conference of the Finnish Physical Society |
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Country | Finland |

City | Joensuu |

Period | 13/03/02 → 16/03/02 |

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## Cite this

Kotiluoto, P., Laitinen, J., & Helminen, H. (2002). Application of the new MultiTrans SP3 code in coupled photon-electron radiation transport problems. In

*Proceedings of the XXXVI Annual Conference of the Finnish Physical Society*[146]. Selected papers. University of Joensuu, Department of Physics, No. 7