Calculation of dose point kernel values for monoenergetic electrons and beta emitting radionuclides

Abstract

Targeted radionuclide therapy (TRT) and beta-emitting seeds brachytherapy (BSBT) exploit the characteristics of energy deposited by beta-emitting radionuclides. Monte Carlo (MC) modelling of electron transport is crucial for calculations of absorbed dose for TRT and BSBT. However, computer codes capable of providing consistent results are still limited. Since experimental validations show several difficulties, the estimation of electron dose point kernel (DPK) is often used to verify the accuracy of different MC codes. In this work, we compared DPK calculations for various point, isotropic and monoenergetic electron sources and several beta-emitting radioisotopes using the codes MCNP, EGSnrc, PENELOPE and TOPAS with different simulation options. The simulations were performed using latest versions of EGSnrc and Penelope, TOPAS version 3.3.1 and MCNP version 6.1 Monte Carlo codes. In our simulations, the geometrical model consists of a point electron source placed at the center of a water sphere emitting isotropically. The water sphere was divided into 28 shells and the energy deposition was scored within these shells. The radius of the outermost shell was 1.2R0, where R0 is the continuous slowing down approximation (CSDA) range. Five monoenergetic beta sources with energies of 0.05, 0.1, 0.5, 1 and 3 MeV were studied. Six beta-emitting radionuclides were also simulated: Lu-177, Sm-153, Ho-166, Sr-89, I-131 and Y-90. Monoenergetic electron simulations showed large deviations among the codes, larger than 13% depending on the electron energy and the distance from the source. In the cases where beta spectra of radionuclides were simulated, all MC codes showed differences from EGSnrc (used as reference value - RV) less than 3% within rE90 range (radius of the sphere in which 90% of the energy of the spectrum electrons would be deposited). TOPAS showed results comparable to EGSnrc and PENELOPE. DPK values for 0.1 MeV monoenergetic electrons, calculated using MCNP6, led to differences higher than ±5% from RV despite our attempts to tune electron transport algorithms and physics parameters.