Differences among Monte Carlo codes in the calculations of voxel values for radionuclide targeted therapy and analysis of their impact on absorbed dose evaluations

• Published in: Medical physics (Lancaster) Vol. 36; no. 5; pp. 1543 - 1552
• Main Authors: Pacilio, M., Lanconelli, N., Lo Meo, S., Betti, M., Montani, L., Torres Aroche, L. A., Coca Pérez, M. A.
• Format: Journal Article
• Language: English
• Published: 01-05-2009
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.3103401

Several updated Monte Carlo (MC) codes are available to perform calculations of voxel
values for radionuclide targeted therapy. The aim of this work is to analyze the differences in the calculations obtained by different MC codes and their impact on absorbed dose evaluations performed by voxel dosimetry. Voxel
values for monoenergetic sources (electrons and photons) and different radionuclides (
,
, and
) were calculated. Simulations were performed in soft tissue. Three general‐purpose MC codes were employed for simulating radiation transport:
MCNP4C
, EGSnrc, and
GEANT4
. The data published by the MIRD Committee in Pamphlet No. 17, obtained with the
EGS4
MC code, were also included in the comparisons. The impact of the differences (in terms of voxel
values) among the MC codes was also studied by convolution calculations of the absorbed dose in a volume of interest. For uniform activity distribution of a given radionuclide, dose calculations were performed on spherical and elliptical volumes, varying the mass from 1 to 500 g. For simulations with monochromatic sources, differences for self‐irradiation voxel
values were mostly confined within 10% for both photons and electrons, but with electron energy less than 500 keV, the voxel
values referred to the first neighbor voxels showed large differences (up to 130%, with respect to EGSnrc) among the updated MC codes. For radionuclide simulations, noticeable differences arose in voxel
values, especially in the bremsstrahlung tails, or when a high contribution from electrons with energy of less than 500 keV is involved. In particular, for
the updated codes showed a remarkable divergence in the bremsstrahlung region (up to about 90% in terms of voxel
values) with respect to the
EGS4
code. Further, variations were observed up to about 30%, for small source‐target voxel distances, when low‐energy electrons cover an important part of the emission spectrum of the radionuclide (in our case, for
). For
and
, the differences among the various codes have a negligible impact (within few percents) on convolution calculations of the absorbed dose; thus either one of the MC programs is suitable to produce voxel
values for radionuclide targeted therapy dosimetry. However, if a low‐energy beta‐emitting radionuclide is considered, these differences can affect also dose depositions at small source‐target voxel distances, leading to more conspicuous variations (about 9% for
) when calculating the absorbed dose in the volume of interest.