A small-scale anatomical dosimetry model of the liver

Anna Stenvall, Erik Larsson, Sven-Erik Strand, Bo-Anders Jönsson
2014 Physics in Medicine and Biology  
There is a need of reassessment of radiation absorbed dose specification in nuclear medicine, taking the non-uniformities in the distribution of radioactivity into account. This is especially important in radionuclide therapies where very high activities are administered and the radionuclides emit alpha particles, beta particles or low-energy electron and will require absorbed dose calculations on a scale comparable to the ranges of these particles. Although the liver is relatively
more » ... t, this treatment rationale for radionuclide therapy has made the liver to be one of the dose-limiting organs. In this work we developed a small-scale dosimetry model of the liver, for Monte Carlo calculations of detailed and more accurate absorbed dose distributions of the liver microstructure. Material and method: The mathematical model created, based on the microstructure of the hepatic lobule, is used to calculate absorbed doses on a tissue level for different radionuclides and for different source-target combinations within the liver. The Monte Carlo codes MCNP5, Version 1.51 and MCNPX 2.6 were used to create the model, consisting of a close packed hexagonal lattice pattern representing the hepatic lobules with the edge length of 500 µm, each centered around the central vein and in the periphery framed by six portal triads; i.e. the portal vein, the hepatic artery and the bile duct. The lobule section consists mainly of the hepatocytes, the bile canaliculi, the space of Disse and the mononuclear phagocytosis system, i.e. the Kupffer cells. Absorbed energy for different common radionuclides; 125 I, 90 Y, 211 At, 99 Tc m , 111 In, 177 Lu, 131 I and 18 F were in different structures tallied and absorbed dose ratios between absorbed dosed in different targets and the mean absorbed dose to the total volume were calculated. Results: Depending on the particles emitted and their energy the absorbed dose in different microstructures will differ. For nuclides emitting low energy particles, e.g. 125 I, the ratio between the locally absorbed dose and the mean dose the whole organ will be high; in the Kupffer cell acting as source about 135 times whereas in the hepatocytes close to the source the absorbed dose is close to the mean dose but significantly lower in areas between the Kupffer cells. For 90 Y on the other hand, the absorbed dose ratio in all target regions will be close to 1. For 111 In, used in diagnostics, the absorbed dose ratio will be quite comparable to 177 Lu, commonly used for the therapy. For the central vein, a possible target for radiation induced liver disease; the absorbed dose ratio will be as lowest for 177 Lu whereas the use of 90 Y and 18 F will result in an absorbed dose close to the mean dose. Discussion: The small-scale dosimetry model created is a simplification of the organization of the hepatic microstructure and therefore the results will visualize an idealized situation and the numerical values presented may not represent the biological reality. Since the dose-response relation of the non-uniform absorbed dose distribution for different commonly used radionuclides is not known, the biological implications still need to be investigated. Conclusion: A heterogeneous activity distribution will for some radionuclides result in a non-uniform absorbed dose distribution. Compared to the mean absorbed dose to the total volume of the liver, both over-and under-estimations of the absorbed dose can be seen, results which can have implications both in outcome of radionuclide therapies and for radiation risk estimations.
doi:10.1088/0031-9155/59/13/3353 pmid:24874832 fatcat:33jsglundnb3dlfx425u4yu62i