Dosimetric assessment in different tumour phenotypes with auger electron emitting radionuclides: 99mTc, 125I, 161Tb, and 177Lu

Abstract Internal radiotherapy using Auger-emitting radionuclides is a relatively new technique that presents interesting advantages with respect to external radiotherapy, such as localized tumor efficacy. The aim of this study was to assess the dosimetric effectiveness in irradiating a tumor partitioned in different phenotypes, with different radionuclides directed at each tumor phenotype: 99mTc, 125I, 161Tb, and 177Lu. State of the art Monte Carlo PENELOPE code and ICRP adult female reference voxel phantom (AFP) were used in order to mimic a lung tumor volume composed by four different phenotypes. For each radionuclide above mentioned, the decay modes (accessed through ICRP-107 data files) considered encompassed Auger electrons and β, X and ϒ radiation. Two main radiation therapy scenarios were simulated: i) the entire tumor was irradiated homogenously with each of the radionuclides; ii) each tumor phenotype was filled with a different radionuclide. The optimal dosimetric configuration was studied in terms of Dose Efficiency (DE), defined as tumor-to-healthy dose ratio. The Monte Carlo model was validated by comparing the results of SAF values in AFP and cellular S-values for 125I with the ones present in the bibliography. In the first scenario, calculations showed that the highest DE is reached by 125I. Namely, with 125I a gain dose factor (GDF) of about 2.8, 2.7 and 122.5 could be achieved, with respect to 177Lu, 161Tb, and 99mTc, respectively. In the second scenario, a combination of radionuclides directed to each of the phenotypes can act as enhancement for DE or dose in the tumor tissues, with respect to using only one radionuclide in the 4 tumor phenotypes. According to this study, the hypothetical use of different electron beam qualities directed to different tumor phenotypes of the same tumor could act as a radio-sensitizer and, at the same time, minimize dose to the surrounding healthy tissues.