Monte Carlo-based evaluation of S-values in mouse models for positron-emitting radionuclides.

In addition to being a powerful clinical tool, Positron emission tomography (PET) is also used in small laboratory animal research to visualize and track certain molecular processes associated with diseases such as cancer, heart disease and neurological disorders in living small animal models of disease. However, dosimetric characteristics in small animal PET imaging are usually overlooked, though the radiation dose may not be negligible. In this work, we constructed 17 mouse models of different body mass and size based on the realistic four-dimensional MOBY mouse model. Particle (photons, electrons and positrons) transport using the Monte Carlo method was performed to calculate the absorbed fractions and S-values for eight positron-emitting radionuclides (C-11, N-13, O-15, F-18, Cu-64, Ga-68, Y-86 and I-124). Among these radionuclides, O-15 emits positrons with high energy and frequency and produces the highest self-absorbed S-values in each organ, while Y-86 emits γ-rays with high energy and frequency which results in the highest cross-absorbed S-values for non-neighbouring organs. Differences between S-values for self-irradiated organs were between 2% and 3%/g difference in body weight for most organs. For organs irradiating other organs outside the splanchnocoele (i.e. brain, testis and bladder), differences between S-values were lower than 1%/g. These appealing results can be used to assess variations in small animal dosimetry as a function of total-body mass. The generated database of S-values for various radionuclides can be used in the assessment of radiation dose to mice from different radiotracers in small animal PET experiments, thus offering quantitative figures for comparative dosimetry research in small animal models.