Cellular dosimetry in nuclear medicine imaging: training

The radionuclides used in nuclear medicine imaging emit not only diagnostically useful photons, but also energy electron emissions, responsible for dose heterogeneity at the cellular level. The mean dose delivered to the cell nucleus by electron emissions of 99mTc, 123I, 111In, 67Ga, and 201Tl, has been calculated, for the cell nucleus, a cytoplasmic and a cell membrane distribution of radioactivity. This model takes into account both the self-dose which results from the radionuclide located in the target cell, and the cross-dose, which comes from the surrounding cells. The results obtained by cellular dosimetry (Dcel) have been compared with those obtained with conventional dosimetry (Dconv), by assuming the same amount of radioactivity per cell. Cellular dosimetry shows, for a cytoplasmic and a cell membrane distributions of radioactivity, that the main contribution to the dose to the cell nucleus, comes from the surrounding cells. On the other hand, for a cell nucleus distribution of radioactivity, the self-dose is not negligible and may be the main contribution. The comparison between cellular and conventional dosimetry shows that Dcel/Dconv ratio ranges from 0.61 and O.89, in case of a cytoplasmic and a cell membrane distributions of radioactivity, depending on the radionuclide and cell dimensions. Thus, conventional dosimetry slightly overestimates the mean dose to the cell nucleus. On the other hand, Dcel/Dconv ranges from 1.1 to 75, in case of a cell nucleus distribution of radioactivity. Conventional dosimetry may strongly underestimates the absorbed dose to the nucleus, when radioactivity is located in the nucleus. The study indicates that in nuclear medicine imaging, cellular dosimetry may lead to a better understanding of biological effects of radiopharmaceuticals. (authors)