COMPARISON OF CELLULAR AND CONVENTIONAL DOSIMETRY IN ASSESSING SELF-DOSE AND CROSS-DOSE DELIVERED TO THE CELL-NUCLEUS BY ELECTRON EMISSIONSOF (TC)-T-99M, I-123, IN-111, GA-67 AND TL-201

The radionuclides used in nuclear medicine imaging emit numerous mono- energetic electrons responsible for dose heterogeneity at the cellular level. S-self, the self-dose per unit cumulated activity (which resul ts from the radionuclide located in the target cell), and S-cross, the cross-dose per unit cumulated activity (which comes from the surround ing cells) delivered to a target cell nucleus by electron emissions of technetium-99m, iodine-123, indium-111, gallium-67 and thallium-201 w ere computed at the cellular level. An unbounded close-packed hexagona l cell arrangement was assumed, with the same amount of radioactivity per cell. Various cell sizes and subcellular distributions of radioact ivity (nucleus, cytoplasm and cell membrane) were simulated. The resul ts were compared with those obtained using conventional dosimetry. S-s elf and S-cross values depended closely on cell dimensions. While the self-dose depended on the tracer distribution, the latter affected the cross-dose by less than 5%. When the tracer was on the cell membrane, the self-dose was particularly low compared to the cross-dose, as the self-dose to cross-dose ratio was always less than 11%. In the case o f cytoplasmic or cell membrane distribution of radioactivity, conventi onal electron dosimetry slightly overestimated the dose absorbed by th e target cell nucleus (by 1.08- to 1.7-fold). In contrast, conventiona l dosimetry strongly underestimated the absorbed dose (1.1- to 75-fold ) when the radioactivity was located in the nucleus. The discrepancies between conventional and cellular dosimetry call for calculations at the cellular level for a better understanding of the biological effect s of radionuclides used in diagnostic imaging.