MODELING OF THE RELATIONSHIP BETWEEN CELL DIMENSIONS AND MEAN ELECTRON DOSE DELIVERED TO THE CELL-NUCLEUS - APPLICATION TO 5 RADIONUCLIDES USED IN NUCLEAR-MEDICINE

The mean dose delivered to the cell nucleus by electron emissions of T c-99(m), I-123, In-111, Ga-67 and Tl-201 was evaluated at the subcellu lar level. Models were applied assuming uniform distributions of radio activity throughout the nucleus, the cytoplasm or the cell membrane, s nowing computation of the total absorbed fraction phi and S-values to the cell nucleus as a function of cell dimensions. The graphs of phi p lotted according to cell dimensions show that the dose to the cell nuc leus strongly depends on the subcellular distribution of radioactivity , the nucleus radius R(nucl) and the cytoplasmic thickness e. For a nu clear distribution, phi ranges from 0.1 to 0.35 for the radionuclides studied and S from 0.049 cGy Bq(-1) s(-1) to 5.503 cGy Bq(-1) s(-1). I n the case of a cell membrane localization, the maximum is obtained fo r I-123 (phi = 0.016). For a cytoplasmic distribution, the maximum is obtained for Tl-201 with a value of 0.036. To ease future calculations , third-degree polynomials have been separately fitted to the relation ship between the mean absorbed dose to the nucleus for activity accumu lated in the nucleus, cytoplasm or surface of the cell membrane. We fo und a good agreement between our computations and the values obtained by the polynomials. The relative difference between the two methods is always less than 0.7%, 2.8% and 4.5% respectively for nuclear, cell m embrane and cytoplasmic distributions.