Microscopic energy absorption of the DNA molecules from Auger electrons of iodine-125

Dosimetry on sub-cellular level for intracellular incorporated auger-electron-emitting radionuclides: a comparison of Monte Carlo simulations and analytic calculations

A quantitative dosimetric comparison was performed between Monte Carlo (MC) simulations and analytic calculations at the (sub) cellular level (V79 cells) for four nucleus-incorporated radiochemicals ((125)I/(123)I/(77)Br-UdR and A (125)IP) and two radiochemicals that localised mainly in the cytoplasm of cells ((125)I-dihydrorhodamine and Na(2)(51)CrO(4)). A microscopic investigation around the decay site of the three DNA-incorporated radionuclides ((125)I/(123)I/(77)Br-UdR) was also carried out. On the whole, deviations between MC and analytic calculations for the absorbed dose and dose rate to the cell nucleus were within ∼10%. The dose rate to the nucleus for the radiochemicals that mainly localised in the cytoplasm was greater than that for the nucleus-incorporated ones. Also evident was that the dose rate to the nucleus was approximately the same for the three DNA-incorporated radiochemicals. In contrast to the small differences found between MC and analytic calculations for the (average) absorbed dose to the nucleus, the dosimetric analysis at the microscopic level for the three DNA-incorporated radionuclides showed that the two computational approaches lead to a completely different energy deposition pattern around the decay site.

Absorbed dose estimates at the cellular level for 131I

Microdosimetric calculations of 131I have been evaluated for a single cell and for cell clusters. A VsBasic program has been used to calculate stopping power, linear energy transfer, range values and deposited energies per decay for beta particles, Auger and conversion electrons of 131I. The chemical composition of the cell has been taken into account in this model; results were compared with water medium. Besides, total absorbed doses have been calculated for the radionuclides distributed randomly within the cell and clusters. Cross-fire irradiation has been considered for clusters of cells. In this case, absorbed doses per cell within a cluster were found to be significantly higher than absorbed doses per single cell, depending on the cluster size. Results showed that 131I is a promising radionuclide for therapy of tumors from millimeter to centimeter dimensions.

Synkavit and its direct labelling with iodine-125, as a potential anti-cancer drug

The compound known as "synkavit" is a diphosphate derivative of vitamin K3 (menadion), which is capable of being selectively accumulated in certain tumour cells, and covalently bonded to DNA producing considerable DNA damage. On the other hand, iodine-125 nuclide incorporated into the nucleus of living cells causes extreme radiotoxic effects. Consequently, synkavit can be used as a specific carrier of iodine-125 into the nucleus of tumour cells. Thus, its iodo-derivatives have become interesting agents on the potential application of iodine-125 in cancer therapy. 6-Iodo-synkavit is a unique iodo-derivative described in the literature. In addition, its synthesis and radioiodination is still problematic, and consequently the results obtained using 6-iodo-synkavit labelled with iodine-125 remains in question. For this reason, the synthesis of 6-iodo-synkavit was examined in this study. It is finally determined that a mixture of different iodo-isomers of synkavit has been produced rather than its specific 6-iodo-isomer, when the synthetic sequence was begun with the direct sulfonation of 2-methyl-naphthalene. On the other hand, it is also determined that synkavit can directly be radioiodinated using different iodination techniques, and iodogen especially can be successfully used as an oxidative agent.

Microscopic energy absorption in the colloid of thyroid follicles from Auger electrons of iodine-125

In this study the microscopic energy absorption from Auger electrons of iodine-125 in colloid of two different thyroid models, which represent the normal and thyrotoxic thyroid glands, was theoretically calculated. These calculations were based on the method recently given by Unak (1987; Nucl. Intrum. Meth. Phys. Res. A255, 274) and the Auger electron spectrum given by Charlton and Booz (1981; Radiat. Res. 87, 10). Subsequently the range of highest energy Auger electrons of iodine-125 in thyroidal colloid was found to be about 72 microns. This range is considerably shorter than the radius of a normal thyroid follicle, about 150 microns, but is considerably longer than the 25 microns radius of a thyrotoxic thyroid follicle. On the other hand, the comparison of dose distributions in two different thyroid models showed that the similar activity of iodine-125 per gram of thyroid tissue is able to produce more radiotoxic effects on the thyrotoxic follicular cells.