On the biological effectiveness of incorporated 99'mTc

Full text: 99mTc is frequently used in diagnostic nuclear medicine because in addition to an appropriate half-life (T1/2 = 6,01 h) it is supposed to be a pure gamma emitter (140,5 keV). However, the transition from the metastable to the ground state includes internal conversion processes, which cause the release of Auger and conversion electrons. Ultra soft X rays depositing their energy mainly via photoeffect initiating thereby also low energy photoelectrons and an additional Auger electron have shown an increased RBE. Therefore, to quantify the biological efficiency of incorporated 99mTc, these electrons should be particularly considered. Methodically, this paper encloses the following steps: (1) Electron emission spectra of mmcTc have been generated by Monte Carlo simulation; (2) with subsequent track structure calculations the induction of DNA strand breaks with different complexity was modelled, and (3) ultra soft X ray simulation has been used for comparison. The results show an emission of 4 electrons on average per 99mTc decay. The energies are partly in the same range as those produced by ultra soft X rays. Within a spherical volume of 10 μm diameter, typical for a mammalian cell, about 2.5 keV electron energy will be deposited per decay. On the basis of the same number of decays or photo effects, respectively, 99mTc causes a nearly identical spectrum of primary DNA strand breaks as carbon K radiation