Assessment of dose increase after administration of radiopharmaceuticals prepared with cyclotron-produced 99mTc

Technetium-99m (99mTc) is currently available from 99Mo/99mTc generators as the β-decay product of 99Mo (T½=66 h). Nowadays, 99Mo is mostly obtained as a fission product in nuclear reactors by neutron-induced reactions on highly enriched uranium. Alternative production routes, such as direct production of 99mTc via 100Mo(p,2n)99mTc reaction using medical cyclotrons has the potential to be both reliable and relatively costeffective. However, results showed that the extracted 99mTc from the proton-bombarded 100Mo-enriched target contains small quantities of several Tc radioisotopes (93mTc, 93Tc, 94Tc, 94mTc, 95Tc, 95mTc, 96Tc and 97mTc). The aim of this work was to estimate the dose increase (DI) due to the contribution of Tc radioisotopes generated as impurities, after the intravenous injection of four radiopharmaceuticals prepared with cyclotronproduced 99mTc (CP-99mTc) using 99.05% 100Mo-enriched metallic targets. Four 99mTc radiopharmaceuticals (pertechnetate, sestamibi (MIBI), hexamethylpropylene- amine oxime (HMPAO) and disodium etidronate (HEDP)), were considered in this study. The biokinetic models reported by the International Commission on Radiological Protection (ICRP) for each radiopharmaceutical were used to define the main source organs and to calculate the number of disintegrations per MBq that occurred in each source organ (Nsource) for each Tc radioisotope present in the CP-99mTc solution. Then, target organ equivalent doses and effective dose were calculated for each Tc radioisotope with the OLINDA/EXM software versions 1.1 and 2.0, using the calculated Nsource values and the adult male phantom as program inputs. Total effective dose produced by all Tc isotopes impurities present in the CP-99mTc solution was calculated using the fraction of total activity corresponding to each radioisotope generated by the bombardment of 100Mo-enriched (99.05%) metallic target. Finally, the effective obtained dose was compared with the effective dose delivered by the generator-produced 99mTc. The total effective dose increases of CP-99mTc radiopharmaceuticals, calculated with both versions of the OLINDA software, remained within the 10% limit in all cases, from 6 up to 12 hours after end of bombardment (EOB). The Tc radioisotopes with the highest concentration in the CP-99mTc solution at EOB are 94mTc and 93mTc. However, their contribution to DI 6 hours after EOB is minimal, due to their short half-lives. 96Tc is the radioisotope with the largest contribution to the effective DI, followed by 95Tc and 94Tc, although their concentration in the CP-99mTc solution is 5 times less than 94mTc and 93mTc at the EOB. This is due to the types of their emissions and relatively long half-lives. The increase in the radiation dose caused by the other Tc radioisotopes contained in produced CP-99mTc, as described here, is quite low. Although the concentrations of the 94Tc and 95Tc radioisotopes in the CP-99mTc solution exceed the limits established by the European Pharmacopoeia, CP-99mTc radiopharmaceuticals could be used in routine nuclear medicine diagnostic studies if administered from 6 to 12 hours after the EOB; thus, maintaining the effective DI within the 10% limit.