Internal radiation dose assessment of radiopharmaceuticals prepared with cyclotron-produced Tc.

Purpose: Technetium-99m ( Tc) is the radioisotope most widely used in diagnostic nuclear medicine. It is readily available from Mo/ Tc generators as the β decay product of the Mo (T  = 66 h) parent nuclide. This latter is obtained as a fission product in nuclear reactors by neutron-induced reactions on highly enriched uranium. Alternative production routes, such as direct reactions using proton beams on specific target materials [ Mo(p,2n) Tc], have the potential to be both reliable and relatively cost-effective. However, results showed that the Tc extracted from proton-bombarded Mo-enriched targets contains small quantities of several Tc radioisotopes ( Tc, Tc, Tc, Tc, Tc, Tc, Tc, and Tc). 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 cyclotron-produced Tc (CP- Tc) using 99.05% Mo-enriched metallic targets.

Methods: Four Tc 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 (N ) for each Tc radioisotope present in the CP- Tc 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 N values and the adult male phantom as program inputs. Total effective dose produced by all Tc isotopes impurities present in the CP- Tc solution was calculated using the fraction of total activity corresponding to each radioisotope and compared with the effective dose delivered by the generator-produced Tc.

Results: In all cases, the total effective DI of CP- Tc radiopharmaceuticals calculated with either versions of the OLINDA software was less than 10% from 6 up to 12 h after EOB. Tc and Tc are the Tc radioisotopes with the highest concentration in the CP- Tc solution at EOB. However, their contribution to DI 6 h after EOB is minimal, due to their short half-lives. The radioisotopes with the largest contribution to the effective DI are Tc, followed by Tc and Tc. This is due to the types of their emissions and relatively long half-lives, although their concentration in the CP- Tc solution is five times lower than that of Tc and Tc at the EOB.

Conclusions: The increase in the radiation dose caused by other Tc radioisotopes contained in CP- Tc produced as described here is quite low. Even though the concentrations of the Tc and Tc radioisotopes in the CP- Tc solution exceed the limits established by the European Pharmacopoeia, CP- Tc radiopharmaceuticals could be used in routine nuclear medicine diagnostic studies if administered from 6 to 12 h after the EOB, thus maintaining the effective DI within the 10% limit.