Internal radiation dose assessment of radiopharmaceuticals prepared with cyclotron‐produced 99mTc
Authors: Meléndez‐Alafort Laura , Ferro‐Flores Guillermina , De Nardo Laura , Bello Michele , Paiusco Marta , Negri Anna , Zorz Alessandra , Uzunov Nikolay , Esposito Juan , Rosato Antonio 
Published: January 2019

Purpose

Technetium‐99m (99mTc) is the radioisotope most widely used in diagnostic nuclear medicine. It is readily available from 99Mo/99mTc generators as the β−decay product of the 99Mo (T&frac12; = 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 [100Mo(p,2n)99mTc], have the potential to be both reliable and relatively cost‐effective. However, results showed that the 99mTc extracted from proton‐bombarded 100Mo‐enriched targets 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 cyclotron‐produced 99mTc (CP‐99mTc) using 99.05% 100Mo‐enriched metallic targets.

Methods

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 and compared with the effective dose delivered by the generator‐produced 99mTc.

Results

In all cases, the total effective DI of CP‐99mTc radiopharmaceuticals calculated with either versions of the OLINDA software was less than 10% from 6 up to 12 h after EOB. 94mTc and 93mTc are the Tc radioisotopes with the highest concentration in the CP‐99mTc 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 96Tc, followed by 95Tc and 94Tc. This is due to the types of their emissions and relatively long half‐lives, although their concentration in the CP‐99mTc solution is five times lower than that of 94mTc and 93mTc at the EOB.

Conclusions

The increase in the radiation dose caused by other Tc radioisotopes contained in CP‐99mTc produced as described here is quite low. Even though 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 h after the EOB, thus maintaining the effective DI within the 10% limit.