Patient dosimetry for cyclotron produced ^99mTc radiopharmaceuticals

Objectives Direct production of ^99mTc by medical cyclotrons has been proposed as an alternative to ⁹⁹Mo generators. However, both theoretical calculations and experiments show that other technetium radioisotopes, chemically inseparable from ^99mTc, will also be produced. Our objective was to evaluate the impact of these impurities on patient dosimetry when using cyclotron produced ^99mTc radiopharmaceuticals.

Methods Several batches of ^99mTc pertechnetate were produced by irradiating two different lots of ¹⁰⁰Mo with 18-10MeV protons. The purified ^99mTcO₄ solution was assayed by high-resolution γ-spectroscopy up to 32 days after irradiation to determine its radionuclide content. These results were compared with theoretical yields calculated for proton energies ranging from 16 to 24MeV (down to 6 and 10MeV) using the Empire-3 code. Absorbed radiation doses were estimated for ^99mTc-pertechnetate, ^99mTc-MDP and ^99mTc-Sestamibi using the OLINDA software.

Results The optimal time for ^99mTc utilisation was found to be between 2 and 12h after irradiation. For highly enriched ¹⁰⁰Mo material, radiation doses from cyclotron produced technetium exceeded by 1-2% doses from pure ^99mTc. Our results also showed that isotope composition of ¹⁰⁰Mo target is critical as higher enrichment but a less desirable Mo-isotopic composition led to doses exceeding by 10% those from pure ^99mTc with the main contributors being ⁹⁴Tc, ⁹⁵Tc and ⁹⁶Tc. Modeling incident proton energies above 20MeV showed progressively higher radiation doses mainly from (p,3n) reaction products.

Conclusions Cyclotron produced ^99mTc will contain other technetium radioisotopes, with amounts highly dependent on ¹⁰⁰Mo target composition. The yields observed in our experiments closely followed theoretical predictions. With high quality ¹⁰⁰Mo, few contaminants were produced below 20MeV, having minimal impact on patient absorbed radiation doses. However, higher proton energies and unfavorable target composition could lead to significantly increased radiation dose.

Research Support NSERC and CIHR research grants