Assessment of radionuclide impurities in [F]fluoromethylcholine ([F]FMCH).

Purpose: [F]Fluoromethylcholine ([F]FMCH) is a radiopharmaceutical used in positron emission tomography (PET) imaging for the study of prostate, breast, and brain tumors. It is usually synthesized in cyclotron facilities where F is produced by proton irradiation of [O]HO through O(p,n)F reaction. Due to the activation of target materials, the bombardment causes unwanted radionuclidic impurities in [O]HO, that need to be removed during the radiopharmaceutical synthesis. Thus, the aim of this study is to quantify the radionuclide impurities in the F production process and in the synthesized [F]FMCH, demonstrating the radionuclidic purity of this radiopharmaceutical.

Methods: Long-lived radionuclide impurities were experimentally assessed using high-resolution gamma and liquid scintillation spectrometries, while short-lived impurities were monitored analyzing the decay curve of the irradiated [O]HO with an activity calibrator. As spectrometric radionuclide library, a Geant4 Monte Carlo simulation of the F-target assembly was previously performed.

Results: H, Mn, Co, Tc, Cd, and Re were found in the irradiated [O]HO, but no radionuclide was found in the non-irradiated [O]HO neither in the final [F]FMCH solution with an activity concentration greater than the minimum detectable activity concentration. A total impurity activity <6.2 kBq was measured in the irradiated [O]HO, whereas a [F]FMCH radionuclide purity >99.9999998% was estimated. Finally, the decay curve of the irradiated [O]HO revealed a very low maximum of N activity (<0.03% of F) even immediately after the end of bombardment.

Conclusions: This study demonstrated the radionuclidic purity of [F]FMCH according to the EU Pharmacopeia.