Effective dose to immuno-PET patients due to metastable impurities in cyclotron produced zirconium-89

Abstract Immuno-PET is a nuclear medicine technique that combines positron emission tommography (PET) with radio-labeled monoclonal antibodies (mAbs) for tumor characterization and therapy. Zirconium-89 ( 89 Zr) is an emerging radionuclide for immuno-PET imaging. Its long half-life (78.4 h) gives ample time for the production, the administering and the patient uptake of the tagged radiopharmaceutical. Furthermore, the nuclides will remain in the tumor cells after the mAbs are catabolized so that time series studies are possible without incurring further administration of radiopharmarceuticals. 89 Zr can be produced in medical cyclotrons by bombarding an yttrium-89 ( 89 Y) target with a proton beam through the 89 Y(p,n) 89 Zr reaction. In this study, we estimated the effective dose to the head and neck cancer patients undergoing 89 Zr-based immune-PET procedures. The production of 89 Zr and the impurities from proton irradiation of the 89 Y target in a cyclotron was calculated with the Monte Carlo code MCNPX and the nuclear reaction code TALYS. The cumulated activities of the Zr isotopes were derived from real patient data in literature and the effective doses were estimated using the MIRD specific absorbed fraction formalism. The estimated effective dose from 89 Zr is 0.5±0.2 mSv/MBq. The highest organ dose is 1.8±0.2 mSv/MBq in the liver. These values are in agreement with those reported in literature. The effective dose from 89m Zr is about 0.2−0.3% of the 89 Zr dose in the worst case. Since the ratio of 89m Zr to 89 Zr depends on the cooling time as well as the irradiation details, contaminant dose estimation is an important aspect in optimizing the cyclotron irradiation geometry, energy and time.