Accurate activity measurements for 223Ra

Aim: 223Ra-dichloride is a calcium analogue that targets the hydroxyapatite in areas of increased bone turnover and has been shown to improve survival in patients with castration-resistant prostate cancer. It decays via 5 alpha emitting and 2 beta emitting radionuclides which delivers high linear energy transfer radiation, increasing the probability of cell kill whilst minimising the damage to healthy tissue. The widespread adoption of 223Ra necessitated the development and dissemination of traceable activity measurement standards to allow accurate patient administrations. The aim of this study was to generate calibration factors for accurate 223Ra activity measurements for a range of commercially available radionuclide calibrators. Materials and Methods: Two of NPL?s well-characterised secondary standard ionisation chambers (Fidelis and PA782) and two commercially available ionisation chamber systems (Capintec CRC-25R and an Atomlab-500) were used to determine the calibration factors required for accurate activity measurements. These instruments allow comparative measurements using devices commonly found in clinical sites. Standardised solutions of 223RaCl2 were measured in a range of clinically relevant geometries and volumes to determine dial settings or calibration factors along with correction factors for volume dependency. Results: Dial settings and calibrations factors for commonly used vials, syringes and ampoules at specific volumes are presented along with robust uncertainty estimates (dependent on the type of calibrator in use). The effect of volume in each geometry is also presented along with realistic correction factors that may be used in lieu of independent dial factors for each volume. Conclusion: Accurate measurements of 223Ra can be made in a clinical setting using commercially available detectors in a variety of geometries. This will ensure the correct prescribed activity is administered to patients and in turn will improve the accuracy of whole-body and tumour uptake and absorbed dose calculations.