223Ra is an alpha emitter with an half-life of 11.435 days and it decays in 219Rn (t1/2 = 3.96 s). 223Ra-dichloride (Xofigo) is a novel radiopharmaceutical indicated for the treatment of adults with castration-resistant prostate cancer with bone metastases and without symptomatic visceral metastases. Xofigo is provided in a ready to use solution, contained in a sealed vial, with an activity at the calibration date of 6 MBq. In this work we assessed the presence of 219Rn in the air gap of a commercial vial of 223Ra-Xofigo. Moreover transient of liquid-gas equilibrium was evaluated.
Samples of the air on top of the radiopharmaceutical solution were taken by an in-house sampling technique: the vial containing Xofigo and a 11 ml under-vacuum technevial were punched simultaneously using two needles connected each other with a plastic tube. To balance the pressure within the two vials and to collect all the 219Rn trapped in the plastic tube, an additional needle was added to the vial of the radiopharmaceutical preparation just after the first punch. The vial containing the sampled air was then measured in gamma-ray spectrometry using a HPGe N-Type detector with a 30 % relative efficiency and a resolution of 1.8 keV at 1332 keV. The spectrometry system was calibrated in the 59-1836 keV range by means of a multi-radionuclide certified reference solution, obtained from an accredited Standardization Laboratory. Different density of the samples, with respect to the calibration source, was taken into account: the efficiency curve for an equivalent gaseous 5 cm3 calibration source was evaluated using the software Labsocs and applied to the measurements. Finally, measurements were decay corrected taking into account the sampling time. To evaluate the transient of the liquid-gas equilibrium within the vial additional samples were taken after 5 and 15 minutes from the first punch.
Results of the measurements showed an activity of 219Rn in the air gap of a commercial 223Ra-Xofigo vial quantifiable in 1.2 % of the 223Ra activity at the sampling time. Furthermore, measurements of the samples taken after 5 and 15 minutes from the first punch showed an activity of 219Rn comparable, within the measurement uncertainty, with the activity measured in the first sample.
In conclusion, the measurements campaign conducted showed a significant fraction of activity of 219Rn in the air gap of a commercial vial of 223Ra-Xofigo. Patient dose should be prepared just before injection in ventilated fume hood; furthermore the air gap should be collected in a under-vacuum vial. After the preparation of a patient dose liquid-gas equilibrium is reached within the vial in a time comparable with the reaching of the secular equilibrium 223Ra-219Rn.

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