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Radiation protection for the radium/thorium therapy (Xofigo)

Radioactive substances may be released during production and application of 227Th preparations (Xofigo). In that case, the resulting activity concentration in the air cannot be determined by standard algorithms used for long-living radionuclides by air samplers.

The decay chain of 227Th contains a number of short-living alpha emitters reaching very soon the radioactive equilibrium with the mother nuclide. However, the main problem results from the appearance of the radon isotope 219Rn (actinon) within the decay chain. The rare gas emanates very easily from the preparation and will be distributed quickly in a room by diffusion. In practice, the concentration of radon daughter products is always above the ones of 227Th/223Ra — in most cases by magnitudes of order. In case of promptly analysis of the activity collected by the air sampler on a filter, the alpha spectrum will be dominated by the decay products of the various radon isotopes. The requested detection limit for 227Th/223Ra in the range of a few mBq/m³ cannot be obtained by spectroscopic analysis due to the limited resolution of filter spectra and emission energies very close to each other.

For that purpose, SARAD in cooperation with the Bayer AG has developed a time and energy spectroscopic procedure which is based on a sampling cycle of 24 hours. The activity concentration off actinon (219Rn) will be determined during the leading eight hours exposure period using the alpha emission of the daughter nuclide 211Bi. A subsequent interval allows the complete decay of short-living daughters of the radon isotopes 222Rn and 219Rn on the filter. Within a third interval, the activity of the remaining thorium and radium will be measured. The probably occurring activity of thoron daughters will be compensated by alpha spectroscopy. Thoron daughters are also accumulated on the filter due to the half-life of 212Pb.

The algorithm is available for instruments with fixed filter (poCAMon) as well as for ones with filter stepping units (Aer 5000, Aer 5200 and Aer 5400).

The advantages of the filter stepping unit are the stable detection limit and the long maintenance-free operational period of nearly one year. The detection limits of the various instruments are mainly defined by their flow rate. Under ventilated conditions with assumed low radon concentrations, the requested values of 20 mBq/m² are achievable even at 3 l/min (poCAMon).