Investigations of 131I concentration in indoor air using charcoal filters and gamma spectroscopy

Full text: Radiation protection standards require the recording of staff radiation dose in nuclear medicine thyroid radiotherapy. A commonly used method measures the 131I thyroid activity externally with a gamma detector, followed by calculation of the committed equivalent thyroid dose. The main disadvantages are the low sensitivity and the uncertainty of the time of uptake, which can only be compensated by long measurement times and short measurement intervals. The measurements have to be applied to all staff members. An alternative can be provided by the measurement of the cumulated concentration of 131I in indoor air using charcoal filters. The filters are placed in patient rooms at representative locations, exposed for several days and then investigated for absorbed 131I activity. Both above mentioned disadvantages can be overcome: the measurement records the cumulated concentration history of the room and the obtained value can be used for dose calculation for all staff members knowing their working history. Standard charcoal filters (PicoRad vials, Accustar Labs, normally used for indoor radon measurements) were placed into patient rooms in a thyroid therapy department and exposed to room air for 72 hours. The vials were then subjected to high resolution low level gamma spectroscopy using a high purity germanium (hpGe) detector (50% relative efficiency, 10 cm lead shielding). Absolute activity determination was based on the net count rate in the 364 keV gamma peak and absolute counting efficiency obtained mathematically from detector and vial geometry using the commercial Labsocs (Canberra Inc.) program. Using the mean air activity/vial activity calibration factor obtained in a similar study pioneering this application, but using liquid scintillation spectroscopy as detection method (F. Jimenez et al, 2nd. European IRPA Congress, Paris 2006), mean air concentrations between 0.09 and 2.01 Bq/m3 were found. The data correlated well with patient administered activity. The detection limit was calculated to be 0.05 Bq/m3 for a spectrometer counting time of 60000 s. Using standard ICRP dose conversion factors, the applied method would be able to predict an annual staff thyroid dose down to 3 μSv, following exposure to airborne 131I during 1 hour per working day. The results show the applicability of the method for radiation protection purposes without the need of time-consuming and frequent individual thyroid burden measurements. (author)