Iridium-192 high-dose-rate calibration discrepancies: A growing concern?

An increasing discrepancy between the measured reference air kerma rate (RAKR) of high-dose-rate (HDR) iridium-192 (¹⁹²Ir) sources and that reported by the manufacturer on the source certificate was observed at a hospital in Australia. This study aimed to determine whether the drift recorded in a single clinic was a local anomaly or more widespread.

A survey was distributed to physicists at brachytherapy clinics across Australia, New Zealand, the United Kingdom (UK), France, and Ireland, gathering details on HDR ¹⁹²Ir source model and origin, well chamber model, well chamber calibration provider, and each department's history on the discrepancy between locally measured and manufacturer's source certificate RAKR. Data from each clinic was assigned to a separate series, with an additional series if a clinic changed well chamber calibration provider. Linear regression analysis was performed on data from each series.

One thousand one hundred fifty observations from 32 clinics were analyzed, with RAKR measurement histories ranging from 2 to 20 years (mean, 9 years). The manufacturer of all sources included in this analysis was Curium. A trend of increasing local measurement value compared with the manufacturer certificate value was observed for all series, with 20 or more observations. Sites with well chambers calibrated at NPL or UWADCL showed an increase in discrepancy between measured and source certificate RAKR, approximately 0.2% per year on average, from 2015 to present. There is an increasing discrepancy between locally measured ¹⁹²Ir HDR source strength compared with the manufacturer's certificate. The manufacturer's measurement procedure does not meet the same standard set for the user. Unless the agreement between the Curium and user measurements can improve, brachytherapy societies may need to re-examine the requirement for direct comparison between local and manufacturer's measurements.