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

Area of Science:

• Medical Physics
• Radiation Oncology
• Nuclear Medicine

Background:

• Discrepancies in reference air kerma rate (RAKR) for high-dose-rate (HDR) iridium-192 (¹⁹²Ir) sources have been observed.
• This issue was initially noted at a single Australian hospital, raising questions about its prevalence.

Purpose of the Study:

• To investigate if the observed RAKR discrepancy is a localized event or a more widespread problem.
• To assess the trend of RAKR measurements from ¹⁹²Ir HDR brachytherapy sources over time.

Main Methods:

• A survey was distributed to brachytherapy physicists in Australia, New Zealand, the UK, France, and Ireland.
• Data collected included HDR ¹⁹²Ir source details, well chamber models, calibration providers, and historical RAKR measurement discrepancies.
• Linear regression analysis was applied to data from 32 clinics, encompassing 1150 observations over 2 to 20 years.

Main Results:

• A consistent trend of increasing local RAKR measurements compared to manufacturer certificate values was observed across all analyzed series.
• Clinics using well chambers calibrated at NPL or UWADCL showed an average annual discrepancy increase of approximately 0.2% from 2015 onwards.
• All analyzed ¹⁹²Ir HDR sources were manufactured by Curium.

Conclusions:

• There is an increasing discrepancy between locally measured and manufacturer-certified HDR ¹⁹²Ir source strengths.
• The manufacturer's measurement procedures may not align with user standards, necessitating a review.
• Brachytherapy societies may need to re-evaluate the requirement for direct comparison between local and manufacturer measurements if agreement does not improve.