A technique for calibrating a high dose rate ¹⁹²Ir brachytherapy source

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Nuclear Engineering

Background:

• Accurate calibration of high dose rate brachytherapy sources, specifically Iridium-192 (¹⁹²Ir), is crucial for effective cancer treatment.
• Primary standards dosimetry laboratories may not possess calibration standards for all relevant radioisotope qualities, necessitating alternative calibration methodologies.
• The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) faces this challenge as it lacks a primary standard for ¹⁹²Ir quality.

Purpose of the Study:

• To determine the reference air kerma rate for an ¹⁹²Ir high dose rate brachytherapy source.
• To validate a dosimetry method for calibrating ¹⁹²Ir sources when a direct primary standard is unavailable.
• To compare the results obtained from the validated method with vendor-provided calibration data.

Main Methods:

• Calculated the air kerma calibration coefficient for an IBA FC65-G Farmer type ionisation chamber using coefficients derived from 300 kV and Cobalt-60 (⁶⁰Co) qualities, applying the Mainegra-Hing and Rogers methodology.
• Validated this approach through Monte Carlo simulations to determine the chamber's air kerma calibration coefficient specifically at ¹⁹²Ir quality.
• Measured the reference air kerma rate in air using the calibrated Farmer chamber, incorporating necessary correction factors, and compared it to the vendor's ¹⁹²Ir source calibration certificate.

Main Results:

• Monte Carlo simulations showed very good agreement with the calculated air kerma calibration coefficient using the indirect method.
• The measured reference air kerma rate, after applying correction factors, differed by less than 1% from the vendor's stated value for the ¹⁹²Ir source, with one exception.
• Excellent agreement between the study's methodology and vendor calibration data was achieved.

Conclusions:

• The indirect method for determining the air kerma calibration coefficient is valid and reliable for ¹⁹²Ir quality.
• The validated dosimetry methodology provides accurate reference air kerma rates for ¹⁹²Ir brachytherapy sources.
• This approach is suitable for clinical application, ensuring reliable source calibration in the absence of direct primary standards.