Calibration of 192Ir high-dose-rate afterloading systems

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiological Calibration

Background:

• Accurate calibration of high-dose-rate (HDR) brachytherapy systems is crucial for effective cancer treatment.
• The National Institute of Standards and Technology (NIST) does not provide direct calibration for ionization chambers using the iridium-192 (192Ir) gamma-ray spectrum.
• Existing calibration methods for 192Ir HDR brachytherapy afterloading systems require precise procedures due to the unique spectral characteristics of 192Ir.

Purpose of the Study:

• To develop and describe a reliable method for calibrating 192Ir HDR brachytherapy afterloading systems.
• To establish a procedure for accurate dosimetry in HDR brachytherapy utilizing 192Ir sources.
• To introduce a novel well-type ionization chamber designed for routine use and calibration verification of 192Ir sources.

Main Methods:

• An interpolation procedure was employed using calibrations with cesium-137 (137Cs) and x-rays to determine the effective energy for iridium-192 (192Ir).
• A consistent wall + cap thickness of 0.3 g/cm² was utilized for calibrations and 192Ir measurements to ensure charged particle equilibrium.
• Corrections for source-chamber distance and room scatter were applied during inverse-square-law geometry calibration. A new well-type ionization chamber was designed and utilized.

Main Results:

• The interpolation method provides a viable alternative for calibrating ionization chambers with the 192Ir spectrum, compensating for the lack of direct NIST calibration.
• The recommended 0.3 g/cm² wall + cap thickness ensures accurate measurements by establishing charged particle equilibrium and excluding secondary electrons.
• The newly designed well-type ionization chamber facilitates convenient routine use, decay rate verification, and calibration of replacement 192Ir sources.

Conclusions:

• The described interpolation method and the use of a specific wall + cap thickness enable accurate calibration of 192Ir HDR brachytherapy systems.
• The new well-type ionization chamber simplifies the verification of source decay rates and the calibration of new 192Ir sources, enhancing clinical workflow.
• This approach ensures reliable dosimetry for 192Ir HDR brachytherapy, contributing to improved patient safety and treatment efficacy.