Evaluation of an in-situ procedure for calibration of well-type brachytherapy chambers in hospitals

Brachytherapy is a specialized form of radiation therapy in which small encapsulated radiation sources are placed directly within or near the target area, with known ability to deliver a high radiation dose to the tumor tissue while minimizing exposure to surrounding healthy tissues. Dosimetry protocols for photonemitting sources in brachytherapy typically rely on reference air kerma rate (RAKR) or air kerma strength (AKS) standards, along with transfer instruments. A well-type ionization chamber, or re-entrant well-type chamber, is the recommended method for measuring the strength of primary brachytherapy sources. In Serbia, no calibration services for brachytherapy ionization chambers were previously available. To address this, the Secondary Standards Dosimetry Laboratory (SSDL) at the Vinca Institute of Nuclear Sciences has initiated the establishment of a calibration service for high dose rate (HDR) brachytherapy sources using a reference well-type chamber, Standard Imaging HDR-1000 Plus, coupled with a PTW Unidos Webline electrometer. The chamber’s 20 mm thick aluminum outer wall helps reduce the influence of scattered radiation. A unique source holder is provided for each radiation source to ensure reproducible geometry within the chamber well. The calibration of the system can be done either as a whole (system calibration) or separately (component calibration). In system calibration, both the well-type chamber and the electrometer are calibrated together, while in component calibration the chamber and electrometer are individually calibrated and the overall calibration is derived from the two separate calibrations. Due to the lack of a radiation source generator for brachytherapy at the SSDL, calibrations are performed on-site, using sources and dose delivery systems that are available. The calibration procedure for high-dose-rate brachytherapy (HDRBT) is conducted using the substitution method, in which the user’s well-type ionization chamber is calibrated against the reference HDR-1000 Plus chamber. The calibration process includes several steps: prior to measurements the ionization chambers need to be in thermal equilibrium with the ambient conditions, conducting a dummy source test, pre-irradiating the well-type chamber, measuring the first leakage, determining the “sweet spot” of the chamber, recording reference and measured values, measuring the second leakage, and applying corrections to the measured values. If the ionization chamber is used with different holders or catheters, calibration must be performed for each configuration, and it is crucial to specify the conditions under which the calibration coefficient was determined. In accordance with the guidelines outlined in the IAEA document “Calibration of Photon and Beta Ray Sources Used in Brachytherapy (IAEA TRS 492:2023)”, well-type ionization chambers should undergo periodic stability checks to ensure consistent and reliable performance. These checks are an essential part of maintaining the accuracy of brachytherapy dosimetry and should be conducted at regular intervals to adhere to established quality assurance standards.