Long-term response stability of a well-type ionization chamber and its implication on RAKR measurement for...

Crucial step in brachytherapy is the correct characterization of the radioactive source. Regular quality assurance checks of the well-type ionization chamber used for this purpose are therefore mandatory. This study evaluates the IAEA-recommended method for assessing long-term stability of well-type ionization chambers using an external reference beam across 16 Italian hospitals. Experimental data were collected under a 6 MV linac beam (5x5cm²field, 200 MU) using the reference chamber for external beam radiotherapy (EBRT) and the reference well-type ionization chamber for brachytherapy (BRT). The BRT chamber was positioned at maximum distance from the radiation source, while the EBRT chamber was positioned in a plastic phantom at SSD 100 cm and 5 cm depth. Reading ratios remained within 0.5% of the internal average for nearly all hospitals, highlighting the long-term stability of the well-type chamber and the high reliability of the external-source check method. As secondary objective, reference performance levels were established by comparing Reference Air Kerma Rate (RAKR) measurements obtained with BRT chambers-collected by 15 centers over approximately 20 years-against the corresponding calibration certificate values. A total of 417 relative differences were analyzed, all falling within ±2.3% of mean value. Data from this multicentric study indicate that using an external reference beam to assess the stability of the well-type ionization chamber is an easy, robust, and efficient method accessible to all the brachytherapy centers, for routine checks or when RAKR measurements differ from calibration certificate value by more than 2.5%, which represents the proposed tolerance level from our results.