Evaluation of ionization chamber stability checks using various sources

It is important to check stability of ionization chambers in between regular calibration cycles. Stability checks can include individual ⁶⁰Co irradiations, use of a beta-emitting check source, or redundant measurements in megavoltage photon beams. While ⁶⁰Co irradiators are considered stable, they are rarely found in the clinical setting. Thus, this study seeks to compare the precision and efficiency in monitoring chamber stability using ⁹⁰Sr check sources and linear accelerator beams which are both commonly found in the clinical setting, and compare these sources to ⁶⁰Co.

Measurements were made with a ⁹⁰Sr beta-emitting check source and a 6 MV photon beam using a Constancy Check Phantom with three custom inserts to hold the ionization chambers. A comparison of both methods was performed with an Exradin A28 scanning chamber, Wellhofer IC69 Farmer-type chamber, and Exradin A12 Farmer-type chamber. Chamber stability was evaluated with individual charge readings and charge ratios among the three chambers. Results were compared to measurements taken in ⁶⁰Co with three Farmer-type chambers: the NEL 2571, PTW N30001G, and Exradin A12.

Stability of individual charge reading was found to be within ±1.0% for ⁹⁰Sr source measurements and ±0.5% for external beam measurements, including the ⁶⁰Co comparison. Additionally, the standard deviation of the mean charge ratios ranged from 0.15% to 0.40% for ⁹⁰Sr measurements and from 0.10% to 0.30% for the external beam measurements.

This work provides a comparison of techniques used to assess stability of ionization chambers in order to better inform the clinical physicist.