Evaluation of ionization chamber stability checks using various sources.

Purpose: It is important to check stability of ionization chambers in between regular calibration cycles. Stability checks can include individual Co irradiations, use of a beta-emitting check source, or redundant measurements in megavoltage photon beams. While Co 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 Sr check sources and linear accelerator beams which are both commonly found in the clinical setting, and compare these sources to Co.

Methods: Measurements were made with a Sr 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 Co with three Farmer-type chambers: the NEL 2571, PTW N30001G, and Exradin A12.

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

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