Room: Exhibit Hall
Purpose: To provide an evaluation of two different methods for quality assurance checks of ionization chambers used in clinical photon beams.
Methods: Measurements were performed with three ionization chamber models: A12, A28 and IC69. Each chamber was inserted individually into the â?¹â?°Sr check source, and repetitive 15-second charge readings were taken, reinserting the chamber between measurements. The chambers were also placed into custom inserts of a Constancy Check Phantom manufactured by Standard Imaging (Middleton, WI), and simultaneous charge readings were taken in a 6MV photon beam. Individual charge measurements and charge ratios were tracked over a five month period for both methods.
Results: Measurements taken in the linear accelerator showed less variation both over time and within a given trial than the â?¹â?°Sr measurements, as the standard deviations of the mean used to evaluate the spread of individual measurements and charge ratios were consistently lower for each chamber comparison. The â?¹â?°Sr measurements were also found to be more sensitive to positioning variations. The accelerator methods also had a greater efficiency, as time commitment for a series of measurements in the linear accelerator was 15-20 minutes shorter than measurements in the â?¹â?°Sr check source. The use of charge ratios to compare chambers for both methods would increase the probability of detecting a fault in a chamber, as ratios were less sensitive to daily variation in set up or source output.
Conclusion: Ratios of chamber measurements should be used to monitor chamber stability if possible. The use of linear accelerator methods resulted in lower measurement variability, providing a simultaneous QA check of the accelerator output. Overall, the importance of performing appropriate ionization chamber quality assurance checks with available equipment was highlighted in this work.
Funding Support, Disclosures, and Conflict of Interest: Larry DeWerd is an owner of Standard Imaging Inc., which manufactured the phantom purchased for this work to carry out linear accelerator measurements.