Assessment of a self-inspection method and reporting measured electrometer correction factors for reference-class electrometers in radiotherapy.

BACKGROUND AND PURPOSE: The standard dosimetry system of medical accelerators in radiotherapy consists of an ionization chamber, an electrometer, and cables. Guidance for TG-51 reference dosimetry reported that the electrometer correction factor (Pelec ) should be checked every few years. Therefore, continuous Pelec measurements have not been reported. The purpose of this study is to measure the Pelec with a charge generator at our institution and to evaluate variations over time. The measurements are compared with calibration data given by an Accredited Dosimetry Calibration Laboratory (ADCL). MATERIALS AND METHODS: We used four reference-class electrometers: RT521R (RTQM system/EMF Japan), Model 35040 (FLUKE), RAMTEC Duo (Toyo medic), and UNIDOS-E (PTW). Each electrometer was connected to the charge generator, and the required charge was applied. The measurement points used were the same as those used for calibration by the ADCL. From the measured charges at each point, the Pelec was obtained from the slope of the linear regression function. The measurements were repeated over a 3-month period to evaluate variations over time for each electrometer. Additionally, error budgets for the Pelec measurements were estimated, and the overall uncertainty was determined. RESULTS: The measured Pelec values were 1.0000, 0.9995, 1.0009/0.9999, and 0.9995/0.9998 for RT521R, Model 35040, the low/medium (L/M) ranges of RAMTEC Duo, and the L/M ranges of UNIDOS-E, respectively. The measured Pelec values agreed within 0.1% with those given by the ADCL. We found a small drift in the measurements for one electrometer. Additionally, the uncertainty considered was 0.26% for k = 2 (k, coverage factor). CONCLUSION: In this study, stable Pelec values were obtained for four electrometers using a charge generator over a three-month period. The measured Pelec values were within the overall uncertainty stated in the electrometer guidelines. However, performing periodic measurements for the Pelec was able to help in detecting small errors.