(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: NIST-traceable electrometer calibrations are performed using continuous current sources, however, clinical measurements are made in linear accelerator beams which produce a pulsed ionization current. This work investigates the accuracy of measurements made by four different digital electrometer models in pulsed medical accelerator beams.
Methods: Four digital and one analog electrometer were used to measure signal from two ionization chambers including a spherical chamber with an active volume of 30 cc and a Farmer-type chamber with an active volume of 0.6 cc. The spherical chamber was placed in air at source-to-detector distances of 100 cm and 135 cm. The Farmer chamber was placed at dmax in a Virtual Water ™ phantom at 100 cm SSD. A 6 MV Varian Clinac® 21EX was used to expose the ion chambers at repetition rates ranging from 100 MU/min to 600 MU/min. Charge measurements from the digital electrometers were compared to the readings from the analog electrometer for each chamber, since it is assumed that the analog electrometer is not affected adversely by a pulsed beam.
Results: All digital electrometers agreed with the analog electrometer when measuring signal from the Farmer-type chamber for all repetition rates. Three of the four digital electrometers agreed with the analog electrometer when measuring signal from the spherical chamber at all repetition rates. One digital electrometer made erroneous measurements that differed from the analog electrometer by up to 3.73% at high repetition rates, yet still being operated within its rated range.
Conclusion: The four digital electrometers investigated in this study all operated accurately when coupled to a Farmer-type ionization chamber. However, when using certain digital electrometers to measure large, pulsed signals, incorrect readings were observed, even when operated within the rated range of the electrometer.
Keywords
Dosimetry, Linear Accelerator, Ionization Chamber
Taxonomy
TH- Radiation dose measurement devices: General (most aspects)
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