Performance of a device design for the measurement of matched linac beam output

Abstract

The TG-51 protocol recommends using a water phantom at least annually for clinical reference dosimetry purposes. As routine monthly output checks of linear accelerator (linac) beam absorbed dose measurement in water is time consuming, a more efficient and formal robust system for the measurement of output in solid water using a parallel plate ion chamber has been developed. The purpose of this study is to confirm the stability and suitability of the system for routine output consistency measurement across multiple matched machines. This system consists of a modified Capintec parallel plate ion chamber with Fluke electrometer and 5 pieces of 30×30 cm2 solid water. The long term stability of this system was determined by monitoring a Co-60 beam over a period of one year. Calibration factors for this system were derived on matched Varian and Elekta machine separately for 6, 10 and 18 MV photon and 6, 9, 12, 16 and 20 MeV electron beams based on the beam output measured using the TG-51 protocol. The system is corrected for environmental condition and reproducible within 0.3%. Calibration factors varied between 0.0% and 0.7% across nominally matched beams with an average variation of 0.35% for a given beam quality. System experience over a year found an average variation of 0.2% between the outputs measured using this system and that measured in water using TG-51 protocol. Measurement results confirm that this is a stable, and efficient system of the measurement of linac output as well as the cross checks of the matched beam output. This system was used extensively in the clinic to cross check calibration factor of the daily QA device, and also served as a backup system for the daily output checking device.KeywordsMatched beamoutput factorParallel plate ion chamber