Proton dosimetry inter-comparison based on the ICRU report 59 protocol

Background and purpose: A new protocol for calibration of proton beams was established by the ICRU in report 59 on proton dosimetry. in this paper we report the results of an international proton dosimetry intercomparison, wh ich was held at Loma Linda University Medical Center. The goals of the inte rcomparison were, first, to estimate the level of consistency in absorbed d ose delivered to patients if proton beams at various clinics were calibrate d with the new ICRU protocol. and second, to evaluate the differences in ab sorbed dose determination due to differences in Co-60-based ionization cham ber calibration factors. Materials and methods: Eleven institutions participated in the intercompari son. Measurements were performed in a polystyrene phantom at a depth of 10. 27 cm water equivalent thickness in a 6-cm modulated proton beam with an ac celerator energy of 155 MeV and an incident energy of approximately 135 MeV . Most participants used ionization chambers calibrated in terms of exposur e or air kerma. Four ionization chambers had Co-60-based calibration in ter ms of absorbed dose-to-water. Two chambers were calibrated in a Co-60 beam at the NIST both in terms of air kerma and absorbed dose-to-water to provid e a comparison of ionization chambers with different calibrations. Results: The intercomparison showed that use of the ICRU report 59 protocol would result in absorbed doses being delivered to patients at their partic ipating institutions to within +/-0.9% (one standard deviation). The maximu m difference between doses determined by the participants was found to be 2 .9%. Differences between proton doses derived from the measurements with io nization chambers with N-K-, or N-W-calibration type depended on chamber ty pe. Conclusions: Using ionization chambers with Co-60 calibration factors trace able to standard laboratories and the ICRU report 59 protocol, a distributi on of stated proton absorbed dose is achieved with a difference less than 3 %. The ICRU protocol should be adopted for clinical proton beam calibration . A comparison of proton doses derived from measurements with different cha mbers indicate: that the difference in results cannot be explained only by differences in Co-60 calibration factors. (C) 1999 Elsevier Science ireland Ltd. All rights reserved.