Issues encountered with kilovoltage x-ray reference dosimetry when changing codes of practice from TRS 277 to TRS 398

A difference of 12% has been observed in the output of an 80 kV, 2.2 mm A1 HVL x-ray beam when comparing measurements made according to the TRS 398 medium energy protocol with measurements made according to the TRS 277 low energy protocol. Absorbed dose to water chamber calibration factors used for the TRS 398 measurements were derived from standards of air kerma with the use of the TRS 277 medium energy protocol, and given this fact the discrepancy observed can be considered in terms of a difference between the TRS 277 low and medium energy protocols for this beam. Repeat measurements using the TRS 277 low and medium energy protocols have been made to confirm this. The most likely origins for the discrepancy observed are the chamber perturbation correction, p(u), obtained from TRS 277, and the value of the measured percentage depth dose at a depth of 2 g.cm(-2) for this beam. Given these findings, reference dosimetry for this beam will be performed according to the TRS 398 low energy protocol.