NPL REPORT CIRM 20 Trial calibrations of therapy level electron beam ionisation chambers in terms of absorbed dose to water

Over several years NPL has been developing a calibration service for electron beam radiotherapy. To test this service, a trial calibration of therapy level electron beam ionisation chambers was carried out in 1996, involving fourteen radiotherapy centres supplying a total of eighteen chambers and twelve electrometers. All chambers were either of the NACP 02 (Scanditronix) or Markus (PTW) type. A second trial was carried out in 1997. This second trial involved nine radiotherapy centres supplying a total of seventeen chambers of the NE2571 Farmer, NACP 02 (Scanditronix), Markus (PTW) and Roos (PTW) types. Measurements were carried out at seven energies in the range 4-19 MeV together with an investigation of ion recombination. Electrometers were also calibrated. The instruments were returned to the radiotherapy centres for measurements to be carried out comparing the NPL direct calibration with the 1996 IPEMB air kerma based Code of Practice. The trials indicated that not all chambers of the same type exhibit the same behaviour. The results of the comparison between the NPL calibration and IPEMB Code are somewhat unexpected. The NACP and Farmer chambers show agreement between the two methods at the ±1% level but there is a significant difference for the Markus chambers of around 2%, with a higher dose according to the NPL calibration. There is no obvious reason for this discrepancy and further measurements are required to identify the source of the problem.