Characterisation of two new ionisation chamber types for use in reference electron dosimetry in the UK.

The IPEM Code of Practice (IPEM 2003) for electron dosimetry for radiotherapy beams recommends design requirements for parallel-plate ionisation chambers used to determine the absorbed dose to water in an electron beam. The Classic Markus design has been found not to meet these requirements. The Advanced Markus and the Exradin A10 ionisation chambers have been designed to rectify the problems associated with the Classic Markus ionisation chamber. The response of one Exradin A10 and three Advanced Markus ionisation chambers was investigated. Absorbed dose to water calibration factors were derived at the National Physical Laboratory (NPL) for each ionisation chamber at seven electron energies in the range 4 to 19 MeV. Investigations were carried out into chamber settling, polarity effects, ion recombination and the chamber perturbation. The response of the ionisation chambers in a clinical beam was also investigated. In general all three Advanced Markus ionisation chambers showed the same energy response. For the three Advanced Markus and one Exradin A10 ionisation chamber investigated the magnitude of the polarity effect was typically 5% at an energy of 4 MeV. There was discrepancy between the polarity measurements made at the NPL and in the clinic. Until further measurements are made the recommendation of this study is that these chambers are not suitable for use for reference dosimetry in electron beams.