A dosimetric intercomparison of electron beams in UK radiotherapy centres.

A dosimetry intercomparison has been carried out for all 52 radiotherapy centres in the UK which possess electron treatment facilities. The intercomparison was carried out on one treatment unit in each centre and for three energies across the range of available energies. The position of the depth of maximum dose for a standard field size was independently determined and a subsequent beam calibration made. The factor to convert the reading on a calibrated ionization chamber to absorbed dose in an electron beam is energy dependent, and hence to carry out an independent calibration measurement also requires the beam energy to be determined. In addition a quantitative measure of the difference in the calibration chains between the intercomparison equipment and the host department's field instrument was carried out. In order to provide a follow-up to the initial IPSM national photon intercomparison, a photon beam calibration was measured in one photon beam in each centre. For 156 electron beam measurements, a mean ratio of intercomparison measured dose to locally measured dose of 0.994 was obtained with a standard deviation of 1.8%. For the 52 photon beam measurements, a mean ratio of intercomparison measured dose to locally measured dose of 1.003 was obtained with a standard deviation of 1.0%.