A DOSIMETRIC INTERCOMPARISON OF ELECTRON-BEAMS IN UK RADIOTHERAPY CENTERS

A dosimetry intercomparison has been carried out for all 52 radiothera py centres in the UK which possess electron treatment facilities. The intercomparison was carried out on one treatment unit in each centre a nd for three energies across the range of available energies. The posi tion of the depth of maximum dose for a standard field size was indepe ndently 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 a n independent calibration measurement also requires the beam energy to be determined. In addition a quantitative measure of the difference i n the calibration chains between the intercomparison equipment and the host department's field instrument was carried out. In order to provi de 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 st andard deviation of 1.8%. For the 52 photon beam measurements, a mean ratio of intercomparison measured dose to locally measured dose of 1.0 03 was obtained with a standard deviation of 1.0%.