A dosimetric intercomparison of kilovoltage X-rays, megavoltage photons and electrons in the Republic of Ireland.

BACKGROUND AND PURPOSE A comprehensive dosimetry intercomparison has been carried out involving all the radiotherapy centres, all external beam modalities and every radiotherapy treatment unit in the Republic of Ireland. MATERIALS AND METHODS Reference point measurements were made for all megavoltage photon beams. Doses were also investigated in planned three-field distributions. One of these was in a homogeneous epoxy resin solid water phantom, whilst the second included a lung equivalent insert. The intercomparison was also carried out for three electron energies in each centre. The position of the depth of maximum dose for a standard field size was independently determined, as was the beam energy and a subsequent beam calibration was made. In addition, a kilovoltage X-ray intercomparison was carried out on every kilovoltage quality. RESULTS For 13 megavoltage photon beams a mean ratio of intercomparison measured dose to locally measured dose of 1.002 was obtained (standard deviation 1.2%). For 12 electron beam measurements a mean ratio of intercomparison measured dose to locally measured dose of 1.018 was obtained (standard deviation 0.8%). For four kilovoltage beams a mean ratio of intercomparison measured dose to locally measured dose of 0.997 was obtained (standard deviation 1.9%). CONCLUSIONS The intercomparison has given confidence in the basis of clinical delivery of radiation dose in radiotherapy treatment and in the consistency (precision) of dosimetry between different centres within the Republic of Ireland. In addition, it has established a methodology for subsequent ongoing routine radiotherapy dosimetry audit and a baseline set of results to act as an initial reference point.