A regional audit of kilovoltage X-rays--a single centre approach

A dosimetric comparison between therapeutic kilovoltage (kV) equipment in the South West of the UK was carried out. The aim was to detect dosimetric errors and examine consistency across the region. This work also investigated the use of a single ionization chamber to minimize errors introduced through multicentre auditing and assessed the potential impact of the "Addendum to the IPEMB Code of Practice" for the determination of absorbed dose for X-rays below 300 kV. Comparisons were carried out at low and medium energies, with additional measurements made "in-air" at medium energy in accordance with the addendum. The measurement of output with a non-standard applicator was also assessed by calculating the ratio of the two outputs or applicator factor. The mean ratio of visitor's measured dose to that measured locally was 0.994 (standard deviation, 0.006) for nine low-energy beams and 0.991 (standard deviation, 0.011) for four medium-energy beams. The mean ratio of visitor's measured dose in-air and in-phantom at medium energy was 0.981 (standard deviation, 0.024). Applicator factors were within +/-4% for low and medium energies. In conclusion, the use of a single ionization chamber and single centre to carry out the audit gives a smaller deviation between measured values, although there are both advantages and disadvantages to the method. There are differences between doses calculated from the in-air and in-phantom protocols for medium energy, although these could be within quoted uncertainties.