title: A dosimetric intercomparison of megavoltage photon beams in UK radiotherapy centres.
abstract: A dosimetry intercomparison has been carried out for all 64 radiotherapy centres in the UK. Doses were measured with an ionization chamber in an epoxy resin water-substitute phantom of relatively simple geometry. Reference-point measurements were made for all MV photon beams. For 61 Co-60 beams, a mean ratio of measured-to-stated dose of 1.002 was observed with a standard deviation of 0.014, whilst for 100 MV x-ray beams, the corresponding figures were 1.003 and 0.015. 97% of beams lay within a +/- 3% deviation. One measurement was instrumental in discovering a large discrepancy. Doses were also investigated in two planned three-field distributions at one beam quality in each centre. One of these was in a homogeneous phantom, whilst the second included a lung-equivalent insert. Doses were measured at the central point and at four other points in the high dose volume. In both situations, the mean ratio of measured-to-calculated doses for all points was 1.008, with standard deviations of 0.027 and 0.035 for the uniform and non-uniform phantoms, respectively. Discrepancies over 5% were followed up. The work must be viewed in the context of other international intercomparisons and is an essential part of wider radiotherapy audit processes.
authors.affiliation: Department of Medical Physics and Medical Engineering, University of Edinburg, UK.
authors.forename: D I
authors.initials: DI
authors.lastname: Thwaites
authors.forename: J R
authors.initials: JR
authors.lastname: Williams
authors.forename: E G
authors.initials: EG
authors.lastname: Aird
authors.forename: S C
authors.initials: SC
authors.lastname: Klevenhagen
authors.forename: P C
authors.initials: PC
authors.lastname: Williams
mesh_terms: D006801:Humans; D008961:Models, Structural; D011785:Quality Assurance, Health Care; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy / Q000592:standards*; D006113:United Kingdom