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The purpose of this work is to present the results of a dosimetry intercomparison that has been performed between four radiotherapy clinics in the south east region of Sweden. A dosimetry intercomparison of absorbed dose to water under reference conditions is a method to investigate possible systematic errors in the procedure implemented at the clinic and includes all equipment used, applied correction factors, the measurement technique and the interpretation of the dosimetry protocol used. Dosimetry intercomparisions or audits can be performed on different levels: in reference conditions, non-reference conditions or end-to-end tests. The different audit types have different purposes and acceptance criteria. This intercomparison is a complement to external dosimetry audits based on mailed TLD or OSLD:s such as IROC Audits (Houston Quality Assurance Center) where agreement within 5% is considered a satisfactory check.

The dose determinations were all performed at Linköping University Hospital at three newly installed Varian TrueBeam accelerators (Varian, Paulo Alto, San Francisco, US), including both photon and electron beams. The results determined by the external physicists were compared to the results determined by the physicists in Linköping. The measurements took place two days in a row. All clinics used the code of practice IAEA TRS-398 for absorbed dose determination. In order to be as independent as possible, each clinic used their own equipment regarding ionization chambers, electrometer, phantom, cables, thermometer and barometer. All measurements were made in water. For photons the field size  was 10x10 cm2. Clinic 1 used SSD 100 cm and depth 10 cm, clinic 2 used SSD 90 cm and depth 10 cm and clinic 3 used SSD 95 cm and depth 5 cm. The electron measurements were made at zref, SSD 100 cm and applicator size 10x10 cm2 (clinic 1) or 15x15 cm2 (clinic 2 and 3).

The results of the dosimetry intercomparison are shown in Table 1 for photon and electron beams. The percentage difference is the difference between the specific clinics (numbered 1-3) compared to Linköping. For photons, the largest deviation was 0.8% and for electrons 1.1%.

Table 1: The difference in percent for each clinic compared to the host clinic

The comparison of absorbed dose determination under reference conditions shows good agreement between the included clinics, regardless of setup and equipment used. Participation in a dosimetry intercomparison improves the credibility of the established methods implemeted at the clinic and reduces the possible systematic errors in the measurements.