The influence of the IAEA standard holder on dose evaluated from TLD samples.

The EROPAQ quality assurance project started in 1994 with TLD postal dose intercomparisons for photon beams used in 47 radiotherapy centres in the middle and eastern European countries. The photon dose intercomparisons include beam output checks and beam quality checks. Since an acceptance limit of +/- 3% was chosen for the EROPAQ intercomparisons, any systematic error in dose evaluation by the measuring centre should be minimized. The standard IAEA TLD holder is used in the intercomparisons. In this work the magnitude of the holder attenuation is evaluated and holder corrections derived both for output and photon beam quality checks for beam qualities ranging from Co-60 up to 18 MV. The correction of the dose at the depth of reference does not exceed 1% and decreases with increasing beam energy. A correction of the beam quality ratio of the order of 1% and that is independent of the photon energy has been determined by both calculation and measurements. These corrections are consistent with the preliminary data published by the IAEA and EC QA network.