Quality assurance network in central Europe. External audit on output calibration for photon beams.

The EROPAQ project for TLD monitoring of photon beams started in June 1994 with the set-up of the TLD system: calibration, reading and evaluation procedures. The acceptance level of +/- 3% was set for the TLD intercomparisons. The policy of the project was to check all beams in 47 participating radiotherapy centres and to recheck all the beams in those centres, where a deviation exceeding +/- 3% occurred in one or more of the beams. Out of 129 beams checked, 100 beams (78%) were found within the +/- 3% limit. Eleven beams show deviations larger than +/- 6%, and immediate corrective action was undertaken. Out of 47 centres checked, 22 did not participate in any external audit in a preceding 5 years. In these centres 68% (34/50) of the total number of gamma and x-ray beams checked but only 59% (20/34) of gamma beams were within the acceptance level, while in the 25 centres, which participated in an external audit before, these figures were 84% (66/79) and 88% (35/40) respectively. The sources of discrepancies were thoroughly investigated, discussed with the participants and the errors corrected. Poor results were in several cases associated with very old design of radiotherapy units and old dosimetry systems, equipped with inadequate ionization chambers. In several centres, an insufficient training of the physicists in clinical dosimetry was observed. Thanks to the corrective action, a great improvement of calibration of the beams was achieved. Standard deviation of the distribution of the results for all x and gamma beams checked decreased from SD = 7.4% at the first check to SD = 2.5% at the second check.