Clinical impact of dosimetry quality assurance programmes assessed by radiobiological modelling of data from the thermoluminescent dosimetry study ofthe European Organization for Research and Treatment of Cancer

The European Organization for Research and Treatment of Cancer (EORTC) Radi otherapy Group initiated its mailed thermoluminescence dosimetry (TLD) prog ramme in 1986. The aim of the present study was to evaluate the clinical re levance of variations in beam output detected in the period 1993 to 1996. A total of 140 beam outputs were checked (26 for cobalt-60 units and 114 for linear accelerators) in 35 centres. Clinical dose-response data for tumour control and normal tissue morbidity were used to assess the variation in c linical outcome resulting from variability in beam output. For 75 checked b eams with nominal accelerating potentials (n.a.p.) of 6 MV or less the mean ratio, +/- standard deviation (S.D.) of measured to stated output was 1.00 4 +/- 0.020. For 65 beams with n.a.p. of 8 MV or more, the ratio was 1.009 +/- 0.021. Even with this relatively high level of precision, broad distrib utions of estimated tumour control or normal tissue morbidity were found. I n the 10% of the beams with the most pronounced underdosage, the loss in tu mour control probability was estimated at 7-8 percentage points. Likewise, in the 10% of the beams with the most pronounced overdosage, the increase i n mild/moderate morbidity was 19-22 percentage points. For severe morbidity the same beams raised the estimated incidence of severe complications from 5% to 9-10%. An estimation of the loss of uncomplicated cure probability w as about 1% for both high and low energy beams. Sequential mailings conside rably improved the uniformity of clinical outcome. We conclude that small d eviations in beam output may lead to clinically important variations in out come. Substantial reductions in the variation between measured and stated o utput can be achieved by sequential mailings. Mailed TLD checks should be a n integral part of a continuously ongoing quality assurance activity in rad iotherapy. (C) 2000 Elsevier Science Ltd. All rights reserved.