ESTRO 2021 Abstract Book

S1467

ESTRO 2021

For each beam audit, the radiotherapy (RT) centre is required to irradiate two dosimeters in reference conditions with a D w value of 2 Gy. Audit results are firstly evaluated in terms of percentage differences between D w values stated by the RT centre and measured by the TL dosimeter, then the normalized error, E n [1], is determined, being passing results in the range [-1,1]. Results Reference output was checked for 41 beams across 20 RT centres enrolled on a voluntary basis. The photon beam qualities are reported in Fig. 1 with 6 MV and 6 MV FFF being the most frequent. The audit results do not show dependence on the beam quality. Fig. 2 represents a global view of the percentage difference between the dose values for all the irradiated dosimeters. 66% of points result within ±2% and nearly all the data are within ±3.5%. Considering that the combined standard uncertainty of D w values obtained by each TL dosimeter is typically 1.7% almost all the results lie within the expanded uncertainty (k=2) of the TLD measurements. On the basis of E n analysis, 74% of results are of optimal level with E n in the range [-0.5,0.5] and only one beam does not pass the audit. This unsatisfactory result is ascribed to an error in the dosimeter positioning during irradiation.

Figure 1: Photon beam qualities involved in the audit.

Figure 2: Audit results in terms of percentage differences between delivered and measured D w values.

Conclusion Reference dosimetry audits were successfully carried out in 20 RT centres. Next goal will be to increase the number of Italian RT centres involved in the audit and to develop procedures for non-reference conditions. References: [1] ISO/IEC 17043:2010 Conformity assessment – General requirements for proficiency testing. PO-1745 A pre-trial quality assurance for salvage radiotherapy of the multi-institutional PERYTON-trial F. Staal 1 , K. Ng Wei Siang 2 , C. Brouwer 2 , B. Vanneste 3 , M. de Jong 4 , T. Budiharto 5 , D. Haverkort 6 , M. van de Sande 7 , H. Langendijk 2 , R.J. Smeenk 8 , S. Aluwini 1 1 University of Groningen, University Medical Center Groningen , Department of Radiation Oncology, Groningen, The Netherlands; 2 University of Groningen, University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands; 3 GROW - School for Oncology and Developmental Biology, Department of Radiation Oncology , Maastricht, The Netherlands; 4 Radiotherapeutic Institute Friesland, Department of Radiation Oncology, Leeuwarden , The Netherlands; 5 Catharina Hospital, Department of Radiation Oncology, Eindhoven, The Netherlands; 6 Radiotherapiegroep , Department of Radiation Oncology, Arnhem, The Netherlands; 7 Institute Verbeeten, Department of Radiation Oncology, Tilburg, The Netherlands; 8 Radboud University Medical Center , Department of Radiation Oncology, Nijmegen, The