ESTRO 36 Abstract Book

S498 ESTRO 36 2017 _______________________________________________________________________________________________

deliveries, slightly higher standard deviation was observed than IMRT.

likelihood of occurrence (O), severity of effect (S) and lack of detectability (D). Forty tests were examined just above the expected tolerance levels and indexes O, S, and D were scored from 1 (lowest risk) to 10 (highest risk) using two methods: 1) A survey was submitted to each of the medical physicists of our institute involved in the linac QC 2) The QC data over a period of three years were analyzed and some FMs were simulated with the treatment planning system. The average RPN for each test was obtained taking into account both the methods. For each linac, the tests were then sorted by their frequency (daily, monthly or annual) and RPN value.Two different Varian linacs (DBX, Unique) were considered, the first used only for conformal therapy and the second one used essentially with volumetric modulated arc therapy (VMAT) technique. Results A high variability was found in the O-D-S scores of the survey, as shown in the box plots of figure 1 for the dosimetric tests of the Unique linac. Nevertheless, a lower variability was obtained for RPNs, highlighting at the same time the more relevant tests.

Figure 1- Planar dose assessment of the centres for head and neck and post-prostatectomy plans: a) IMRT delivery, b) VMAT delivery

Figure 2- 3D dose assessment of the centres for head and neck and post-prostatectomy plans and: a) IMRT delivery, b) VMAT delivery Conclusion All linacs were equipped with EPIDs so a consistent detection system was used by the centres. The method was significantly less expensive and faster than conventional audits due to its remote nature and use of virtual phantoms. All measured data were analysable with relatively high pass rates. Interactive communications with centres was often necessary to ensure quality data were provided. PO-0908 Application of Failure Mode and Effects Analysis to linac quality controls: advantages and limits F. Bonfantini 1 , T. Giandini 1 , S. Meroni 1 , C. Stucchi 1 , M. Carrara 1 , V. Mongioj 1 , I. Veronese 2 , E. Pignoli 1 1 Fondazione IRCCS Istituto Nazionale dei Tumori, Medical Physics, Milan, Italy 2 Università degli studi, Physics, Milan, Italy Purpose or Objective The increased complexity of the modern linac-based radiotherapy requires more thorough quality assurance programs to reduce the risk of errors and ensure patient safety. However, these demands are cumbersome and the efforts should be optimized in order to take maximum advantage of the available resources. In this context, prospective methods for risk analysis, such as Failure Mode and Effects Analysis (FMEA), can be a useful tool. Aim of this work was to evaluate advantages and limits of the application of FMEA for the optimization of linac quality controls (QCs). Material and Methods Each parameter tested by the QC was considered as a potential failure mode (FM) and a Risk Priority Number (RPN) was calculated from the product of three indexes:

Both the FM simulations and the analysis of the QC trend allowed to reduce the subjectivity of the FMEA score. Integration of both evaluations provided the RPN-based ranking of tests: an example is shown in figure 2 for monthly tests for DBX and Unique linacs.