ESTRO 36 Abstract Book

S502 ESTRO 36 _______________________________________________________________________________________________

Conclusion These preliminary data show that fractal and lacunarity analysis may be able to characterise areas of restricted diffusion and non-restrictive diffusion on ADC images. Restrictive diffusion often indicates areas of aggressive prostate tumour. This method could be used in future studies to investigate other MR sequence images where the visual difference between prostate tumour and normal tissue is not so obvious to the naked eye, or where simple analysis of multiparametric data fails to adequately characterise tumour biology. Poster: Physics track: Implementation of new technology, techniques, clinical protocols or trials (including QA &audit) PO-0907 Remote auditing of IMRT/VMAT deliveries N. Miri 1 , K. Legge 2 , J. Lehmann 3 , P. Vial 4 , B. Zwan 5 , P. Greer 6 1 University of Newcastle, School of Mathematical and Physical Sciences, Newcastle- NSW, Australia 2 University of Newcastle, School of Mathematical and Physical Sciences, Newcastle, Australia 3 Calvary Mater Newcastle Hospital, Radiation and Oncology, Newcastle, Australia 4 Liverpool and Macarthur Cancer Therapy Centres, Department of Medical Physics, Sydney, Australia 5 Gosford Hospital, Central Coast Cancer Centre, Gosford, Australia 6 Calvary Mater Newcastle Hospital, Radiation and Oncology departement, Newcastle, Australia Purpose or Objective Purpose: To perform a novel study on remote auditing of dose deliveries of VMAT/IMRT clinical trials of different radiotherapy centres. The assessment is undertaken using EPID images from the centres and a local ‘signal to dose’ conversion model. Material and Methods Methods: The assessment included IMRT deliveries from 12 centres and VMAT deliveries from 6 centres. The centres downloaded benchmarking CT data sets and instructions to produce IMRT/VMAT trial plans, a head and neck (H&N) and post-prostatectomy (P-P) plan. Two virtual phantom data sets were provided for a flat and a cylindrical phantom. Trial plans were transferred to the phantoms; individual field/arcs at gantry zero on the flat phantom and the trial plan at actual gantry angles to the cylindrical phantom. EPID images acquired from a calibration plan were used to align and calibrate the EPID systems and model/correct EPID-linac sag. Integrated images were acquired for IMRT fields and cine images for VMAT arcs each cine image encompassing approximately 5 degrees. For 2D and 3D analysis, the images were converted to dose inside respectively the virtual flat and cylindrical phantom. The dose conversion was performed using an established model. To assess the delivered doses, the modelled dose was compared with corresponding TPS dose using the gamma function with all doses greater than 10% of the global maximum dose assessed. Results At 3%/3mm, 2D analysis of the H&N plan resulted in 99.6% (SD: 0.1) and 99.1% (SD: 0.1) mean pass rates for respectively IMRT and VMAT deliveries. Similarly, the P-P plan analysis resulted in 99.7% (SD: 0.2) and 99.6% (SD: 0.3) mean pass rates for corresponding deliveries over the centres. 3D analysis, on the other hand, resulted in slightly lower pass rates. H&N deliveries resulted in 98.3% (SD: 0.2) and 96.4% (SD: 2.6) mean pass rates. The P-P plan assessment resulted in 98.3% (SD: 1.5) and 97.2% (SD: 1.3) mean pass rates. Using a more stringent criteria, 3%/2mm, the H&N analysis resulted in 92.2% (SD:1.9) and 93.3% (5.4) mean pass rates and the P-P plan resulted in 94.0% (SD:4.3) and 95.6% (SD: 1.8) mean pass rates for

respectively IMRT and VMAT deliveries. For VMAT deliveries, slightly higher standard deviation was observed than IMRT.

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