ESTRO 38 Abstract book

S933 ESTRO 38

criteria), including noise reduction (integrated to PRIMO) in order to reduce underestimation of GI due to statistical noise in reference dose. Results For all 4 PDD comparisons (2 beam sizes, 2 linacs), GI 1%/1mm is 99.7%-100%. With 3%/3mm criteria, all plans pass GI. 1 plan fails with DPM, but passes with PENELOPE. With GI 2%/2mm, 42/50 plans pass with DPM (6/8 failing plans are lung cases). Combining with PENELOPE recalculation, 45/50 plans pass GI. For 8 recalculated plans, mean GI is 90.5% and 95.2% for DPM and PENELOPE respectively. Average PTV median dose difference is +0.95% between PRIMO (DPM) and Acuros. Calculation time varies from 19 min to 2 h 17 min (DPM), with an observed correlation with both PTV volume and Acuros calculation time. Calculation time with PENELOPE is ~6-8 times longer. Conclusion PRIMO can perform systematic VMAT plan dose verification for TrueBeam, using Varian Phase Space files. This requires low setup effort. With enhancement regarding automation (PRIMO side and in-house developments), user time required is reduced to a few seconds. EP-1731 Using plane-parallel ionization chamber PPC40 to measure Dose Area Product Ratio D. Mateus 1,2 , G. Mora 3 , M.J. Cardoso 1,2 , A. Martins 1 , A. Rocha 1,2 , C. Greco 1 1 Fundação Champalimaud, Radiotherapy, Lisboa, Portugal ; 2 Mercurius Heath, Radiotherapy, Lisboa, Portugal ; 3 Faculty of Sciences- University of Lisbon, Institute of Biophysics and Biomedical Engineering, Lisboa, Portugal Purpose or Objective The introduction of Dose Area Product Ratio ( DAPR 20,10 ) as a new beam quality parameter for small field dosimetry to replace the traditional TPR 20,10 have been suggested by several authors. These studies have been investigated the characteristics of DAPR 20,10 for FF photon beam energies using large-area plane-parallel ionization chambers (IC), such as LACs, with diameter between 96.5 and 39.6mm. But LACs are not commonly used in the radiotherapy clinical centers and there is a lack of research about DAPR 20,10 for FFF photon beam energies. In the present work, it was investigated the feasibility of using the common plane-parallel ionization chamber PPC40 (diameter of 16mm) instead of LACs for the DAPR 20,10 measurements. In addition, it was studied the variation of DAPR 20,10 with field size, beam energy and the diameter of the active area of IC. Finally, it was calculated correction factors to compensate the observed dependence of DAPR 20,10 on the beam diameter of the IC (see Fig.1). Material and Methods Measurements of DAPR 20,10 and TPR 20,10 for photon beams emerging from the linear accelerator Edge , Varian, were performed using the PPC40 and PTW60019 microDiamond detector, respectively. Beam collimation included: 1) cones of different diameter (7.5mm, 10mm, 12.5mm, 15mm and 17.5mm) with jaws opening of 5x5 cm 2 ; and 2) square field size of 1x1 cm 2 . Several energies were studied: 6MV (FF and FFF) and 10MV (FFF). Results Our preliminar results indicate 1) using the PPC40, it is possible to properly measure DAP for the circular fields with diameter up to 12.5 mm and also the square field of 1x1cm 2 defined by the jaws; 2) differences between DAPR 20,10 and TPR 20,10 are around 2% (see Table 1); 3) DAPR 20,10 does not depend practically on field size (maximum difference of 0.5%); 4) DAPR 20,10 and TPR 20,10 have similar variation with energy (difference around 7% between the energies 6MV-FFF and 10MV-FFF).

Conclusion The characteristics of DAPR 20,10 determined for the circular fields up to 12.5 mm and square field of 1x1 cm 2 using the common clinical PPC40 are similar to those reported by previous studies which used LACs. We are conducting more research to validate our calculated correction factors by Monte Carlo method. EP-1732 The effect of different table top models on patient-specific QA L. Paelinck 1 , E. Wuyts 1 , B. Vanderstraeten 1 , C. De Wagter 1 , Y. Lievens 1 1 University Hospital Ghent, Radiotherapy, Gent, Belgium Purpose or Objective The purpose of this study was to investigate the effect of different table top models on the agreement between calculations and measurements on the Delta4 phantom (Scandidos) for different beam configurations. Also, 8 full arc prostate plans were measured and compared with calculations without the table model and with the three different table models included. Material and Methods Our Elekta linear accelerators are equipped with the iBeam evo carbon fiber table top. Three different models for this table were considered in our TPS RayStation 6 (RaySearch): a CT-based, a simple and an advanced geometric model. For the CT-based model the treatment table was removed from the linear accelerator and scanned with a Toshiba Aquilion LB CT simulator. Thereafter, the part of the CT-images where the table- pixels are located, were extracted and injected in the artificial CT-images of the Delta4 phantom with a home- made java program. The result of this operation is seen in fig 1a. The simple geometric model is represented by a slab with a density of 0.25 g/cm³ (fig 1b). The advanced geometric model is represented by a thin outer surface layer mimicking the table contour composed of carbon fiber (1.18 g/cm³) and an inner core composed of foam (0.055 g/cm³) (fig 1c). Different beam configurations of 3x3 cm², 5x5 cm² and 10x10 cm² at angles of 0°, 140°, 160° and 180° as well as a half arc of 180° crossing the table and a full arc of 360° for 6 and 15 MV were measured and calculated on the Delta4 phantom without and with the three table models included. The same was done for the 8 full arc prostate plans. All measurements were performed on the same day and a proper daily correction factor was applied. For the analysis of the prostate plans, a gamma criterion of 3%/3mm was used.