ESTRO 37 Abstract book

S962

ESTRO 37

beams 1, 2 . The focus of this work was to characterise WE- EPID response under flattening filter free (FFF) beams and implement a new model for transit dosimetry using a conventional treatment planning system (TPS). Material and Methods A standard amorphous silicon (a-Si) EPID was modified to a WE-EPID 2 configuration by replacing the metal- plate/phosphor screen situated above the photodiode detector with a 3 x 40 x 40 cm 3 solid water slab. The dose response linearity of the WE-EPID was evaluated using the integrated pixel value per monitor unit (MU) over 5–1000 MU at three different dose rates (nominal, 1000 MU/min and 550 MU/min). A clinical TPS was used to calculate dose to the WE-EPID in its conventional EPID position behind the phantom/patient, with the ‘’extended phantom’’ concept 3 enabling dose calculation at the EPID position. The accuracy of TPS dose calculations at the EPID plane in transit geometry was first evaluated for different field sizes and distance from the beam axis by comparison with dose measured using a 2D ion-chamber array (ICA) and then the WE-EPID. Following basic dose response tests, clinical volumetric modulated arc fields (VMAT) with gantry collapsed to zero were measured. The EPID images were corrected for dark signal and pixel sensitivity 4 and converted to dose using a single dose calibration factor. The 2D dose evaluation was conducted using 3%/3 mm gamma index criteria. All the experiments were conducted with 10MV FFF beam at 150 cm source to detector distance. Results The pixel dose response down to 5 MU was within 2 % of the response at 1000 MU. The WE- EPID agreed with ICA and TPS to within 2 % for field size and off-axis response with open fields in the transit configuration (figure 1). The WE-EPID does not exhibit dose rate response. The average percentage Gamma pass rates for the absolute dose images of the WE-EPID and ICA for all VMAT fields were > 96%(3%/3mm criteria) respectively. The TPS calculated absolute dose for the same VMAT fields also show similar gamma pass rates. Conclusion The accuracy of transit dose measurements with the WE- EPID design was confirmed by close agreement with reference ICA measurements. The advantage of having a WE-EPID for FFF beams is that it does not saturate, unlike conventional EPIDs for high dose rate FFF beams. This study also demonstrates the feasibility of incorporation of the WE-EPID into a commercial TPS for in vivo dosimetry of FFF photon beams. Reference 1.Vial et al Med Phys 35 , 4362-74 (2008) 2.Deshpande et al Med Phys 42, 1753-64 (2015) 3. McNutt et al Med Phys 23 , 1381-92 (1996) 4.Greer Med Phys 32 , 3558-68 (2005) EP-1792 A non-measuring way to compare pre- treatment QA devices and set gamma analysis parameters. M. Gizynska 1 , D. Blatkiewicz 1 , M. Bukat 1 , M. Gil- Ulkowska 1 , S. Maluszczak 1 , A. Paciorkiewicz 1 , D. Szałkowski 1 , A. Walewska 1 1 The Maria Sklodowska-Curie Memorial Cancer Center, Medical Physics Department, Warsaw, Poland Purpose or Objective The IMRT technique is widely used in radiotherapy of many cancer sites. Pre-treatment verification of such plans is very important and in some countries even obligatory. The pre-treatment verification is done to test the machine performance in order to detect errors that would be clinically relevant. Rangel et al. (2010) and Steers et al. (2016) showed the methodology of introducing known errors into the RT plans in order to select gamma criteria on the basis of clinical relevance. The weak point of their studies was unknown

treatment plans of the patients treated within the Fist- in-Man (FIM) study. Material and Methods Patients with painful lumbar spine bone metastases were treated within the FIM study. The vertebrae and the spinal cord are delineated using the pre-treatment CT and MRI. For the actual treatment the delineations were propagated to the online MRI, also a pseudo CT was created using deformable registration. Both were used for on-line treatment planning. Step-and-shoot IMRT plans were generated using the Monaco planning system (version 5.19, Elekta AB, Sweden) which includes the effects of the magnetic field on the dose distributions. The beam model was commissioned for the 7 MV MRI Linac. All plans were automatically created using the same template, which includes 3 or 5 beams depending on the location of the vertebrae. The prescribed dose to the PTV was 8 Gy in a single fraction. The QA result presented are for: 1) test plans created prior to clinical introduction using patient data of 20 existing patients and 2) pre-treatment and online generated plans of the 4 patients in the FIM study. All plans were recalculated on a polystyrene slab phantom (30x30x10 cm3) and on the ArcCheck-MR phantom (Sun Nuclear GmbH, Germany). The ionization chamber measurements were performed at the center of the slab phantom using an IBA CC04 ionization chamber (IBA dosimetry GmbH, Germany). Film measurements were also performed by placing radiochromic EBT3 films in the center of the phantom (isoc) in the coronal plane. To evaluate the absolute dose distribution, the film was normalized to the ionization chamber dose at isoc. For both the film and ArcCheck dose distributions, a gamma analysis was performed using a 3% and 3 mm criteria. Results The QA results of the test plans and the plans created for the FIM study are very similar as shown in the table. The average difference between the calculated dose and dose measured with the ionization chamber was small (< 0.5%). The gamma analyses of the film measurements also showed that the measured absolute dose is in good agreement with calculated dose distribution with gamma pass rates better than 98 %. The gamma analysis of the measured dose obtained with the ArcCheck-MR also resulted in high gamma pass rates (>98 %). Conclusion The results of the QA test performed with various detectors and phantoms for the IMRT plans generated for the FIM plans show that the calculated dose and the measured dose distribution are in good agreement. EP-1791 Evaluation of a water equivalent EPID model for flattening filter free (FFF) beam transit dosimetry S. Deshpande 1,2,3 , S. Blake 3,4 , L. Holloway 1,2,3,4 , P. Vial 1,3,4 1 Liverpool and Macarthur Cancer Therapy Centre, Department of Medical Physics, Sydney, Australia 2 University of New South Wales, South Western Sydney Clinical School, Sydney, Australia 3 Ingham Institute, Applied medical research, Sydney, Australia 4 University of Sydney, Institute of Medical Physics- School of Physics, Sydney, Australia Purpose or Objective Our research group has developed and benchmarked water-equivalent electronic portal imaging devices (WE- EPID) for dosimetry of conventional flattening filter (FF)