ESTRO 36 Abstract Book

S797 ESTRO 36 2017 _______________________________________________________________________________________________

and 10% in relative deviations for five key energies within the treatment area. As an example, FWHM in function of the air gap for three key energies are reported in Figure 1. Deviations observed are presented in Figure 2. Agreement achieved in terms of ranges in water is within 0.1 mm in absolute deviation for all the energies We extended a preliminary beam model based on a first predictions at nozzle entrance. The final beam model describes spot sizes within clinical tolerances of 1 mm/10%, for the treatment area considered. Detailed validation of this MC beam model is on-going and is based on beam scattering of the core pencil beam, transverse dose profiles in the low dose region (nuclear halo), absolute dose in reference conditions, evaluation of the delivery of 3D cubes (depth-dose and transverse profiles). Special emphasis will be given to non-isocentric set-up, including the use of range shifters . considered. Conclusion

treatment were enrolled in this study. Patient plan was delivered after correcting set up error and transmitted images were acquired by the EPID aS 1200 during the treatment. The transmitted dose images were acquired by EPID after the beam passes through patient. Images were acquired in continuous mode at source to imager distance SID = 150cm on the 1,2,3,5,10,15,20,25 fraction number. Before measuring transmitted dose images cone beam CT was performed to eliminate any set up error. Day one transmitted dose images were defined as base line images. On an average 8 images were acquired during treatment for each patient. These images were compared with base line image. Gamma index evaluation was performed with 1mm and 1% parameter using Varian portal dosimetry software. Results For the first five images i.e. up to tenth fraction we got average gamma index passing 98.3% which is within action level threshold of 97%. Depending upon the site of treatment we observed gamma passing percentage varies during fag end of treatment Conclusion Dosimetric measurement during treatment is good tool to investigate error during the treatment. Portal vision is mostly used for patient set up and pre treatment QA of patient. We found that portal dosimetry is useful tool for checking consistency of treatment delivery and monitoring changes in patient contours. EP-1506 Temperature dependent dose readout of Gafchromic EBT3 and EBT-XD film and clinical relevance in SRT K. Buchauer 1 , L. Plasswilm 1 , J. Schiefer 1 1 Kantonsspital St. Gallen, Departement of Radiation Oncology, St Gallen, Switzerland Purpose or Objective Modern radiation therapy modalities regularly produce SRT/SRS/SBRT plans with highly irregular and steep dose gradient distributions consisting of many small beam apertures. Accurate verification of such complex treatment fields is still challenging and Gafchromic EBT3 and EBT-XD films play a key role as dosimeter with the highest spatial resolution. Purpose of this work is to evaluate whether well-known temperature dependences of former Gafchromic film media are present with EBT3 and EBT-XD film. The observed systematic patterns of temperature dependence are characterized with respect to relevance in the pre-treatment verification. Material and Methods An Epson V750 pro flatbed scanner was used to perform scan studies with 125 consecutive scans to purposely warm up the scanner bed. During all scans two temperatures probes were used to measure an average scanner bed temperature. Square film pieces with irradiation dose form 0 Gy to 64 Gy of 1 cm size were placed in the central axis of the scanner bed. Evaluation was performed with the software 'Image J” in all three colour channels in 8100 measurements in total. Results Temperature dependent relative transmission (%T) readout differences known from former type Gafchromic film media are found to still be present with EBT3 and EBT- XD film type. Higher temperature results in most cases in darker film readout. Interestingly, EBT3 red colour channel changes temperature dependence direction around 16 Gy irradiation dose, meaning that a higher temperature results in less dose readout. Figure 1 and Figure 2 illustrate the relation between the temperature dependent transmission error and irradiation dose for EBT3 and EBT-XD respectively.

EP-1505 Use of Portal dosimetry to monitor treatment consistency throughout the course of treatment S. Deshpande 1 , A. Sutar 1 , S. Naidu 1 , M. Vikram 1 , V. Anand 1 , R. Bajpai 1 , V. Kannan 1 1 P.D. Hinduja National Hospital, oncology, Mumbai, India Purpose or Objective Use of portal dosimetry software to check treatment delivery consistency and to monitor changes in patient anatomy during course of treatment. Material and Methods Varian portal dosimetry software and Electronic Portal Imaging Device (EPID) aS1200 were used to study consistency of treatment. Patients undergoing VMAT