ESTRO 2020 Abstract book

S18 ESTRO 2020

The number of FF due to imager position or calibration has decreased from 31% to 17%, but the number of FF due to technical imager or machine problems increased from 8% to 18%. The number of FF due to problems with patient positioning decreased from 40% to 32%. Those due to patient anatomy increased from 19% to 28%. Actions for FF were comparable to the 1 st year with an increase of the number of plan adjustments from 1% to 4%. Because of the introduction of absolute verification more errors were caught such as: weight loss at start of treatment, problem with bellyboard, errors in planning, problems at simulation with 4DCT artefacts or contrast agents in bowel, pleural effusions cleared up by the time of treatment, poor breathing for gated breast patients…

Conclusion We have successfully proven the linearity, dose rate and energy independence of organic based technology in reference conditions to 6MV photons. The radiation hardness of these devices up to 40kGy has proven their longevity for applications in radiation environments and when coupled with a plastic scintillator have the potential to be used as real-time dosimeters for radiotherapy treatment. PH-0050 Results of 2 years of automated pre- treatment and absolute transit in vivo dosimetry. E. Bossuyt 1 , R. Weytjens 1 , D. Nevens 1 , S. De Vos 1 , D. Verellen 2 1 Iridium Kankernetwerk, Radiation Oncology Department, Antwerpen, Belgium ; 2 Antwerp University, Faculty of Medicine and Health Sciences, Antwerpen, Belgium Purpose or Objective Being a busy department with 5700 new patient plans/year and several satellite centers, efficiency, standardization and automation are key for a QA program. A web-based system was installed early 2017, for pre-treatment and in- vivo QA based on phantomless EPID and log files (PerFRACTION TM , Sun Nuclear Corporation). Images and log files are actively retrieved. Calculation and analysis occur automatically in the background. A clinical validation of the system’s performance on detection of errors and a comparison with the 1 st year results will be reported, along with an analysis of causes and actions taken for failed fractions. Material and Methods In this study results are reported for all patients treated between Sept 2018 and Aug 2019, compared with the results from the 1 st year (Oct 2017 to Aug 2018). For all patients receiving photon treatment, logfiles were automatically analyzed. For all VMAT plans, pre-treatment QA was performed (‘Fraction 0’). Transit EPID images were generated the first 3 days of treatment and weekly thereafter. In the 1 st year these had been compared using relative (image to image) analysis. In the 2 nd year absolute verification was introduced allowing comparison of the images to calculated data, enhancing detectable errors. Results 56542 fractions were analyzed: 91% passed, 7% failed and 2% were not calculated. As in the 1 st year, no relevant patient or machine errors were detected with analysis of log files alone. Pre-treatment QA showed results comparable to the 1 st year, with clearly better results for TrueBeam than for Clinac (Fig 1). In 43% of the treatment fractions, transit EPID dosimetry was performed. 16% of these fractions failed, compared to 15% in the 1 st year. When analyzing the 3619 failed fractions (FF) of non- stereotactic patients, the number of false positives has slightly decreased from 41% to 38%, with much better results for TrueBeam (19%) than for Clinac (49%), Fig 2.

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