ESTRO 2020 Abstract book

S1039 ESTRO 2020

typical dose rates between 2.5 and 10 kGy/s. Here we aim to do a simple comparison between EBT-XD film and alanine in a modified linear accelerator, to test their performance under ultra-high dose rate electron irradiation. Material and Methods For alanine and film irradiated simultaneously at four SSDs, giving dose rates between 25 and 400 Gy/s (0.1 and 2 Gy/pulse, respectively), the dosimeters showed no indication of dose rate depending effects within this range. The DD curves for alanine and film are in reasonable agreement on the slope after the maximum doses, D max , see Figure 1. However, for the build-up region, there is up to 12 % difference between the curves. This is possibly due to a contribution from a low energy electron component of the beam.

a maximum dose of 140-150 % to the centre of the GTV and a constraint of V 18Gy below 30 cc to the brain excluding the GTV and ventricles. Further, the margins were changed from a total of 6 mm from GTV to PTV to a total of 3 mm with the new protocol. The margin was further reduced if the brain V 18Gy criteria could not be met. For patients where the V 18Gy criteria for brain could not be fulfilled the GTV to PTV margin were reduced in steps of 1mm down to 0 mm. The dose to the (normal) brain with respect to the size of the tumour was compared between plans treated prior to and following the new protocol. Significance testing was performed using the Mann Whitney U-test. Results Regression analysis of V 18Gy versus the logarithm of metastasis size shows that V 18Gy was significant lower after implementation of the treatment protocol, figure 1. The median GTV volume was 3.5 cc and 3.9 cc prior and after the new protocol, with no significant difference between the two groups. The median V 18Gy was 24.5 cc and 17.7 cc, median D 2% was 30.9 Gy and 40.4 Gy, median D 98% was 28.6 Gy and 29.0 Gy before and after the new protocol. Further, there is a notable difference between helical plans an VMAT, but the difference was not significant. This difference could be attributed to the absence of a normal tissue function in the treatment planning system used for the helical plans, as compared to the planning system used for VMAT planning.

Results For alanine and film irradiated simultaneously at four SSDs, giving dose rates between 25 and 400 Gy/s (0.1 and 2 Gy/pulse, respectively), the dosimeters showed no indication of dose rate depending effects within this range. The DD curves for alanine and film are in reasonable agreement on the slope after the maximum doses, D max , see Figure 1. However, for the build-up region, there is up to 12 % difference between the curves. This is possibly due to a contribution from a low energy electron component of the beam. Conclusion Neither of the dosimeters exhibited any dose rate effects between 25 and 400 Gy/s, and their depth dose shape agree after D max . However, the observed differences at shallow depths were not anticipated and need to be further investigated. From these results, and in particular due to the robustness of the alanine dosimeter over a wide range of conditions including insensitivity to time between irradiation and measurement, it appears that alanine is an interesting candidate as an audit dosimeter for FLASH beams and should be considered further. PO-1773 A Comprehensive and feasible QA Program for Surface Guided Radiotherapy V. Batista 1 , J. Fischer 1 , B. Rhein 1 1 Heidelberg University Hospital, Department for Radiation Oncology and Radiation Therapy, Heidelberg, Germany Purpose or Objective Implementing new technology into the clinical routine requires not only the establishment of a commissioning and quality assurance (QA) program but also the definition of workflows adapted to each clinic. However, the increase in the complexity of equipment/processes associated with limited resources makes essential to define a QA program built on a quality management (QM) structure, where

Conclusion Implementation of the new protocol, with reduced margins, inhomogeneous dose distribution and dose restrictions on V 18Gy brain significant lowered the dose to brain with VMAT planning. In addition, this relationship between GTV volume and V 18Gy can be used as a planning criteria to predict a reasonable V 18y to the brain. PO-1772 Towards auditing FLASH electron beam dosimetry: clinical film vs. alanine C. Ankjærgaard 1 , E. Konradsson 2 , J.B. Christensen 1 , K. Petterson 3 , C.E. Andersen 1 , C. Ceberg 2 1 DTU, Center for Nuclear Technologies, Roskilde, Denmark ; 2 Lund University, Department of Medical Radiation Physics, Lund, Sweden ; 3 Skåne University Hospital, Department of Hematology- Oncology and Radiation Physics, Lund, Sweden Purpose or Objective In the past years, FLASH radiotherapy has gained increased attention due to a remarkable sparing of normal tissue after irradiation with high energy electrons at ultra-high dose rates (>40 Gy/s). Auditing of such systems is not straightforward, as the high dose rates cause ionization chambers to suffer from ion recombination. Therefore, the preferred choice of dosimeter in the clinic is often EBT films despite their limited dose linearity and delicate nature. Alanine is an easy to handle reference dosimeter with a combination of near water equivalence, precision, stability, and a linear response between 5 Gy and 120 kGy. It is routinely used in industrial e-beam sterilization with