ESTRO 36 Abstract Book

S777 ESTRO 36 _______________________________________________________________________________________________

EP-1455 IGRT kV-CBCT dose calculations using Virtual Source Models and validated in phantoms using OSL G. Boissonnat 1 , H. Chesneau 1 , J. Garcia-Hernandez 1 , D. Lazaro 1 1 CEA-LIST, DM2I/LM2S, GIF SUR YVETTE, France Purpose or Objective With the growing use of X-ray imaging equipment in Image-Guided RadioTherapy (IGRT), the need to evaluate the dose-to-organs delivered by kV-CBCT imaging acquisition increases. This study aims to propose accurate Monte Carlo (MC) calculations of the patient dose-to- organs delivered by two commercially available kV-CBCT systems: the XVI from Elekta’s VERSA HD accelerator and the OBI from Varian’s TrueBeam system. Simulations are to be validated using in phantom OSL measurements. Material and Methods For both kV-CBCT systems, the kV irradiation head geometry was implemented in the MC simulation code Penelope. As a first step, the resulting photon distributions were expressed as Virtual Source Models (VSM) for every standard irradiation condition (kVp, filtration, collimation); it was then validated and adjusted using in water-phantom measurements performed wit a calibrated Farmer-type ionization chamber. In a second step, the validated VSMs were used to simulate the dose delivered by both the XVI and OBI systems in anthropomorphic phantoms, using standard clinical imaging protocols. Simulated dose-to-organs were then confronted to dose measurements performed using OSL inserted into the same phantoms, following a dosimetric protocol for OSLs previously established [1]. In addition, VSM results were confronted to their direct MC counterparts in order to evaluate the benefit of using such technique. Results The current study highlights the possibility to reproduce OSL dose-to-organ measurements using VSM-driven Monte Carlo simulation with an overall agreement better than 20 %. In addition, the use of VSM in the MC simulation enables to speed-up the calculation time by a factor better than two (for the same statistical uncertainty) compared to direct MC simulation. Nevertheless, if direct and VSM calculations are in agreement inside the irradiation field, outside, VSM results tend to be significantly lower (10- 30%). Conclusion The use of a VSM was demonstrated to simplify and fasten MC simulations for personalized kV-CBCT MC dose estimation. In addition, OSLs enable to perform the low dose measurement in the kV range needed for in phantom X-ray imaging equipment dose QA. This study is to be completed in the near future by the addition of other standard X-ray imaging equipment dedicated to IGRT. EP-1456 In-vivo dosimetry using Dosimetry Check: 5- year experience on 345 prostate cancer patients W.H. Nailon 1 , D. Welsh 1 , K. MacDonald 1 , D. Burns 2 , J. Forsyth 2 , G. Cooke 1 , F. Cutanda 1 , D.B. McLaren 3 , J. Puxeu-Vaque 1 , T. Kehoe 1 , S. Andiappa 1 1 Edinburgh Cancer Centre Western General Hospital, Department of Oncology Physics, Edinburgh, United Kingdom 2 Edinburgh Cancer Centre Western General Hospital, Department of Radiography, Edinburgh, United Kingdom 3 Edinburgh Cancer Centre Western General Hospital, Department of Clinical Oncology, Edinburgh, United Kingdom Purpose or Objective It is recommended that all radiotherapy centres in the United Kingdom (UK) have a protocol for in vivo dosimetry

(IVD) and in several European countries IVD is now mandatory. Electronic portal imaging devices (EPIDs), which although developed primarily for the purposes of imaging, are now widely used for IVD and consequently for treatment quality assurance (QA). Here we present results from 5-year clinical experience of using IVD for verification of prostate cancer patients. Material and Methods Between 2011 and 2016 IVD was performed by Dosimetry Check (DC) (Math Resolutions LLC, Columbia, MD, USA) on 345 prostate cancer patients. Treatment plans were prepared in Eclipse (Varian Medical Systems, Inc., Palo Alto, CA, USA) with 285 patients treated with a volumetric modulated arc therapy (VMAT) technique and 60 patients treated with a three-dimensional conformal radiotherapy (3DCRT) technique. The difference between the dose calculated by Eclipse at a reference point and the dose measured by DC at the same reference point at time-of- treatment was recorded. In cases where the dose difference exceeded ±10% an alert was triggered and a full three-dimensional gamma analysis (4%/4mm) performed on the treatment plan. This led to either the measurement being repeated or further positional and patient-specific QA checks being performed. Results Figure 1 shows the percentage difference in point doses calculated by Eclipse and measured by DC for the 3DCRT and VMAT treatments monitored. The mean and standard deviation (µ±σ) of the percentage difference in dose obtained by DC and calculated by Eclipse was 1.23±4.61% in VMAT and −3.62±4.00% in 3DCRT. A total of 12 plans exceeded the ±10% alert criteria accounting for 3.5% of all prostate cancer treatments monitored. In all of these cases further investigation using 3D gamma analysis and additional patient-specific QA found no reportable treatment errors.

Figure 1 : IVD point dose measurements on all prostate cancer patients treated between 2011 and 2016. Conclusion The preliminary results of this pilot study show that EPID- based IVD using the DC software has the potential to detect errors and identify sub-optimal treatments. With the addition of more data it may also be possible to establish site-specific alert levels, which could improve the quality of radiotherapy. EP-1457 Introducing the fraction of penumbra dose in the evaluation of VMAT treatment plans A. Bäck 1 , F. Nordström 1 , M. Gustafsson 1 , J. Götstedt 2 , A. Karlsson Hauer 1 1 Sahlgrenska University Hospit, Therapeutic Radiation Physics, Göteborg, Sweden 2 University of Gothenburg, Radiation Physics, Göteborg, Sweden