ESTRO 37 Abstract book

S968

ESTRO 37

Results Results are in agreement with the IEC 60731 document, which states that the angular dependence of an ion chamber should not exceed a 1% deviation in the angular interval comprised between ±40º. However, for gantry angles greater than 40º an increasing angular dependence was found. In figure 2 the percent deviations of the normalized values as a function of the gantry angle is depicted. Some symmetry with respect to 0º is observed mainly due to the cylindrical geometry of the chamber as well as due to the irradiation geometry. Deviations up to 5.2% with respect to the 0º value were observed. For each angle the standard deviation did not exceed 1%, so it was decided not to perform a higher number of measurements per gantry position.

Material and Methods For each VMAT treatment site in turn (Prostate, Head & Neck and Rectum), a comprehensive review of several hundred previous verification results was performed to demonstrate the stability of the relevant class solutions. Normal MU ranges were determined for each VMAT treatment site to aid identification of atypical plans, which would still require a physical dosimetric verification measurement. A commercial system, Mobius3D (Mobius Medical Systems) was commissioned to perform independent dose checks for plans lying within the relevant normal MU range. Site specific tolerances were determined for independent dose checks using Mobius3D based on samples of approximately 100 treatment plans per site, which had all successfully passed dosimetric verification. In-house software was developed to check the electronic transfer of every VMAT plan parameter from Pinnacle to MOSAIQ. Risk assessments were produced to demonstrate all risks had been considered and mitigated. Routine VMAT plan verification measurements were added as 6- monthly tasks to linac QC schedules. Results Until 2015 patient plan specific verification measurements were performed for every VMAT treatment plan at our institute. Since then such measurements have been halted for the majority of prostate VMAT, head & neck VMAT and rectum VMAT plans (in March 2015, May 2016 and August 2017 respectively). The graph shows the resultant significant reduction in these measurements with prostate VMAT reducing by 87% and head & neck VMAT verifications by 68% compared to the twelve months prior to the introduction of Mobius3D as an independent dose check for each site. Similar results are expected for rectum VMAT plans in the coming months.

Figure 2: Pin Point angular dependence in percentage deviation as a function of gantry angle. Conclusion The angular dependence of a Pin Point ion chamber was obtained for an angular interval between 90º and 270º. This dependence is in accordance with the IEC 60731 document, but it increases as the angle deviates from 0º up to 5.2% at 90º. Considering these results it is recommended that the angular dependence of the detectors used be known by the user, being this data provided by the manufacturer or measured by the user itself. A proper characterization of this dependence could be employed to calculate correction factors to be applied when using an ion chamber. EP-1800 Evidence based large-scale reduction in patient specific VMAT verifications A. Pooler 1 , J. Wood 1 , L. Freear 1 , P. Whitehurst 1 , G. Budgell 1 1 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering, Manchester, United Kingdom Purpose or Objective VMAT was first introduced into clinical use at our institute in 2012. The number of patients treated using VMAT techniques per year has steadily risen since then to over 1500 in 2016 and is expected to reach at least 1750 in 2017. The expansion in VMAT treatments resulted in a corresponding increase in dosimetric verification measurements, which were initially performed on a patient specific basis. These served many purposes, including as an independent MU check and to check the transfer of the plan from the planning system (Philips Pinnacle) to the record and verify system (Elekta MOSAIQ). However dosimetric verification measurements normally have to be performed outside of clinical treatment hours, which constrained further expansion of VMAT techniques. Therefore it was desirable to achieve a large-scale reduction in these measurements for established VMAT techniques.

Conclusion A safe, evidence-based method of reducing the number of patient specific verification measurements has been implemented for certain VMAT treatment sites. This has resulted in a mean reduction of 44 patient plan specific verification measurements per month for established VMAT techniques. EP-1801 Prompt gamma imaging for the identification of proton range deviations due to anatomical change Y. Xie 1 , J. Petzoldt 2 , G. Janssens 2 , L. Yin 1 , E.H. Bentefour 2 , J. Smeets 2 , D. Prieels 2 , R.L. Lustig 1 , A. Lin 1 , B.K. Teo 1 1 Hospital of the University of Pennsylvania, Department of Radiation Oncology TRC 2 West, Philadelphia- PA, USA 2 Ion Beam Applications SA, Advanced Technology Group, Louvain-la-Neuve, Belgium Purpose or Objective Prompt Gamma (PG) imaging has previously been demonstrated for use in proton range verification of a brain treatment with a homogeneous target region. The mean range shifts were measured to be smaller than the applied margins [Xie et al, IJROBP 99:210-218 2017]. In