ESTRO 36 Abstract Book

S955 ESTRO 36 2017 _______________________________________________________________________________________________

quality have been assessed. Subsequent real-time monitoring issues have been considered. Results The presence of IQM chamber in the beam path changed the pass rate of MapCheck (IMRT), and ArcCheck (VMAT) within ±1% when %Dose–DTA criteria were employed, as shown in Table-1. The calculated and measured IQM signals on a Varian TrueBeam Unit for 340 randomly chosen IMRT field segments from clinical plans show good agreements (Fig.1): 95% of segments are within ±3; total cumulative signals for VMAT delivery were within± 2%. By introducing the system as a pre-treatment QA tool 700 hours of staff time and 180 hours of machine time can be saved annually, for a facility treating 240 IMRT and VMAT patients per day. Additionally, the segment by segment dosimetry and independent gantry angle monitoring provides added quality values for pre-treatment QA. Daily monitoring of beam delivery may save more machine and staff time by eliminating pre-treatment QA, while improving patient safety. However, a number of issues need to be addressed, such as: (1) Modification of TPS beam model to include the effect of the IQM chamber on the beam (2) The TPS should allow an accessory code in IMRT and VMAT (3) The Linac manufacturer should make an accessory code available for the on-line monitor.

The leaf position is defined as the position of the 50% of the dose profile. This measurement depends on relative position of the beam source and of the leaves. In order to validate the EPID measurements of the absolute leaf positions, 10 dose profiles, at the center of different leaf pairs of the same MLC field, were acquired with an Elekta iViewGT EPID and with a diode positioned in a water phantom. The comparison between the two detectors was performed by Matlab. Garden Fence (GF) was chosen as test of the leaf position accuracy and a preliminary study on the gap width was conducted. Leaf position accuracy was checked automatically with DC by acquiring GF at the 4 cardinal gantry angles and with all the beam energies (6, 10 and 15MV), while the reproducibility was tested with 5 GF repeated in one day and 6 repeated in a time interval of 70 days. Results The difference between EPID and diode absolute measurement of the leaf positions was less than 0.8mm for all the analyzed leaves, resulting from the summation of an error due to the isocenter identification (0,5mm) plus the leaf positioning error (0.2mm). The gap width study revealed that, because of the penumbra widening observed in small fields, the leaf position could be accurately measured as the 50% of the edge profile, only if the gap width is equal or larger than 16mm with 6MV beam. Therefore, GF with 20mm gap was chosen as leaf position accuracy test for all the energies in order to distinguish the effect of beam source from that of leaf positioning. For the GF at different gantry angles the difference between the measured and the prescribed position was well within 1.0mm for all the leaves. Moreover, reproducibility of each leaf position resulted to differ from its average value less than 0.4mm. Conclusion This work permitted to assess the accuracy and the repeatability of the Elekta Agility MLC leaf positioning by the combined use of the Elekta IviewGT EPID and the Dosimetry Check software through the acquisition and the analysis of Garden Fence test. This system was validated comparing the EPID with a diode in a water phantom and assessing the minimum gap width necessary for an accurate leaf position measurement at all energies which is useful to distinguish issues related to beam symmetry from those related to leaf positioning. EP-1760 A simple method for estimating the longitudinal isocentre shift due to gantry motion R. Hudej 1 , D. Brojan 1 , S. Pulko 2 , P. Peterlin 1 1 Institute of Oncology Ljubljana, Department of Radiophysics, Ljubljana, Slovenia 2 University Clinical Centre Maribor, Department of Oncology- Radiotherapy Unit, Maribor, Slovenia Purpose or Objective The isocentre as a point of intersection of the three rotational axes (gantry, collimator and treatment couch) ideally remains fixed in space during the rotation of gantry, collimator, or the treatment couch. Due to the mechanical limitations, gantry sags slightly, and consequently the radiation isocentre shifts slightly towards the treatment couch when the gantry rotates from the uppermost to the lowermost position. The purpose of this study is to assess this shift. Material and Methods A strip of radiochromic film embedded in a suitable water- equivalent phantom is irradiated with a cross-line half-slit field from the top (0°). Then the gantry is rotated to the lowermost position (180°) without moving the jaws and the phantom is irradiated again. The film is scanned and analysed with an image analysis script. The central lines of both half-slit images are determined, then the intersection angle between them is calculated, and finally the distance between the intersections of extrapolated lines with the 'sagittal” plane is calculated.

Conclusion Clinical implementation of the system in multiple phases helps understanding the performance characteristics of the system, allows smooth transition of QA practices, make overall clinical workflow safe and effective for real- time beam monitoring. EP-1759 MLC positioning study based on EPID images analyzed with the Dosimetry Check software C. Avigo 1 , M. Mignogna 2 , S. Linslata 3 1 National Research Council, Institute of Clinical Physiology, Pisa, Italy 2 Azienda USL Toscana nord ovest- S. Luca Hospital, Radioterapia, Lucca, Italy 3 Azienda USL Toscana nord ovest- S. Luca Hospital, Fisica Sanitaria, Lucca, Italy Purpose or Objective The IMRT requires extensive knowledge of the MLC position accuracy and repeatability since when accurate leaf positioning is lost significant dose delivery errors can occur. Therefore, the MLC QA is crucial for a complete control of the patient treatment. The use of EPID for this scope can be very helpful in saving time providing images with high spatial resolution and directly digitalized. Dosimetry Check is a commercial software which uses EPID images for pre-treatment verification, in vivo-dosimetry and also MLC QA. The aim of this work was to validate the combined system EPID-Dosimetry Check, in order to control the leaf positioning of an Elekta Agility MLC. Material and Methods