ESTRO 36 Abstract Book

S968 ESTRO 36 _______________________________________________________________________________________________

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.

Results The differences in leaf positions compared with film and light field are beyond 0.1 mm and 1 mm (light field edge detection has a much bigger uncertainty). The acquisition and analysis for one strip-test take less than 4 min. Conclusion The methodology employed analyzes a MLC strip-test in an Elekta LINAC in a fast and accurate way. EP-1758 Towards Clinical Implementation of an Online Beam Monitoring System M. Islam 1 , M. Farrokhkish 2 , Y. Wang 2 , B. Norrlinger 2 , R. Heaton 1 , D. Jaffray 1 1 Princess Margaret Cancer Centre and University of Toronto, Medical Physics, Toronto, Canada 2 Princess Margaret Cancer Centre, Medical Physics, Toronto, Canada Purpose or Objective Continual advancement of Radiation Therapy techniques and consequent complexity in planning and delivery require constant vigilance. To address this, the idea of independent real-time beam monitoring has been proposed. In this presentation, we describe initial steps towards introducing the Integral Quality Monitoring (IQM) system into clinical practice. Material and Methods The IQM system (manufactured by iRT, Germany) consists of a large-area ion chamber mounted at Linear Accelerator’s (Linac) accessory slot, which provides a spatially dependent “dose -area- product” per field segment. The system monitors beam delivery in real-time by comparing the expected and measured signals. Initial evaluation of the system included: reproducibility and stability, agreements between calculated vs. measured signals, sensitivity and specificity for errors. A multiphase approach was considered for clinical implementation. First, IQM data are collected during conventional dosimetric QA tests. Results of the QA tests with and without the IQM chamber in the beam are compared. During this phase a reference dataset of measured IQM signals vs. calculated is generated to help determine the tolerance in the predicated signals. Second, IQM system is introduced as the primary pre-treatment QA tool. Gain in work-flow efficiency and

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.