ESTRO 36 Abstract Book

S787 ESTRO 36 _______________________________________________________________________________________________

profiles showed qualitatively good agreement between the gel dosimeter, EBT film and RTP data for all PTVs. Conclusion The results indicate that those processes could effectively evaluate geometric and dosimetric accuracy of brain SRT. This study using 3D dosimetry system was useful to validate the 3D dose distributions for patient-specific QA. EP-1473 Improving the accuracy of dosimetry verification by non-uniform backscatter correction in the EPID Y. Md Radzi 1,2 , R.S. Windle 2 , D.G. Lewis 2 , E. Spezi 1,2 1 Cardiff University, School of Engineering, Cardiff, United Kingdom 2 Velindre Cancer Centre, Department of Medical Physics, Cardiff, United Kingdom Purpose or Objective Challenges in improving the accuracy of EPID-based patient dose verification have been widely discussed and remain a key topic of interest for patient safety, as exemplified in the UK by the ‘Towards Safer Radiotherapy’ 2008 report[1]. In particular, one of which is for every radiotherapy centre to have protocols for in vivo dosimetry (IVD) to be used for most patients as recommended in the Annual Report of the Chief Medical Officer for 2006 and it is already a legal requirement in many European Countries [2]. In this presentation, we report on commissioning and implementation of the commercially available Dosimetry Check (DC) [3, 4] system. Particular emphasis has been given to addressing the significant non-uniform backscatter effect from the VARIAN aSi-1000 EPID arm [5, 6]. Material and Methods A backscatter correction matrix was developed by combination of dosimetric information from a set of segmented fields sampling on different positions around the active area of the imager. The matrix was then used to correct EPID images using MATLAB programming scripts. The corrected image was created in DICOM format and exported to Dosimetry Check to read and analyse. Example treatment fields were generated in our Oncentra MasterPlan (OMP) Treatment Planning System (TPS), with several equidistant dose reference points relative to central axis included. A dose comparison given by DC with reference to the TPS was recorded in an auto-generated report. Assessment and comparison undertaken included the (i) asymmetry evaluation of equidistant points before and after correction being applied with respect to TPS, (ii) improvement in segmented IMRT dose profiles after correction, and (iii) OMP-DC pass rate with gamma criterion 3%/3mm[7], as well as 2-D Gamma Volume Histogram (GVH) evaluation on outlined PTVs. Results (i) Correction for non-uniform backscatter improved with overall agreement between fields generated in OMP and those recorded in DC from within 3% to better than 1%. (ii) Agreement between OMP and DC for IMRT dose profiles with a sample Head & Neck case was improved by approximately 3% using the correction methodology ( Table 1 ). (iii) For gamma comparison of fields in OMP and DC with 3%/3mm, pass rates were improved from around 80% to around 90% by the correction method. Similarly in GVH evaluation for the outlined PTVs, pass rate has increased from around 80% to 90% after correction being applied.

Conclusion The correction method implemented herein for the Dosimetry Check system has proved to be an effective way to reduce verification inaccuracy caused by backscatter from the Varian EPID arm and can be used to enhance the previously established portal verification method for IMRT using this technology.

EP-1474 Feasibility of dose delivery error detection by a transmission detector for patient-specific QA H. Honda 1,2 , K. Kubo 1 , R. Yamamoto 1 , Y. Ishii 1 , H. Kanzaki 1 , Y. Hamamoto 1 , T. Mochizuki 1 , M. Oita 3 , M. Sasaki 4 , M. Tominaga 5 , Y. Uto 6 1 Ehime University, Department of Radiological Technology, Toon, Japan 2 Tokushima University, Graduate School of Advanced Technology and Science, Tokushima, Japan 3 Okayama University, Department of Radiological Technology- Graduate School of Health Sciences, Okayama, Japan 4 Tokushima University Hosipital, Department of Radiological Technology, Tokushima, Japan 5 Tokushima University, Institute of Health Sciences, Tokushima, Japan 6 Tokushima University, Institute of Bioscience and Bioindustry, Tokushima, Japan Purpose or Objective Dose delivery error detection of on-line treatments is an important issue for clinical QA practices. The goal of this study was to evaluate a feasibility of the delivery error detection by a new type of on-line transmission detector compared to a 3D detector in patient-specific QA measurements for VMAT treatment. Material and Methods The Delta 4 Discover system is a transparent, p-type semiconductor diodes detectors, placed in the accessory holder of the treatment head. The system measures the