ESTRO 36 Abstract Book

S778 ESTRO 36 2017 _______________________________________________________________________________________________

(Varian) with 10 MV photon beams and a QA plan which underwent prostate VMAT. For the QA plan, we prepared seven modified MLC files in which MLC positions were manually shifted between 0 mm to 3 mm, respectively. Then, the dose delivery errors were measured and analyzed dose deviation, distance-to-agreement (DTA) and gamma index by the Delta4 Discover system as well as to the Delta 4 3D dosimetry system. All measurements were compared against the points that received less than appropriate percentage dose (<10%) were excluded from the gamma index calculation. Results The results of the Delta4 Discover system and the Delta 4 3D dosimetry system using all available points for the gamma calculation, 95.7±5.9% and 97.8±3.5% of them passed the criteria (3%/2mmDTA/Th10%), respectively. The two systems also had high correlations of dose deviation and DTA, permitting the routine verification of VMAT patient-specific QA plan as well as permanent in- vivo dosimetry during the patient’s treatment course. Conclusion In this study, we have found that there was high correlation between the pass rate and the intentional dose delivery error, as respect to dose deviation, DTA, and gamma index of the Delta 4 Discover system and the Delta 4 3D dosimetry system. It was suggested that the dosimetric verification system under investigation could be useful for routine patient specific QA. EP-1475 RBE estimation of different Brachytherapy sources based on micro- and nanodosimetry M. Bug 1 , T. Schneider 2 1 Phys. Techn. Bundesanstalt PTB, 6.5 Radiation Effects, Braunschweig, Germany 2 Phys. Techn. Bundesanstalt PTB, 6.3 Radiation Protection Dosimetry, Braunschweig, Germany Purpose or Objective Depth-dependent RBE values of typical photon-emitting Brachytherapy (BT)-sources were determined by a microdosimetric and a nanodosimetric approach. The microdosimetric approach considers a biological endpoint while the nanodosimetric approach is entirely based on the track structure, given by the interactions of the photons and secondary electrons. The track structure characterizes the radiation quality on the nanometric scale. Material and Methods Within a cylindrical water phantom, isotropically emitting BT-sources were positioned 4 cm below the surface. Studied were Co-60 and Ir-192 representing high-energy photon-emitting sources, I-125 being a low-energy photon- emitting source, and Intrabeam ® - and Axxent ® -devices as examples for electronic BT X-ray sources (EBX). Resulting photon spectra were calculated at several points along the cylindrical axis within cylindrical voxels of 0.5 mm depth and 2 mm radius up to a depth of 10 cm. The microdosimetric calculations of RBE are based on yield coefficients α dic, representing the linear component of the dose-effect relationship for the dicentric chromosome aberration yield after an irradiation with monoenergetic photons. The RBE for a given source in a given point was determined by convoluting the respective spectrum with the function α dic (E), obtained previously by microdosimetric calculations. The same depth-dependent photon spectra were used to determine nanodosimetric quantities by Geant4-DNA calculations. For each initial photon track, target volumes in size of one DNA convolution which experienced at least 4 ionizations (F4) were identified. Such quantities were previously shown to describe the DSB yield. Based on the average minimum distance between these volumes within each photon track, the RBE was estimated by normalization to the distance for Co-60 at 0.125 mm depth.

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 dose by the accelerator directly and evaluates the dose delivery error to the plan. We have used True-Beam