Abstract Book

S924

ESTRO 37

actual LINAC used to determine the detector-reading ratios and (ii) differences between the detector blueprints, from which the detector was modelled in MC, and the actual detector geometry. Machine-to-machine analysis showed differences in detector-reading ratios between LINACs of the same model by 5 % for 0.5 x 0.5 cm 2 and 2 % for the 0.8 x 0.8 cm 2 and they were also attributed to differences in beam characteristics between different LINACs and to jaw/detector positioning uncertainties. Analysis resulted in a combined uncertainty of detector and jaw collimators positioning of 2% for 0.5 x 0.5 cm 2 and 1 % for 0.8 x 0.8 cm 2 whereas for larger fields it decreased below 1 %.

Conclusion The in-house developed phantom is a versatile 4D antropomorphic lung phantom, well suited for (routine) QA measurements of all techniques involved in lung stereotactic treatments, and also for end-to-end tests of such treatment. It can be applied for the QA of 4D-CT, 3D- and 4D-CBCT as well as dosimetry. With material costs of less than 200 euros, the phantom has an unparalleled price/quality ratio. EP-1728 Small field dosimetry formalism implemented using various detectors on several linear accelerators M. Ghazal 1 , A. Carlsson-Tedgren 1 , H. Benmakhlouf 1 1 Karolinska University Hospital, Department of Radiation Physics and Nuclear Medicine, Stockholm, Sweden Purpose or Objective The objective of the study was to implement the new small photon field dosimetry formalism by Alfonso et al 2008 to linear accelerators (LINAC) at Karolinska University Hospital. The detector response in small photon fields was analysed. Monte Carlo (MC) calculated output correction factors (OCF) were applied to experimentally determined detector-reading ratios. Machine-to-machine differences in small photon field’s output factors (OF) were also investigated by comparing detector-reading ratios measured with several LINACs of the same model. Uncertainties related to the reproducibility of the positioning of the detector and the jaw collimators were also investigated. Material and Methods Detector-reading ratios were measured for field sizes set by the jaw collimators, ranging from 10 x 10 cm 2 down to 0.5 x 0.5 cm 2 using six 6 MV Varian Clinac LINACs and one 6 MV Varian TrueBeam LINAC. The detectors used in this study were two IBA silicon diodes EFD (unshielded) PFD (shielded), one PTW unshielded silicon diode T60017 and one PTW micro-diamond detector T60019. OCF calculated by Benmakhlouf et al 2016 using PENELOPE were applied to detector-reading ratios. Results Field sizes smaller than 2 x 2 cm 2 showed large differences between different types of detectors (Table 1). OCF applied to detector-reading ratios successfully corrected for differences between detectors for 1 x 1 cm 2 . However, for 0.5 x 0.5 cm 2 and 0.8 x 0.8 cm 2 the application of OCF to detector-reading ratios did not result in reducing the differences between detectors. This was attributed to: (i) differences in beam characteristics (electron beam energy, electron beam width and electron beam divergence) between the MC model, from which the OCF are calculated, and the

Conclusion OCF cannot be used to correct for differences in detector-reading ratios in field sizes smaller than 1 x 1 cm 2 set by the jaw collimators. OCF are successfully implemented to detector-reading ratios for 1 x 1 cm 2 field size and larger set by the jaw collimators. LINACs of the same model, Varian Clinac, cannot be considered to be dosimetrically tuned in terms of OF for field sizes below 1 x 1 cm 2 (Figure 1).

EP-1729 Dosimetric characterization of a novel phantom for cell irradiation in active proton beam scanning M. Clausen 1,2 , S. Khachonkham 1,2,3 , P. Kuess 1,2 , B. Knäusl 1,2 , D. Georg 1,2 , W. Dörr 1,2 , S. Gruber 1 1 Medical University of Vienna, Department of Radiotherapy, Vienna, Austria 2 Medical University of Vienna, Christian Doppler Laboratory for Medical Radiation Physics for Radiation Oncology, Vienna, Austria 3 Faculty of Medicine Ramathibodi Hospital- Mahidol University, Division of Radiation Therapy- Department of Diagnostic and Therapeutic Radiology, Bangkok, Thailand Purpose or Objective In proton therapy a constant value of 1.1 for the relative biological effectiveness (RBE) is traditionally considered. However, the existing experimental and theoretical studies demonstrate different cell response and RBE variations along the spread-out Bragg peak (SOBP). The purpose of this study was the detailed dosimetric characterization of a novel phantom designed for in vitro