ESTRO 37 Abstract book

S925

ESTRO 37

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 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 %.

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 biological experiments. The phantom enables simultaneous cell irradiation at up to 16 positions along the Bragg curve for a horizontal research beam line with active scanning technology. Cell responses and spatial variation of RBE values along the beam direction can be investigated with this phantom for proton and carbon ion beams. The dosimetric characterization of the phantom was performed with cylindrical ionization chambers (IC) and EBT3 films under conditions where no reference dosimetry applies. Additional challenges arise from the fact that the phantom does not exclusively consist of 'standard' materials commonly used in particle therapy. Material and Methods The phantom is composed of 16 compartments of 2.5 cm in depth, where plastic mini-flasks with seeded cells can be inserted. The phantom depth of 40 cm covers the entire clinically relevant proton energy range of 60 – 250 MeV. The water equivalent thicknesses of the phantom components were determined experimentally using the Peakfinder water column system (PTW, Germany). Plans with a SOBP of 8 cm length were created based on CT data sets of the phantom using the RayStation treatment planning system (TPS, RaySearch, Sweden). Absolute dose and dose homogeneity were assessed in each compartment with PinPoint IC (PTW 31015, Germany) and EBT3 radiochromic films. Films were cut into pieces of 2x6 cm 2 to fit exactly into the compartment and were positioned at the same place where the cells would be irradiated.

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).