ESTRO 36 Abstract Book

S84 ESTRO 36 2017 _______________________________________________________________________________________________

OC-0166 Fast 3D CBCT imaging for Lung SBRT: Is image quality preserved ? B. De Rijcke 1 , R. Van Geeteruyen 1 , E. De Rijcke 1 , Y. Lievens 1 , E. Bogaert 1 1 Ghent University Hospital, Radiation Oncology, Gent, Belgium Purpose or Objective Irradiation of Early Stage Non-Small Cell Lung Cancer (ES- NSCLC), through Stereotactic Body Radiotherapy (SBRT) requires image guidance. At our institute double pre- treatment CBCT, with manual registration is performed at every fraction. Speeding up CBCT gantry rotation and implementation of automated registration allows for faster decision taking. It also offers the possibility of intrafraction CBCT, without severe prolongation of treatment time. In a first step we investigated the image quality and performance of a CBCT protocol with lower dose and faster acquisition time. Material and Methods Standard (S) and Fast (F) scan protocols only differed in gantry speed (180°/min (S) and 360°/min (F)) and were performed on XVI Elekta ® CBCT. For six patients receiving lung SBRT (60Gy in 3 or 4 fractions) for upper lobe ES- NSCLC, dual pre-treatment imaging consisted of a S scan followed by a F scan. This resulted in 17 useful S and F image sets. Tumor movement amplitude stayed below 1cm (1) , removing the necessity for 4D-CBCT. All CBCT images were retrospectively exported to Raystation ® (RaySearch Laboratories, Sweden) for easy and blended side-by-side evaluation. The resolution was 1x1x1mm 3 for all scans. All CBCT images were matched to planning CT. WW/WL was set fixed per patient. Zooming was allowed. Visual Grading Analysis (VGA) comprised well defined criteria over the three planes (T, C, S), categorized in three Image Quality (IQ) Focus groups: bony anatomy (N=11), tumor characteristics (N=3) and anatomical landmarks (N=7). Examples are: visualization of corpus vertebrae (C, S plane), tumor edge (3 planes); carina bifurcation (C, T plane). Scoring was done independently by 3 routined RTTs. Possible answers were: equal, better or worse for ‘upper’ scan (randomly assigned to F or S). Data were analyzed using SPSS software v24 (IBM Corp., New York, NY). Results In 73.7 % of all cases, visualization of anatomical structures was appreciated equally on S and F scans. When differences emerged, visualization on F scan was appreciated more in 71.3 % of the cases (71.8 % for bony anatomy, 75.0 % for tumor characteristics and 67.2 % for anatomical landmarks). Binary Logistic Regression in these cases did not reveal significant dependence on patient (for which BMI or tumor location are most relevant; however not evaluated separately) (p=0,638), not on IQ focus group (p=0,540) and not on reader (p=0,883). Thus, in 92.4 % of all cases, image quality was scored equal or better for fast imaging protocol compared to the standard protocol (Figure 1).

Conclusion Fast CBCT imaging can be safely used for ES-NSCLC tumors with tumor movement amplitude < 1cm. In 73.7 % of the cases there is no image quality loss and even more, in 18.8 % of the cases IQ of the fast scan is preferred compared to the standard scan. (1) Rit, S., et al., Comparative study of respiratory motion correction techniques in cone-beam computed tomography. Radiotherapy and Oncology, 2011. 100(3): p. 356-359 SP-0167 The ESTRO initiative on biological effects of particle therapy B.S. Sørensen 1 1 Aarhus University Hospital, Exp. Clin. Oncology, Aarhus C, Denmark Particle therapy as cancer treatment, with either protons or heavier ions, provide a more favourable dose distribution compared to x-rays. While the physical characteristics of particle radiation have been the aim of intense research, less focus has been on the actual biological responses particle irradiation gives rise to. One of the biggest challenges for the radiobiology is the RBE, with an increasing concern that the clinical used RBE of 1.1 is an oversimplification, as RBE is a complex quantity, depending on both biological and physical parameters, as dose, LET, biological models and endpoints. Most of the available RBE data is in vitro data, and there is very limited in vivo data available, although this is a more appropriate reflection of the complex biological response. There is a need for a systematic, large-scale setup to thoroughly establish the RBE in a number of different models, in a clinical relevant fractionated scheme. The aim of the ESTRO initiative is to form a network of the research and therapy facilities. This would open for the possibility of standardising radiobiological experiments, and coordinating the research in order to deliver the needed experimental data. SP-0168 RBE of protons B. Jones 1 1 Jones Bleddyn, CRUK-MRC Oxford Institute- Department of Oncology, Oxford, United Kingdom Introduction . Increasing clinical use of proton therapy (PT) is not simply an extension of photon radiotherapy (RT), but requires more detailed knowledge of clinical physics and radiobiology in order to achieve optimal outcomes. A critical difference is that megavoltage RT has linear energy transfer (LET) of around 0.22 keV.µm -1 , but LET further increases towards and within proton Bragg peaks. ‘Spread-out’ Bragg peaks (SOBP), depending on their volume, normally have LET of 1-2 keV.µm -1 , but Symposium: Novel approaches in particle biology