ESTRO 36 Abstract Book

S824 ESTRO 36 _______________________________________________________________________________________________

waveform to amplitudes from 2 to 10mm in 2mm steps. We built the actual 4D dose distribution for every PLAN z considering all combinations of breathing patterns/amplitudes. We first copied the original treatment (planned at CT 0mm ) to the remaining CT z scans and recalculated them by using fixed MU. Then we copied the resulting dose matrices back to the CT 0mm scan and we shifted them axially by -z. Later, we summed the dose matrices using weights derived from the PDF of the underlying waveforms. We defined a Quality Index which balances GTV coverage and healthy tissue-sparing as QI = (V 100%,GTV ) 2 /V PD , where V 100%,GTV and V PD stand for the percentage of GTV covered with the Prescribed Dose (PD) and the volume of the PD isodose respectively. The optimal plan, PLAN opt , was the highest QI scoring plan for each breathing pattern/amplitude. Finally, we assessed the PDF’s measure of central tendency that best predicts PLAN opt irrespective of the breathing pattern/amplitude. Results Figure 1a shows the QI for the sinusoidal movement. Every breathing pattern’s maximum QI scores project an optimal curve to the x-y plane (figure 1b). For any breathing pattern/amplitude, PLAN opt was found to be conformed to an optimal ITV smaller than the purely geometric ITV. We found that the integral of the PDF between ±x, i.e., the time fraction on which the GTV is on the central part of the respiratory excursion, was the best predictor of PLAN opt. Irrespective of the breathing pattern/amplitude all PDF’s integrals collapsed to a unique curve for x = 3mm (figure 2). Conclusion Based on 4D dose calculations, we propose a QI to reduce the ITV while maintaining the GTV coverage for SBRT lung treatments. We provide a model to predict the optimal ITV from the integral of the PDF of the breathing waveform. Partially financed by FIS PI15-00788 grant. EP-1533 Modulation complexity assessment in VMAT plans from different treatment planning systems. P. Winkler 1 , A. Trausnitz 2 , J. Schroettner 2 , A. Apfolter 1 , K. Kapp 1 1 Medical University of Graz, Department of Therapeutic Radiology and Oncology, Graz, Austria 2 University of Technology, Institute of Health Care Engineering, Graz, Austria Purpose or Objective Modulation complexity (MC) in Linac-based VMAT plans might influence the accuracy of dose calculation and dose delivery as well as the precision of dose delivery. However, MC is not a single-parametric property, but rather consists of several different influencing factors, e.g. average leave speed (ALS), leave sequence variability (LSV), mean field aperture area (FAA), aperture area variability (AAV), gantry acceleration (gantry-speed variation, GSV) and dose rate variability (DRV), which might predominantly be correlated with uncertainties in either dose calculation or delivery. In our clinical treatment plans we observed, that different TPS accomplish strong modulation in a noticeably different way, forcing either ALS, LSV and AAV whilst retaining moderate GSV and DRV, or vice versa. The aim of this study is to present several distinct modulation complexity indices (MCI), describing the different aspects of modulation complexity in VMAT plans, and to assess the characteristic ranges of these MCI for different TPS. Material and Methods We established six MCI, parameterising the magnitude of ALS, LSV, FAA, AAV, GSV and DRV in a VMAT-arc, and implemented their calculation in our automated plan-QA software tool (in-house development). The MCI for 200 randomly selected clinical beams were calculated for the TPS Eclipse (Varian) and Pinnacle (Philips),

respectively. Additionally 20 phantom plans (37 arcs) with increasing modulation complexity were generated for both of the two TPS using best possible matching of optimization criteria, and were subsequently analysed. Results In the phantom plans, the Pinnacle-optimized arcs showed significantly higher average leaf speed compared to Eclipse-optimized arcs (10.9 and 6.7 mm/sec, respectively). Aperture – opening was 40% larger for the Eclipse-arcs. Consequently, the number of monitor units was smaller in the Eclipse plans (-32%). Whereas the differences for LSV and AAV were rather small (figure 1), DRV and GSV differed significantly, revealing a more pronounced modulation in the Pinnacle plans as far as dose rate and gantry acceleration are concerned. Findings for retrospectively analysed clinical plans and (non-biased) phantom plans were similar.

Figure 1: Modulation complexity scores (LSV: leave sequence variability, AAV: aperture area variability, GSV: gantry-speed variation, DRV: dose rate variability) for 200 VMAT plans, calculated with Eclipse and Pinnacle TPS, respectively. Box-plots showing median, first and third quartile and range. Conclusion Modulation complexity in VMAT plans has a potential impact on dose-calculation and –delivery accuracy. We found considerable differences for two different TPS in multi-parametric assessment of MC features, indicating the diverging algorithms of the different optimizers. A further investigation of the correlation between particular MC-scores and dosimetric accuracy can be the basis for the definition of tolerance criteria to identify potentially problematic plans. EP-1534 Automate the Complex Stuff: Pathways, Pitfalls and Results of Planning Automation in Raystation B. Archibald-Heeren 1,2 , M. Byrne 1 , Y. Wang 1 , Y. Hu 3 1 Radiation Oncology Centres, Wahroonga, Sydney, Australia 2 University of Wollongong, Clinical Medical Physics, Wollongong, Australia 3 Radiation Oncology Centres, Gosford, Sydney, Australia Purpose or Objective Automation provides the real possibility of providing exceptional plan quality to an enormous population of patients where time constraints or staffing levels may form a barrier. It is thus the authors hope that by openly sharing the constructed methodologies incorporated at Radiation Oncology Centre, Sydney, they may in some way expedite adoption of automation across the greater community. The work will focus on prostate and breast deliveries, but touch on other areas and solutions