ESTRO 37 Abstract book

ESTRO 37

S565

Conclusion A homogeneous calculation and extraction method of DVH data was applied to a grouped analysis of SBRT plans. A consistent multiple DVH analysis was performed. The bias due to different DVH calculation algorithms was eliminated by employing a single independent calculation method. The observed differences suggest that comparable standards in patient treatment among different centers can be obtained if a consistent high- level data sharing capability is granted. In the strive to harmonize the planning process, this analysis constitutes a first step toward the creation of a platform of crowd- knowledge-based planning guidelines. This platform could give an high-quality benchmark to less experienced centers that are willing to implement SBRT techniques. PO-1011 Precision of the optically controlled Multi Leaf Collimator (MLC) on a high field MR linac A. Bertelsen 1 , I. Hanson 2,3 , S. Nill 2,3 , U. Bernchou 1,4 , V.N. Hansen 2 , C. Brink 1,4 , U. Oelfke 2,3 1 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark 2 NHS Foundation Trust, The Royal Marsden Hospital, London, United Kingdom 3 The Institute of Cancer Research, Radiotheraphy, London, United Kingdom 4 University of Southern Denmark, Institute of Clinical Research, Odense, Denmark Purpose or Objective Online MR guidance in radiotherapy makes it possible to distinguish small tumors from adjacent organs at risk (OAR) within soft tissue regions. Treating such tumors to high doses in few fractions calls for high precision dose delivery. This study investigates random and time dependent positional variations of the optically controlled 160 leaf MLC operating in the magnetic field of the Elekta Unity MR linac Material and Methods An onboard EPID, with a pixel size of 0.21 mm at isocenter, was used to measure a series of rectangular fields with MLC leaf positions moving in steps of 20 mm from -30 mm to 30 mm. Only the central 30 MLC leaf pairs within the field of view of the EPID were evaluated. For each field the position of the individual leafs was assessed by in-house developed software using steepest gradient analysis. Random variations were assessed by repeating the above described procedure sequentially five times. This was repeated after 35 and 55 days to be able to evaluate the time dependent drift. No MLC calibration was performed during the 55 days. For each leaf and prescribed position the random variation was defined as the SD of the five repeated measurements. The overall random variation was calculated as the root mean square of all the individual leaf and position specific SDs.Time dependent drift per leaf and prescribed position was defined as the change of the average values of the five repeated measurements from the first to the 2 nd and 3 rd set of measurements; creating two distributions of drifts. The SD and mean value of these distributions were calculated to characterize the drift of the MLC over time. Statistical differences between distributions were assessed using Wilcoxon rank sum test, the significance of the mean drift value was tested by Student t-test, and Spearmans Rho was used to investigate correlations. Results with p-values below 0.05 was considered statistical significant. Uncertainties of all reported values are calculated using bootstrapping with 1000 samples.

to generate the DVH. This can bias a grouped analysis due to possible poor data consistency. In order to make data comparable, a homogeneous method for data extraction is necessary. In this work we used a consistent method to present a preliminary analysis of multiple data coming from a national survey on stereotactic body radiotherapy (SBRT) planning. Material and Methods A single spine case was shared among 9 radiation oncology centers. The dose prescription was 30 Gy in 3 fractions with specific constraints on target coverage and dose to nearby organs at risk. The VMAT delivery technique was employed by each center. All data were collected in DICOM-RT format. A script was developed in R language using the RadOnc R-Package for DVHs recalculating using a homogeneous algorithm. Specific DVH points collected from the 9 centers were compared with those recalculated with RadOnc. Using the data recalculated with RadOnc, a consistent multiple-DVH analysis was performed and relevant dose parameters were compared. This preliminary analysis was focused only on dose parameters relative to the planning target volume (PTV). Results Differences between collected and recalculated DVHs were minimal, however in some cases the deviations were up to 1.5%. The multiple-DVH analysis showed a notable variability on target dose level (Fig.1). The D2% to the target deviated from its median value up to 34%. Deviations of comparable magnitude were found for the mean dose. These differences were caused mainly by different planning optimization strategies, rather than by the use of a specific treatment technology. The variability was reduced when considering the dose level corresponding to the prescribed target coverage ( i.e. D90% in Fig.2). This could be attributed to a clear indication on the coverage constraint given in the planning guidelines.