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.

Results Small but statistically significant fluctuations in random variation of less than 0.01 mm were observed between the three measurements (p<0.001). The average random variation observed was 0.075+/-0.005 mm. The strongest, but not significant, correlation in random leaf position variation measurements was observed between the 2nd and 3rd measurement (figure 1).

Between the 2nd and 3rd measurement a small but statistical significant drift was measured, see figure 2.

The outliers observed after 55 days are coursed by a positional displacement of one leaf of less than 0.5 mm during one of the sequences. After 55 days the width of the time dependent drift distribution was found to be 0.08+/-0.05 mm Conclusion The leaf positioning accuracy of the optically controlled MLC is stable over time with time dependent drift and random variation below 0.1 mm, which is small compared to the EPID pixel size. These results support the use of the Elekta Unity MR linac for high precision stereotactic treatments E. Villaggi 1 , M. Esposito 1 , C. Marino 1 , V. Hernandez 2 , J. Saez 3 , A. Bruschi 1 , G. Borzì 1 , C. Carbonini 1 , R. Consorti 1 , D. Fedele 1 , E. Moretti 1 , B. Nardiello 1 , S. Russo 1 , M. Stasi 1 , P. Mancosu 1 1 SBRT Working Group, AIFM - Italian Association of Medical Physics, Italy, Italy 2 Hospital Universitari Sant Joan de Reus, Medical Physics, Tarragona, Spain 3 Hospital Clinic de Barcelona, Radiation Oncology Department, Barcelona, Spain Purpose or Objective Dosimetric and plan complexity quality metrics have been demonstrated to be essential for compliance assessment. Improving homogeneity between centers in multi- PO-1012 Can re-planning improve standardization? A multi-institutional SBRT prostate comparison