ESTRO 2020 Abstract book

S815 ESTRO 2020

PO-1439 Comparison of two algorithms for leaf motion calculation in eclipse treatment planning system A. Seoane 1 , D. Sánchez-Artuñedo 1 1 Hospital Universitario Vall d'Hebron, Medical Physics Dpt, Barcelona, Spain Purpose or Objective Eclipse treatment planning system (v15.6, Varian Medical Systems) includes two algorithms that convert fluence into leaf movement for IMRT fields. Varian reports a reduction of MU and less tongue and groove effect using the newer algorithm Smart LMC (SLMC) in comparison to the algorithm Varian Leaf Motion Calculator (VLMC). The aim of this work is to compare the MU and the leaf sequence of IMRT fields these two algorithms. Material and Methods Sixteen IMRT breast plans, with a total of 79 fields, were optimized using PO (v 15.6. 04). All the plans were calculated with 6 MV photons and a dose rate of 600 MU/min. The dosimetric leaf gap configured in Eclipse was 2 mm. The optimal fluence was converted into leaf motion of a MLC Millenium 120 using SLMC and VLMC. When leaves motion travel exceeds a maximum value fixed by manufacturer, the fields are split into two or more subfields. In our patients, this effect turned out in 123 fields with VLMC and 117 with SLMC. The following parameters were evaluated for the two algorithms: 1. Monitor units of the 79 fields. 2. For every split field and every control point, the leaf gap and tongue-and-groove was calculated according to [1]: - The leaf gap was calculated as the distance between the leaf-ends of every pair of leaves. - Tongue-and-groove of every leaf pair was obtained as the difference between the position of a leaf and the two adjacent leaves. More details can be found in [1]. Closed leaf pairs were excluded from the analysis. 3. A complexity index to describe the distribution of leaves speed over control points (MI s ), and one to consider not only the leaves speed but also the acceleration (MI a ). Further details are described in [2]. As the fluence modulation increases, so do the values of the indices. Statistical analysis of the parameters derived for VLMC and SLMC were assessed using a Wilcoxon-Mann-Whitney test. Results • Although a mean MU reduction of -6.9% was obtained using SLMC, no statistical difference was obtained.

Conclusion Smart LMC algorithm results in a reduction of leaf gap and less change in leaf accelerations, but an increase in tongue-and-groove. A MU reduction was observed for this algorithm, although it was not statistical significant. References PO-1440 Influence of heterogeneities on the dose calculation accuracy in proton beam therapy S. Ruangchan 1,2 , B. Knäusl 1,3 , H. Fuchs 1,3 , D. Georg 1 , M. Clausen 1 1 Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria ; 2 King Chulalongkorn Memorial Hospital, Department of Radiology, Bangkok, Thailand ; 3 EBG MedAustron GmbH, Medical Department, Wiener Neustadt, Austria Purpose or Objective The treatment planning system RayStation (TPS, RaySearch Laboratories, Sweden) offers two clinical dose calculation algorithms for scanned proton beam therapy, i.e. the Pencil Beam (RS-PB) and Monte Carlo (RS-MC) algorithms. The purpose of this study was to experimentally validate these two algorithms against measurements for complex geometries in general and for the following tissue interfaces specifically: bone-lung, bone-soft tissue. These represent clinical scenarios for the head, lung, and head and neck patients. Besides validating the algorithm’s performance in the treated volume, the dose calculation accuracy was studied in surrounding non- To mimic complex geometries, tissue-equivalent slabs (bone-lung or bone-soft tissue) were enclosed in a watertight holder and placed side by side into a water phantom. Respective treatment plans were generated in the TPS for a target volume of 4x4x4 cm 3 , which was located behind the tissue interfaces. Treatment plan optimization was performed using the RS-MC algorithm, and all plans were subsequently recalculated with the RS- PB algorithm. Dose measurements within and behind the target were performed in a water phantom using a 3D holder with 24 PinPoint chambers (T31015, PTW, Germany) and compared to the dose calculations. For various heterogeneous test geometries, two different target depths (SOBP1 and SOBP2), an oblique beam incidence of 30 degrees as well as the presence of a range shifter were investigated. Results For both the RS-PB and RS-MC algorithms, good agreement with dosimetric measurements was found for points of target tissue volumes. Material and Methods [1] YAO et al., JACMP. 16(4) (2015) [2] PARK, Phys.Med.Biol. 59. (2014)

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The number of control points obtained with SMLC is fixed, 166 control point per field or 83 per split-field. Conversely, the number of control points in VLMC fields is variable. Because of that, the number of leaf pairs analyzed (N) differs between both algorithms. Leaf gap resulted to be larger for VLMC while tongue and groove turned out to be smaller, being significant for both parameters (table).

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No statistical significant differences were found in MI s . Graph shows statistical significant differences (p<0.001) of MI a using SMLC (mean=31.3) instead of VLMC (mean=43.2).

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