ESTRO 2020 Abstract book
S102 ESTRO 2020
For entities highly influenced by these effects we propose a method, in which a reference plan is being verified by a measurement and its verified recalculation seen as ground truth. Due to measurement restrictions for rectal cancer, the recalculation is in better agreement with the TPS calculation than the measurement.
dose distribution to the artifact-free baseline scan. The proton plans copied onto the OMAR and the uncorrected scans presented an underdosing of the target in the posterior region, further represented by the 77.9% and 84.7% of prescribed target dose coverage respectively. In comparison, the plans on the baseline and AMPP presented comparable prescribed target coverages. PD-0185 Development of a Monte-Carlo head model for a fast online validation of 1.5 T MR-linac plans M. Nachbar 1 , O. Dohm 2 , M. Friedlein 3 , J. Winter 1 , D. Mönnich 1 , D. Zips 2 , D. Thorwarth 3 1 University Hospital Tübingen, Section for Biomedical Physics- Department of Radiation Oncology, Tuebingen, Germany ; 2 University Hospital Tübingen, Department of Radiation Oncology, Tuebingen, Germany ; 3 University Hospital Tübingen, Section for Biomedical Physics, Tuebingen, Germany Purpose or Objective MR-linac (MRL) systems are a promising technology in which online plan adaptation during treatment is required. For a fast control of new plan characteristics, we developed an automatic verification routine using a Monte- Carlo (MC) head model for the 1.5 T MRL (Unity, Elekta AB, Stockholm, Sweden) which neglects the influence of the magnetic field. In this work we evaluate the physical differences and validate the clinical usability of the proposed method by comparison against experimental plan verification. Material and Methods An independent MC head model for the 1.5 T MRL was developed in our in-house treatment planning software Hyperion. It was designed based on measurements of the MRL and evaluated based on PDD, square fields and output factors (OF) without the magnetic field effect (B = 0 T). In the automatic online verification, Monaco (Elekta AB, Stockholm, Sweden) dose distributions (3 mm grid, MC- uncertainty 1%) are compared with a global γ-analysis (6mm/3%) to the recalculated dose (3 mm grid, MC- uncertainty 5 %). The γ-values are calculated evaluating all voxel doses >40% D max . To benchmark this fast online procedure against experimental plan QA, we compare it for n=100 IMRT plans against measurements evaluated with a local γ-criterion of 3mm/3% for 6 different tumor entities (table 1). Additionally, interpolation of the grid to 1mm is performed to compare dose distributions and measurements with a global γ-criterion of 3mm/3%. Results The developed MRL head model (B = 0 T) shows reasonably good agreement with the measured MRL beam data (B = 1.5 T) with mean absolute differences of 1.6% (inplane), 3.1 % (crossplane) and 2.2 % (PDD) in a 10 x 10 cm 2 field. Figure 1 shows OFs and PDD, cross- and inplane profiles of an exemplary 10 x 10 cm 2 field in comparison to the TPS (B = 1.5 T). Differences are visible due to the magnetic field effects (shorter build-up region, shifted profile in crossplane direction). The analysis shows a good mean agreement over all plans between the TPS and the measurement (98.67 %), the original voxel grid (97.34 %) and the interpolated grid (95.58 %). The mean calculation time above all entities was 01:27 min. Additional entity specific analyses are depicted in table 1. Based on a two-sided wilcoxon rank test only the measured data in mamma, H&N and rectal cancer was significantly different (p < 0.01) compared to the MC-based recalculations. Conclusion In this work we propose a fast method for an online verification of treatment plans, showing good agreement between measurement and calculation for entities treated with multiple beam angles and little air-tissue interfaces.
PD-0186 Impact of effective spot size parameter on MU calculation of Acuros algorithm in small MLC fields A. Fogliata 1 , A. Girardi 2 , L. Cozzi 3 , A. Stravato 1 , G. Reggiori 1 , M. Scorsetti 3 1 Humanitas Research Hospital, Department of Radiation Oncology, Milan-Rozzano, Italy ; 2 Universitair Ziekenhuis Brussel- Vrije Universiteit Brussel, Department of Radiotherapy, Brussels, Belgium ; 3 Humanitas Research Hospital and Humanitas University, Radiation Oncology and Faculty of Medical Science, Milan-Rozzano, Italy Purpose or Objective To evaluate the “effective spot size” value to configure Acuros dose calculation algorithm in the Eclipse TPS (Varian) for small fields shaped by the HD-MLC for 6MV, flattened and flattening filter free (FFF) beams. Material and Methods Absorbed dose delivered by small fields, from 0.5x0.5 cm 2 to 4x4 cm 2 , square and rectangular, shaped by an HD-MLC (2.5 mm leaf width at isocentre) for 6 MV and 6FFF MV from a Varian TrueBeam STx linac were measured. Jaws were set at 4x4 cm 2 in all cases. Measurements were acquired with an Exradin W1 scintillator detector (Standard Imaging) in a 3DS water phantom (Sun Nuclear corporation), at SSD=100 cm and 10 cm depth. The same doses were estimated by Acuros calculations in the same conditions, by varying the effective spot size parameters, in the X and Y directions, from 0 to 2 mm in steps of 0.5 mm, for a total of 25 configurations for each selected energy. All calculations were at 1 mm grid size in all the directions. The dosimetric leaf gap was set, for all the configurations, to 0.070 and 0.031 mm for 6 and 6FFF MV, respectively. Comparison between measurements and calculations for the above field sizes range was conducted to obtain the best effective spot size parameter to set. Results Calculated doses for square fields from 1x1 cm 2 and larger agreed with measurements within 1.5% and 2.5% for 6 and 6FFF MV, respectively, in all configurations with spot sizes less than 2 mm. Variations were more evident in rectangular small fields. The following table reports the
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