ESTRO 2020 Abstract Book

S92 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.

mean dose difference between calculated and measured output in the beam central axis, for rectangular fields with one side at 0.5 cm, and the other varying from 1 to 3 cm in a number of different spot size configurations.

The configuration with effective spot size X and Y set to 1.5 and 0 mm is recommended from the vendor. A better compromise in the here presented extreme conditions would suggest to set an X spot size smaller than the recommendation. Conclusion The effective spot size has a strong impact on the MU calculation for very small fields shaped by the MLC. The adjustment of those parameters according to accurate measurements is advisable. PD-0187 Converting absorbed dose-to-medium to dose-to-water in heterogeneous media A. Delbaere 1 , T. Younes 2 , M. Chauvin 1 , L. Simon 1,2 , C. Khamphan 3 , L. Vieillevigne 1,2 1 Centre de Recherche et de Cancérologie de Toulouse, UMR1037 INSERM - Université Toulouse 3 - ERL5294 CNRS, Toulouse, France ; 2 Institut Claudius Regaud - Institut Universitaire du Cancer de Toulouse, Department of Medical Physics, Toulouse, France ; 3 Institut Sainte Catherine, Department of Medical Physics, Avignon, France Purpose or Objective The conversion from absorbed dose-to-medium (D m ) to dose-to-water (D w ) usually based on the Bragg-Gray cavity theory has been recently discussed. Andreo 1,2 proposed to modify this theory by adding a correction factor to take into account the difference of the electron fluence between water and the various media. In Reynaert et al 3 , it was proposed to use average mass energy absorption coefficient ratios instead of average mass collisional stopping power ratios for bone. In this context, we investigated the absorbed dose calculated with Acuros XB (AXB) and Geant4/GATE 4 Monte Carlo (MC) in terms of dose-to-medium and dose-to-water. Material and Methods The study was carried out on heterogeneous phantoms constituted of a multilayer slab of low (lung) and high- density media (bone and PTFE). The AXB and GATE calculations were investigated and compared with experimental measurements using Gafchromic EBT3 (External Beam Therapy 3) films. These latter were chosen because they were supposed to generate low fluence perturbation due to their small thickness. Film measurements were also compared to D w obtained by applying the method conversion suggested by Andreo 1,2 and Reynaert et al 3 . Results For D m , a good agreement was found between AXB and MC for all configurations (±1%). For D w , AXB agreed with MC in lung (0.7%) whereas significant differences were observed in both bone (4.5%) and PTFE (3.5%). D w calculated by AXB and MC were greater than film measurements in high- density media: average discrepancies of 9% and 5% were observed in bone for AXB and MC, respectively. These differences decreased significantly (<1%) by applying the conversion method suggested by Andreo 1,2 or Reynaert et al 3 .

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