ESTRO 2023 - Abstract Book

S1464

Digital Posters

ESTRO 2023

system. Finally, we tested the applicability of the system to facilitate gated beam deliveries using the Dynamic Platform model 008PL in combination with surface guidance. Results Using global gamma 3 % / 3 mm at 10 % dose threshold, delivered clinical plans had passing rates higher than 98 % after implementing a homogeneous density overwrite for the anthropomorphic surrogate within the TPS. The VisionRT system correctly tracked the misalignment of the setup within sub-millimeter accuracy in all directions, indicating the suitability of the surrogate towards enhancing surface guidance QA. Gated beam deliveries were also feasible with the surrogate in place.

Figure 1: (a) Anthropomorphic surrogate mounted on StereoPHAN and (b) Setup on motion platform, prior to surrogate placement, to investigate SGRT and gated dose deliveries. Conclusion The anthropomorphic surrogate enhances the use of the StereoPHAN in combination with the SRS MapCHECK towards quality assurance and end-to-end tests in stereotactic deliveries. The excellent detectability of the surrogate extends the applicability towards surface guidance and gated radiotherapy deliveries.

PO-1744 Multicentric characterisation of lateral beam profiles from 6 MV FFF beam of three 0.35T MR-linacs

A. Fogliata 1 , R. El Gawhary 2 , D. Cusumano 3 , L. Placidi 4 , F. Quaranta 3 , M. Nardini 4 , M. Rago 2 , L. Indovina 4 , S. Menna 3

1 Humanitas Research Hospital IRCCS, Radiotherapy, Rozzano-Milan, Italy; 2 San Pietro FBF Hospital, Radiotherapy, Rome, Italy; 3 Mater Olbia Hospital, Radiotherapy, Olbia, Italy; 4 Fondazione Policlinico Universitario Agostino Gemelli IRCCS , Radiotherapy, Rome, Italy Purpose or Objective The advent of MR-guided radiotherapy technologies, combining flattening filter free (FFF) linear accelerators with on-board MR scanners, opened new clinical opportunities and physical challenges. The physical characterisation of FFF beam profiles in presence of a magnetic field is a new and relevant topic in the field of radiation dosimetry, for which new standardisation procedures and formulation are needed. The aim of this multicentric study is to propose new normalisation factors by analysing such lateral beam profiles, to allow for the calculation of standard parameters typical of flattened beams, such as penumbra and symmetry. Materials and Methods The experimental measurements were carried out on three 0.35 T MRgRT MRIdian (ViewRay) systems. In all the Institutions an equal set of measurements was acquired using the same equipment, consisting of a 3D motorised water phantom (Tales 3D MR, LAP) and a 0.125 cc vented ion chamber with an inner diameter of 5.8 mm (Exradin A28MR). Lateral profiles, in the direction perpendicular to the magnetic field, of the 6 MV FFF beam were measured with fixed fine step resolution at seven different depths: 2, 5.5, 10, 15, 20, 25 and 28.5 cm. For each depth, six beam dimensions were acquired: 4.15x4.15, 6.64x6.64, 9.96x9.96, 14.94x14.94, 19.92x19.92 and 24.07x24.07 cm2. All the measurements were carried out at a source surface distance (SSD) of 85 cm. The integration time was adjusted to balance the time for measuring and the noise reduction, and a reference detector was used during all the experimental measurements. A post-processing procedure consisted of data smoothing within the in-field region, beam centering and mirroring. The position of the inflection point, minimum and maximum of the second derivative in the fall-off region, and the extremes of the third derivative were calculated. The profile normalization was determined by imposing the 50% dose level at the inflection point. The fitting parameters describing the renormalization considering the following formula were estimated: where FS is the field (for square fields) in cm, depth is the measuring depth in mm, and a, b, c, d, e are the fitting parameters. Results The position of the inflection point averaged over the three datasets, for all the fields and depths, is reported in Table 1, together with the percentage dose value relative to the central axis at the same inflection point. Renorm=(a+b·FS+c·depth)/(1+d·FS+e·depth)