ESTRO 2020 Abstract Book

S239

ESTRO 2020

PD-0436 Dose simulations of a novel aluminium-alloy 3D range-modulator for proton therapy Y. Simeonov 1 , U. Weber 2 , C. Schuy 2 , P. Penchev 1 , R. Engenhart-Cabillic 3 , H. Krause 4 , J. Weißer 5 , K. Zink 1 1 Technische Hochschule Mittelhessen - IMPS, LSE, Gießen, Germany ; 2 GSI Helmholtzzentrum für Schwerionenforschung GmbH, Biophysics division, Darmstadt, Germany ; 3 University Hospital Giessen - Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany ; 4 PORTEC GmbH, Rapid Prototyping, Zella-Mehlis, Germany ; 5 Messtronik GmbH, Scan- Analyse- Reverse Engineering, St. Georgen- Langenschiltach, Germany Purpose or Objective The 3D range-modulator (3D RM), has been shown in previous studies to be a viable instrument for the very fast treatment of moving targets with a high degree of dose conformity and homogeneity. The use of a single energy leads to a significant decrease of irradiation time, thus mitigating interplay effects. While PMMA-like resin has demonstrated very good manufacturing accuracy and optimal dose distribution, utilising a metal alloy will improve the mechanical stability, the aspect ratio of the base structures and the overall robustness. It could also lead to better manufacturing accuracy and reproducibility. This work investigates the feasibility of an aluminium (Al) alloy 3D RM for a real tumour form and compares the simulated dose distribution with the one, previously obtained with a PMMA-like resin. Material and Methods Latest generation 3D metal printing technique has enabled the production of models with very high accuracy. Using a high-quality machine a test model with 3x3 fine pyramid- shaped base structures (pins), optimised for a 5cm SOBP, was manufactured (Fig. 1). 3D imaging with a µCT proved the high degree of production accuracy and high relative density of the test model, suggesting that the manufacturing of a complex 3D RM is possible. In order to keep the scattering in the RM within a limit, Al alloy was chosen. Its composition was implemented in the Monte Carlo (MC) code FLUKA and the specific material water equivalent factor was determined. A complex 3D RM (Fig. 1), previously designed for PMMA- like resin and 151.77 MeV 1 H, was scaled for the Al alloy. The modulator, developed on the basis of a patient CT data and a lung tumour, consists of many pins with a 3x3 mm 2 base area, whose heights and shapes are optimized and adjusted to the shape of the target. The FLUKA MC package was used to simulate the resulting dose distributions from the original PMMA-like RM and the newly optimised one. In order to mitigate the effect of increased lateral scattering in Al, the new RM was positioned 5 cm in front of the patient, as opposed to 20 cm for the PMMA one. Results There is very good agreement between the dose distributions produced by both modulators (Fig. 1). Fig. 2 shows a homogeneous dose distribution conformed to the distal and the proximal edge of the target. The dose distribution resulting from the Al RM does not show deterioration of the DVH or broader lateral penumbra.

At couch angle 0 o (for 10 different translations), mean ΔD POSITION in the lateral direction was 0.3 ± 0.2 mm (mean ± STD), whereas in the longitudinal direction -0.7 ± 0.1 mm, and 0.6 ± 0.1 mm in the vertical direction. For couch rotations of 60 o and 315 o and 6 different translations each, the measured mean deviations in the lateral, longitudinal and vertical directions were, respectively, -0.1 ± 0.2 mm, -0.3 ± 0.2 mm, -0.0 ± 0.1 mm, -0.1 ± 0.3 mm, -0.2 ± 0.4 mm, and -0.1 ± 0.3 mm. All measured rotational parameters (pitch, roll, and yaw) were below 0.2 o . When the heat signature of the phantom was turned off (at couch angle 0 o ), the mean deviation in any direction was always below 0.7 mm (mean). Figures 1 and 2 show the difference between the position assessed by the monoscopic and the stereoscopic localisation (ΔD X-RAY POS ) at couch angle 60 o in 10 different couch translations.

Conclusion No deviations larger than 1 mm between surface and X-ray based position were found at all evaluated couch angles. In addition, measuring on a phantom without a distinct heat signature does not affect the positioning accuracy of the surface based registration algorithm. Therefore, a distinct heated phantom may not be necessary for most regular routine QA measurements. The positioning error for monoscopic X-ray imaging increases with the deviations from the planned isocenter.