ESTRO 2021 Abstract Book

S737

ESTRO 2021

simple model of the leaf tip that was able to approximate the trailing effect and improved the agreement with measured doses. In particular, with a prototype version of RayStation (11P) that assigned a higher transmission at the leaf tip the agreement with measured doses was within ±3% even for the 5 mm gap. The five Halcyon systems behaved very similarly but differences in the DLG around 0.2 mm were found across different treatment units and between MLC layers from the same system. The DLG for the proximal layer was consistently higher than for the distal layer, with differences ranging between 0.10 mm and 0.24 mm. Conclusion The trailing distance between the leaves from different layers substantially affected the doses delivered by sweeping gaps and the measured DLG values. Stacked MLCs introduce a new level of complexity in TPSs, which ideally need to implement an explicit model of the leaf tip in order to reproduce the trailing effect. Dynamic tests called ‘trailing sweeping gaps' were designed that are useful for characterizing and commissioning dual- layer MLC systems. PD-0897 Experimental validation of absorbed dose-to-medium reporting algorithms in heterogeneous media A. Delbaere 1,2 , T. Younes 1,2,3 , M. Chauvin 1 , J. Labour 1,2 , V. Fonteny 1,2 , L. Simon 1,2 , G. Fares 3 , L. Vieillevigne 1,2 1 Centre de Recherches en 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 Laboratoire de ‘Mathématiques et Applications’, Unité de recherche ‘Mathématiques et Modélisation’, Centre d’analyses et de recherche, Faculté des sciences, Université Saint- Joseph, Beyrouth, Lebanon Purpose or Objective Dose calculation algorithms that report absorbed dose-to-medium are increasingly being implemented in the treatment planning systems in clinical routine. One of the main issues is to experimentally validate this reporting dose mode. At this stage, all international dosimetry protocols in broad and small megavoltage photon fields are based on absorbed dose-to-water. This work was divided into two parts: (i) to understand the physical aspect of detector response in heterogeneous media (ii) to develop a new formalism for the experimental validation of algorithms that report absorbed dose-to-medium in megavoltage photon beams using Monte Carlo method. Materials and Methods Fluence spectra were computed using GATE/Geant4 Monte Carlo code in the sensitive volume of two simulated detectors (T31016 PinPoint 3D ionization chamber and EBT3 radiochromic film) placed in different media (water, RW3, lung and bone) and were compared to those computed in the undisturbed media. Then, Monte Carlo absorbed dose-to-medium simulations were compared with the T31016 PinPoint 3D and EBT3 radiochromic film measurements in heterogeneous phantoms constituted of a multilayer slab of RW3, lung and bone equivalent media. We determined a heterogeneity correction factor h that takes into account the difference between the detector perturbation in the medium and under reference conditions and was calculated for both detectors. Results It was pointed out that the total electron (+positron) fluence perturbations caused by both detectors in RW3 and lung were close to that in water (≤1.5%). By contrast, the perturbation in bone was greater to that in water ( ∼ 4%) and modified both the magnitude and the shape of the fluence spectra. Hence, it was emphasized that detectors readings should be corrected by the h factor that ranged from 0.932 in bone to 0.985 in lung. Conclusion This formalism introduces the concept of applying a heterogeneity correction factor to detectors readings when the perturbation in the heterogeneous medium is different from that in water under reference conditions. Further studies on the determination of this factor for different detectors, media, beam qualities and field sizes are still ongoing. PD-0898 Experimental determination of detector specific lateral dose response functions in proton beams J. Kretschmer 1,2 , L. Brodbek 1,2 , H.K. Looe 1 , E. van der Graaf 2 , M.J. van Goethem 2 , H. Kiewiet 2 , C. Meyer 2 , B. Poppe 1 , S. Brandenburg 2 1 Carl-von-Ossietzky University Oldenburg, University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Oldenburg, Germany; 2 University of Groningen, Department of Radiation Oncology, University Medical Center Groningen, Groningen, The Netherlands Purpose or Objective Dose measurements using an extended point detector may be perturbed by the volume effect, which inherits on the one hand the volume-averaging effect caused by the extended sensitive volume of the detector; and on the other hand the disturbance of the charged particle fluence caused by the non-water-equivalent detector components. The volume effect can be characterized by lateral dose response functions K ( x,y ) acting as the convolution kernel transforming the dose profile D ( x,y ) into the measured signal profile M ( x,y ) according to equation 1 (Looe et al. PMB 60 (2015) 6585). As a result of the volume effect, M ( x,y ) is perturbed in the penumbra regions in relative profile measurements, as well as along the central axis in output measurements of small fields. The aim of this work is to experimentally determine K ( x,y ) for five point detectors in proton beams as they can be used to derive correction strategies for the above-mentioned perturbation effects.

Materials and Methods