ESTRO 2023 - Abstract Book

S1532

Digital Posters

ESTRO 2023

Purpose or Objective The aim of the present study is to investigate the effect of tumor motion on various imaging strategies as well as treatment plan accuracy for lung stereotactic body radiotherapy treatment (SBRT) cases. Materials and Methods Materials and methods: The ExacTrac gating phantom and paraffin were used to represent respiratory motion and lung tumor. Four dimensional computed tomography (4DCT) imaging was performed while the phantom was moving sinusoidally with 4 sec cycling time with three different amplitudes of 8,16 and 24 mm. The reconstructions were done with maximum(MIP) and average intensity projection(AIP) methods. The density of target and volume comparisons were performed between two reconstruction techniques and references values. Volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy(IMRT) were planned on reconstructed computed tomography (CT) sets and it was examined how density variation affects the dose volume histograms (DVH) parameters. The 4D cone beam computed tomography (CBCT) acquisitions were performed with Elekta Versa HD linac imaging system before irradiation and compared with 3D CBCT. Thus, various combinations of 4D CT reconstruction methods and treatment alignment methods have been investigated. The point, 2D, and 3D dose measurements were done by optically stimulated luminescence (OSL), gafchromic film, and electronic portal imaging devices (EPID), respectively. Results The mean volume reduction was 7.8% for the AIP and 2.6% for the MIP methods. The obtained Hounsfield Unit (HU) values were lower for AIP and higher for the MIP technique when compared with the reference volume density. In DVH analysis, there were no statistical differences for D95%, D98%, and Dmean (p>0.05). However, D2% was significantly affected by HU changes(p<0.01). The positional variation was obtained up to 2 mm in moving direction when 4D CBCT was applied after 3D CBCT. Dosimetric measurements showed that the main part of the dose deviation was due to movement. Conclusion In lung SBRT treatment plans, D2% doses differ significantly according to the reconstruction method. Additionally, it has been observed that setups based on 3D imaging can cause a positional error of up to 2 mm compared to setups based on 4D imaging.

PO-1807 Which carbon couch model can better quantify the influence on radiation dose?

L. Nesterenko 1 , M. Berg 1 , B. Mortensen 1 , J.S. Rasmussen 1

1 Hospital Lillebælt, Medical Physics, Vejle, Denmark

Purpose or Objective Carbon couch tops are commonly used for patient support in external beam radiotherapy (RT). Accurate couch modeling in the treatment planning system (TPS) and validation of the dosimetric impact of the couch is important to deliver accurate RT. The purpose of this study is to find a model, which can quantify the carbon couch influence on radiation dose sufficiently. Materials and Methods The iBEAM® evo couch top HP is a sandwich design couch, consisting of a Rohacell IG 71 foam core and surrounded by carbon fiber layers. Three models of couch layers were contoured in the TPS (RayStation, RaySearch Laboratories AB). The mass densities (MD) were assigned based on Hounsfield Units (HU). Model 1 is a uniform model (MD of 0.152 g/cm3), model 2 consists of the outer carbon fiber layers (MD of 0.428 g/cm3) and inner foam core (MD of 0.089 g/cm3) while model 3 also assigns MD (0.732 g/cm3) to the lateral edges, where carbon fiber layers match each other and the ratio of carbon fiber to epoxy is higher due to monolithic compression in this area [Fig. 1]. Finally, model 4 is based on HU only. The attenuation (ATT) of photon beams (6MV, 10MV, and 10MV FFF energies) by the couch top was measured and compared to the calculated data in the TPS for all four models. Three pelvic plans with different models of the couch were recalculated, and changes in PTV coverage and OAR dose were analyzed.