ESTRO 2022 - Abstract Book

S1419

Abstract book

ESTRO 2022

Compared to the full FOV acquisition, a decrease in imaging dose was observed for each of the four VOI cases in the anteroposterior direction (see Figure 2). Imaging dose decreased with VOI dimension, with dose reductions within the VOI ranging from 7% to 20%, compared to the full FOV case. Outside of the VOI, imaging doses decreased to 23%, 20%, 15% and 12% of the full FOV maximum dose.

Conclusion This study demonstrates the dose sparing capabilities of 2.5 MV low-Z VOI CBCT. Compared to the full FOV acquisition, imaging dose within the VOI decreased by up to 20% by collimating the beam with the MLCs. This dose reduction results from decreased scatter in the treatment head and within the phantom compared to the full FOV case. While this study focused on VOI acquisition using a standard EPID, it is anticipated that imaging doses could be further reduced using an efficient multilayer imager in combination with this technique.

PO-1628 Evaluation of material properties retrieved with a radiotherapy specific CT scanner

E. Pettersson 1,4 , A. Lindberg 2 , N. Drugge 3 , A. Bäck 3,5

1 The Sahlgrenska University Hospital, Department of Therapeutic Radiation Phyiscs, Gothenburg, Sweden; 2 The Sahlgrenska University Hospital, Department of Therapeutic Radiation Physics, Gothenburg, Sweden; 3 The Sahlgrenska University Hospital, Department of Therapeutic Radiation Physics, Gothenburg, Sweden; 4 Institute of Clinical Sciences, Sahlgrenska Academy at the University of Gothenburg, Department of Medical Radiation Sciences, Gothenburg, Sweden; 5 Institute of Clinical Sciences, Sahlgrenska Academy at the University of Gothenburg, , Department of Medical Radiation Sciences, Gothenburg, Sweden Purpose or Objective CT images are commonly used for modelling of the patient for the absorbed dose calculation in external beam radiotherapy. The dose calculation is sensitive to variations in density and effective atomic number (EAN) of the tissue. The mass density (MD) and relative electron density (RED) images are usually established from CT-numbers (Hounsfield units) using pre- defined CT-calibration curves. New techniques are now available where information about material properties can be provided directly from the CT software. This study compares MD and RED obtained with single-energy CT (SECT) as well as RED and EAN obtained with dual-energy CT (DECT) to theoretical reference values for materials with known MDs and elemental compositions. Materials and Methods The head sections of two electron density phantoms (Model 1467, Sun Nuclear, SN) and (062M, CIRS) were placed inline in the holder belonging to the first phantom (Figure 1). The SN phantom was setup with its tissue surrogate rods, and a cylindrical container with 5 mg/cm 3 iodine solution. The CIRS phantom was setup with its tissue surrogates, as well as some non-tissue equivalent materials (Table 1). The phantom setup was scanned with a radiotherapy specific CT scanner ( SOMATOM go.Open Pro , Siemens Healthineers) with a SECT (120 kV) and a dual-spiral DECT (80 kV/Sn140 kV) protocol. The SECT images were reconstructed with the Sd40 and Sm40 ( DirectDensity ™ (DD), Siemens Healthineers) kernels which provide RED and MD images, respectively. The RED was also estimated from conventional SECT images (Qr40 kernel) in a more traditional way using a pre-defined CT-calibration curve created based on the CIRS phantom. The DECT images were converted to RED and EAN images in the Rho/Z application in ( syngo.via.VB50A , Siemens Healthineers). The values obtained with the different methods were compared to the reference MD and theoretical reference RED and EAN of the phantom inserts.