ESTRO 2025 - Abstract Book

S2944

Physics - Image acquisition and processing

ESTRO 2025

58

Digital Poster Evaluating the effects of field-of-view on stopping power ratio and proton therapy dose calculation Mananchaya Vimolmoch, Sornjarod Oonsiri, Puntiwa Oonsiri, Nichakan Chatchumnan, Sakda Kingkaew Division of Radiation Oncology, Department of Radiology, King Chulalongkorn Memorial Hospital, Bangkok, Thailand Purpose/Objective: CT simulators provide an extended field-of-view (eFOV) to ensure imaging covers all patient anatomy and immobilization devices beyond the scan FOV (sFOV). However, the accuracy of proton dose calculation using eFOV is challenging since it significantly depends on converting HU to a stopping power ratio (SPR). This study investigated the effect of eFOV on SPR and dose calculation in proton therapy. Material/Methods: All phantoms were scanned using GE Revolution CT, with images reconstructed into sFOV (50 cm) and eFOVs (60, 70, 80 cm). The HU-to-SPR calibration curves were generated for each FOV using the electron density phantom (062M, CIRS). To simulate objects outside the sFOV, the homogenous IMRT QA phantom (002H9K, CIRS) was positioned between two QUART phantoms. The Lung, bone, and water-equivalent plugs were scanned individually at the center of the phantom. For inhomogeneity simulations, the IMRT QA phantom was replaced with the electron density phantom. The SPR of various material plugs were compared between eFOVs using sFOV as a reference. Proton dose calculations were performed on six breast cancer patients whose body or immobilization extended beyond the sFOV. The prescription dose was 43.2 Gy and 52.8 Gy in 16 fractions. Proton plans were recalculated on sFOV and maximum eFOV CT images using Eclipse v16.1 with the PCS algorithm and IMPT technique (5mm/3.5% robust optimization). The dosimetric parameter was assessed by comparing dose differences and 3D gamma analysis Results: The differences in HU-to-SPR calibration curves between the sFOV and eFOVs were within 0.6% (Figure 1). The calibration curve obtained from the sFOV can be applied to other eFOV CT images. With the extended phantom, the IMRT QA phantom demonstrated SPR differences of 1.4%, 0.1%, and 0.2% for lung, bone, and water-equivalent material, respectively, across all extended FOVs. The inhomogeneous materials showed SPR variation within 1.0% for all materials, except lung tissue, which exhibited a 2.4% variation among eFOVs. Patient dose calculations showed an average dose difference within 0.6%±0.01 for CTV and 2.0%±0.03 for OARs. The 3D gamma results with criteria of 3%/2 mm, 2%/2 mm, and 1%/1 mm were within 99.2%, 99.2%, and 95.9%, respectively. The example of dose calculation is shown in Figure 2.