Impact of extended field-of-view on image quality and stopping power ratio in proton therapy dose calculation

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Abstract

CT simulators offer an extended field-of-view (eFOV) to include immobilization devices and anatomy beyond the scan FOV (sFOV). This study evaluated its impact on image quality, stopping power ratio (SPR), and proton dose calculations. CT images were reconstructed into the sFOV and three eFOVs. Image quality was assessed using the CATPHAN phantom. Hounsfield unit (HU)-to-SPR curves were generated using the stoichiometric method. HU and SPR variations were evaluated using homogeneous and heterogeneous phantoms extending beyond the sFOV. Proton dose calculations using 50-cm sFOV and 80-cm eFOV images were compared in breast and lung cases using dose differences and three-dimensional gamma analysis. Geometric distortion remained below 0.5 mm at the center but increased to 1.7 mm near the FOV edge. Spatial resolution, uniformity, and contrast were comparable across FOVs. HU-to-SPR calibration curves differed by ≤0.6%. Homogeneous phantom SPR differences were 1.4%, 0.1%, and 0.2% for lung-, bone-, and water-equivalent materials, respectively. Heterogeneous phantom SPR differences were within 1% for most materials and reached 2.4% for lung-equivalent material. Mean clinical target volume dose differences were 0.6% for breast and 0.8% for lung cases; organ-at-risk dose differences were within 2.0% and 3.2%, respectively. Gamma pass rates exceeded 99%, 98%, and 92% for 3%/2 mm, 2%/2 mm, and 1%/1 mm criteria, respectively.Extended FOV images were reliable for proton dose planning when the treatment area was centered in the scanning field.

Abstract as published, via PubMed.