Proton dose calculation on scatter‐corrected CBCT image: Feasibility study for adaptive proton therapy

<jats:sec><jats:title>Purpose:</jats:title><jats:p>To demonstrate the feasibility of proton dose calculation on scatter‐corrected cone‐beam computed tomographic (CBCT) images for the purpose of adaptive proton therapy.</jats:p></jats:sec><jats:sec><jats:title>Methods:</jats:title><jats:p>CBCT projection images were acquired from anthropomorphic phantoms and a prostate patient using an on‐board imaging system of an Elekta infinity linear accelerator. Two previously introduced techniques were used to correct the scattered x‐rays in the raw projection images: uniform scatter correction (CBCT<jats:sub>us</jats:sub>) and <jats:italic>a priori</jats:italic> CT‐based scatter correction (CBCT<jats:sub>ap</jats:sub>). CBCT images were reconstructed using a standard FDK algorithm and GPU‐based reconstruction toolkit. Soft tissue ROI‐based HU shifting was used to improve HU accuracy of the uncorrected CBCT images and CBCT<jats:sub>us</jats:sub>, while no HU change was applied to the CBCT<jats:sub>ap</jats:sub>. The degree of equivalence of the corrected CBCT images with respect to the reference CT image (CT<jats:sub>ref</jats:sub>) was evaluated by using angular profiles of water equivalent path length (WEPL) and passively scattered proton treatment plans. The CBCT<jats:sub>ap</jats:sub> was further evaluated in more realistic scenarios such as rectal filling and weight loss to assess the effect of mismatched prior information on the corrected images.</jats:p></jats:sec><jats:sec><jats:title>Results:</jats:title><jats:p>The uncorrected CBCT and CBCT<jats:sub>us</jats:sub> images demonstrated substantial WEPL discrepancies (7.3 ± 5.3 mm and 11.1 ± 6.6 mm, respectively) with respect to the CT<jats:sub>ref</jats:sub>, while the CBCT<jats:sub>ap</jats:sub> images showed substantially reduced WEPL errors (2.4 ± 2.0 mm). Similarly, the CBCT<jats:sub>ap</jats:sub>‐based treatment plans demonstrated a high pass rate (96.0% ± 2.5% in 2 mm/2% criteria) in a 3D gamma analysis.</jats:p></jats:sec><jats:sec><jats:title>Conclusions:</jats:title><jats:p><jats:italic>A priori</jats:italic> CT‐based scatter correction technique was shown to be promising for adaptive proton therapy, as it achieved equivalent proton dose distributions and water equivalent path lengths compared to those of a reference CT in a selection of anthropomorphic phantoms.</jats:p></jats:sec>