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

Abstract

Purpose: To demonstrate the feasibility of proton dose calculation on scatter-corrected cone-beam computed tomographic (CBCT) images for the purpose of adaptive proton therapy. Methods: 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 us ) and a priori CT-based scatter correction (CBCT ap ). 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 us , while no HU change was applied to the CBCT ap . The degree of equivalence of the corrected CBCT images with respect to the reference CT image (CT ref ) was evaluated by using angular profiles of water equivalent path length (WEPL) and passively scattered proton treatment plans. The CBCT ap 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. Results: The uncorrected CBCT and CBCT us images demonstrated substantial WEPL discrepancies (7.3 ± 5.3 mm and 11.1 ± 6.6 mm, respectively) with respect to the CT ref , while the CBCT ap images showed substantially reduced WEPL errors (2.4 ± 2.0 mm). Similarly, the CBCT ap -based treatment plans demonstrated a high pass rate (96.0% ± 2.5% in 2 mm/2% criteria) in a 3D gamma analysis. Conclusions: A priori 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. C 2015 American Association of Physicists in Medicine. [http://dx

ScatterNet: A convolutional neural network for cone‐beam CT intensity correction

“…Due to the considerable interfractional anatomical differences between pCT and daily CBCT in the prostate region, CBCT was used as reference for data evaluation. The accuracy of CBCT has been demonstrated in earlier studies, even in the context of proton therapy 24,25 …”

“…For this, the vCT CT numbers (in HU ) are previously converted to attenuation coefficients μ as in Ref. [24]: . are converted to intensity as follows:and are assumed to be free of scatter and other deviations. The low spatial frequency deviations in the measured projections [Fig.…”

“…In contrast to the latter work, our study was based solely on measured clinical data in the pelvic region. Low‐frequency deviations of the measured projections, such as the detection of scatter and beam hardening, were estimated by using a prior virtual CT (vCT), retrieved from deformable image registration (DIR) of the planning CT to the CBCT 22–25 . Moreover, we provide for the first time a comprehensive evaluation of the dosimetric accuracy of the ScatterNet CBCT in the scope of photon and proton radiotherapy.…”

Current state and future applications of radiological image guidance for particle therapy

“…(c,d,e) Dose distributions for a single field uniform dose ( SFUD ) adapted plan using IMPT . (c,d) The doses were recomputed on CBCT images corrected using (c) deformable image registration ( DIR ) of the planning CT to the daily CBCT image and (d) using the result of the deformation as prior information used to estimate a scatter correction . In (e) an in‐room equivalent control CT acquired 1 day after the CBCT was used to calculate the adapted plan and serves as reference.…”

“…In the last 2 yr, several groups have proposed methods making use of the prior information available from the planning CT scan and deformable image registration to correct CBCT data to allow either WET or dose distribution computation, [59][60][61][62][63][64][65] achieving an accuracy of about 2% relative to planning CT. 62 The in-room CT or corrected CBCT data may also be used for treatment plan adaptation 66 to correct for gross anatomical changes such as weight loss, bowel filling variations or organ motion accompanied by large WET deviations. An example of two CBCT correction methods and potential use of in-room volumetric data for adaptive particle therapy is shown in Fig.…”