Daily Cone-beam Computed Tomography Images Research Articles

Setup Reproducibility for Thoracic and Upper Gastrointestinal Radiation Therapy: Influence of Immobilization Method and On-Line Cone-Beam CT Guidance

We report the setup reproducibility of thoracic and upper gastrointestinal (UGI) radiotherapy (RT) patients for 2 immobilization methods evaluated through cone-beam computed tomography (CBCT) image guidance, and present planning target volume (PTV) margin calculations made on the basis of these observations. Daily CBCT images from 65 patients immobilized in a chestboard (CB) or evacuated cushion (EC) were registered to the planning CT using automatic bony anatomy registration. The standardized region-of-interest for matching was focused around vertebral bodies adjacent to tumor location. Discrepancies >3 mm between the CBCT and CT datasets were corrected before initiation of RT and verified with a second CBCT to assess residual error (usually taken after 90 s of the initial CBCT). Positional data were analyzed to evaluate the magnitude and frequencies of setup errors before and after correction. The setup distributions were slightly different for the CB (797 scans) and EC (757 scans) methods, and the probability of adjustment at a 3-mm action threshold was not significantly different ( p = 0.47). Setup displacements >10 mm in any direction were observed in 10% of CB fractions and 16% of EC fractions ( p = 0.0008). Residual error distributions after CBCT guidance were equivalent regardless of immobilization method. Using a published formula, the PTV margins for the CB were L/R, 3.3 mm; S/I, 3.5 mm; and A/P, 4.6 mm), and for EC they were L/R, 3.7 mm; S/I, 3.3 mm; and A/P, 4.6 mm. In the absence of image guidance, the CB slightly outperformed the EC in precision. CBCT allows reduction to a single immobilization system that can be chosen for efficiency, logistics, and cost. Image guidance allows for increased geometric precision and accuracy and supports a corresponding reduction in PTV margin.

Multiscale registration of planning CT and daily cone beam CT images for adaptive radiation therapy

Adaptive radiation therapy (ART) is the incorporation of daily images in the radiotherapy treatment process so that the treatment plan can be evaluated and modified to maximize the amount of radiation dose to the tumor while minimizing the amount of radiation delivered to healthy tissue. Registration of planning images with daily images is thus an important component of ART. In this article, the authors report their research on multiscale registration of planning computed tomography (CT) images with daily cone beam CT (CBCT) images. The multiscale algorithm is based on the hierarchical multiscale image decomposition of E. Tadmor, S. Nezzar, and L. Vese [Multiscale Model. Simul. 2(4), pp. 554-579 (2004)]. Registration is achieved by decomposing the images to be registered into a series of scales using the (BV, L2) decomposition and initially registering the coarsest scales of the image using a landmark-based registration algorithm. The resulting transformation is then used as a starting point to deformably register the next coarse scales with one another. This procedure is iterated at each stage using the transformation computed by the previous scale registration as the starting point for the current registration. The authors present the results of studies of rectum, head-neck, and prostate CT-CBCT registration, and validate their registration method quantitatively using synthetic results in which the exact transformations our known, and qualitatively using clinical deformations in which the exact results are not known.

Analysis of Interfraction Prostate Motion Using Megavoltage Cone Beam Computed Tomography

Determine the degree of interfraction prostate motion and its components measured by using daily megavoltage (MV) cone beam computed tomography (CBCT) imaging. A total of 984 daily MV CBCT images from 24 patients undergoing definitive intensity-modulated radiotherapy for localized prostate cancer were analyzed retrospectively. Pretreatment couch shifts, based on physician registration of MV CBCT to planning CT data sets, were used as a measure of daily interfraction motion. Off-line bony registration was performed to separate bony misalignment from internal organ motion. Interobserver and intraobserver variation studies were performed on 20 MV CBCT images. Mean interfraction prostate motion was 6.7 mm, with the greatest single-axis deviation in the anterior-posterior (AP) direction. The largest positional inaccuracy was accounted for by systematic deviations in bony misalignment, whereas random deviations occurred from bony misalignment and internal prostate motion. In the aggregate, AP motion did not correlate with days elapsed since beginning therapy or on average with rectal size at treatment planning. Interobserver variation was greatest in the AP direction, decreased in experienced observers, and further decreased in intraobserver studies. Mean interfraction motion during the first 6 days of therapy, when used as a subsequent offset, reduced acceptable AP planning target volume margins by 50%. The MV CBCT is a practical direct method of daily localization that shows significant interfraction motion with respect to conventional three-dimensional conformal and intensity-modulated radiotherapy margins, similar to that measured in other modalities.