Validation of a deformable image registration technique for cone beam CT‐based dose verification. Issue 1 (22nd December 2014)

• Record Type: Journal Article
• Authors: Moteabbed, M.; Sharp, G. C.; Wang, Y.; Trofimov, A.; Efstathiou, J. A.; Lu, H.‐M.
• Is Part Of: Medical physics. Volume 42:Issue 1(2015)
• Page Start: 196
• Page End: 205
• Publication Date: 2014-12-22
• Subjects: biomechanics -- calibration -- computerised tomography -- dosimetry -- image registration -- medical image processing -- phantoms -- public domain software -- radiation therapy; Therapeutic applications, including brachytherapy -- Dosimetry/exposure assessment -- Biomechanics -- Standards and calibration -- Computed tomography; Computerised tomographs -- Radiation therapy -- Testing or calibrating of apparatus or arrangements provided for in groups G01D1/00 to G01D15/00 -- Digital computing or data processing equipment or methods, specially adapted for specific applications -- Image data processing or generation, in general -- Calibrating of instruments or apparatus -- Scintigraphy; deformable image registration -- cone‐beam CT -- image‐guided -- prostate radiotherapy; Cone beam computed tomography -- Medical imaging -- Dosimetry -- Calibration -- Intensity modulated radiation therapy -- Vector fields -- Tissues -- Image registration; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4903292
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: As radiation therapy evolves toward more adaptive techniques, image guidance plays an increasingly important role, not only in patient setup but also in monitoring the delivered dose and adapting the treatment to patient changes. This study aimed to validate a method for evaluation of delivered intensity modulated radiotherapy (IMRT) dose based on multimodal deformable image registration (dir ) for prostate treatments. Methods: A pelvic phantom was scanned with CT and cone‐beam computed tomography (CBCT). Both images were digitally deformed using two realistic patient‐based deformation fields. The original CT was then registered to the deformed CBCT resulting in a secondary deformed CT. The registration quality was assessed as the ability of thedir method to recover the artificially induced deformations. The primary and secondary deformed CT images as well as vector fields were compared to evaluate the efficacy of the registration method and it's suitability to be used for dose calculation.plastimatch, a free and open source software was used for deformable image registration. A B‐spline algorithm with optimized parameters was used to achieve the best registration quality. Geometric image evaluation was performed through voxel‐based Hounsfield unit (HU) and vector field comparison. For dosimetric evaluation, IMRT treatment plans were created and optimized on the original CT image and recomputed on the two warped images to be compared. The dose volume histograms were compared for the warped structures that were identical in both warped images. This procedure was repeated for the phantom with full, half full, and empty bladder. Results: The results indicated mean HU differences of up to 120 between registered and ground‐truth deformed CT images. However, when the CBCT intensities were calibrated using a region of interest (ROI)‐based calibration curve, these differences were reduced by up to 60%. Similarly, the mean differences in average vector field lengths decreased from 10.1 to 2.5 mm when CBCT was calibrated prior to registration. The results showed no dependence on the level of bladder filling. In comparison with the dose calculated on the primary deformed CT, differences in mean dose averaged over all organs were 0.2% and 3.9% for dose calculated on the secondary deformed CT with and without CBCT calibration, respectively, and 0.5% for dose calculated directly on the calibrated CBCT, for the full‐bladder scenario. Gamma analysis for the distance to agreement of 2 mm and 2% of prescribed dose indicated a pass rate of 100% for both cases involving calibrated CBCT and on average 86% without CBCT calibration. Conclusions: Using deformable registration on the planning CT images to evaluate the IMRT dose based on daily CBCTs was found feasible. The proposed method will provide an accurate dose distribution using planning CT and pretreatment CBCT data, avoiding the additional uncertainties introduced by CBCT inhomogeneity and artifacts. This is a necessary initial step toward future image‐guided adaptive radiotherapy of the prostate.