Purpose-To investigate the effect of breathing motion and dose accumulation on the planned radiotherapy dose to liver tumors and normal tissues using deformable image registration.Method and Materials-Twenty one free-breathing stereotactic liver cancer radiotherapy patients, planned on static exhale CT for 27 -60 Gy in 6 fractions, were included. A biomechanical model-based deformable image registration algorithm, retrospectively deformed each exhale CT to inhale CT. This deformation map was combined with exhale and inhale dose grids from the treatment planning system to accumulate dose over the breathing cycle. Accumulation was also investigated using a simple rigid liver-to-liver registration. Changes to tumor and normal tissue dose were quantified.Results-Relative to static plans, mean dose change (range) after deformable dose accumulation (as % of prescription dose) was −1 (−14, 8) to minimum tumor, −4 (−15, 0) to max bowel, −4 (−25, 1) to max duodenum, 2 (−1, 9) to max esophagus, −2 (−13, 4) to max stomach, 0 (−3, 4) to mean liver, and −1 (−5, 1) and −2 (−7, 1) to mean left and right kidneys. Compared to deformable registration, rigid modeling had changes up to 8% to minimum tumor and 7% to maximum normal tissues.Conclusion-Deformable registration and dose accumulation revealed potentially significant dose changes to either a tumor or normal tissue in the majority of cases due to breathing motion. These changes may not be accurately accounted for with rigid motion.

No abstract

The optimum biomechanical model with the smallest registration error is when frictionless contact model and 0.4 Poisson's ratio are applied. The overall accuracies of all bifurcation points in all 16 patients including tumor points are 1.0 +/- 0.7, 1.2 +/- 1.0, and 1.7 +/- 1.4 mm in the LR, AP, and SI directions, respectively.

Purpose To investigate the accumulated dose deviations to tumors and normal tissues in liver stereotactic-body radiotherapy (SBRT), and investigate their geometric causes. Methods and Materials Thirty previously treated liver cancer patients were retrospectively evaluated. SBRT was planned on the static exhale CT for 27 – 60 Gy in 6 fractions, and patients were treated in free-breathing with daily cone-beam CT (CBCT) guidance. Biomechanical model-based deformable image registration accumulated dose over both the planning 4DCT (predicted breathing dose), and also over each fraction’s respiratory-correlated CBCT (accumulated treatment dose). The contribution of different geometric errors on changes between the accumulated and predicted breathing dose were quantified. Results Twenty one patients (70%) had accumulated dose deviations relative to the planned static prescription dose greater than 5%, ranging from −15 to 5% in tumors and −42 to 8% in normal tissues. Sixteen patients (53%) still had deviations relative to the 4DCT-predicted dose, which were similar in magnitude. Thirty two tissues in these 16 patients had deviations > 5% relative to the 4DCT-predicted dose, and residual setup errors (n=17) were most often the largest cause of the deviations, followed by deformations (n=8) and breathing variations (n=7). Conclusion The majority of patients had accumulated dose deviations greater than 5% relative to the static plan. Significant deviations relative to the predicted breathing dose still occurred in over half the patients, commonly due to residual setup errors. Accumulated SBRT dose may be warranted to pursue further dose-escalation, adaptive SBRT, and aid in correlation with clinical outcomes.

No abstract

Purpose The accuracy of deformable image registration tools can vary widely between imaging modalities and specific implementations of the same algorithms. A biomechanical model-based algorithm initially developed in-house at an academic institution was translated into a commercial radiotherapy treatment planning system and validated for multiple imaging modalities and anatomic sites. Methods Biomechanical deformable registration (Morfeus) is a geometry driven algorithm based on the finite element method. Boundary conditions are derived from the model-based segmentation of controlling structures in each image which establishes a point-to-point surface correspondence. For each controlling structure, material properties and fixed or sliding interfaces are assigned. The displacements of internal volumes for controlling structures and other structures implicitly deformed are solved with finite element analysis. Registration was performed for 74 patients with images (mean vector resolution) of thoracic and abdominal 4DCT (2.8 mm) and MR (5.3 mm), liver CT-MR (4.5 mm) and prostate MR (2.6 mm). Accuracy was quantified between deformed and actual target images using distance-to-agreement (DTA) for structure surfaces and the target registration error (TRE) for internal point landmarks. Results The results of the commercial implementation were as follows. The mean DTA was ≤1.0 mm for controlling structures and 1.0–3.5 mm for implicitly deformed structures on average. TRE ranged from 2.0 mm on prostate MR to 5.1 mm on lung MR on average, within 0.1 mm or lower than the image voxel sizes. Accuracy was not overly sensitive to changes in the material properties or variability in structure segmentations, as changing these inputs affected DTA and TRE by ≤0.8 mm. Maximum DTA >5 mm occurred for 88% of the structures evaluated although these were within the inherent segmentation uncertainty for 82% of structures. Differences in accuracy between the commercial and in-house research implementations were ≤0.5 mm for mean DTA and ≤0.7 mm for mean TRE. Conclusions Accuracy of biomechanical deformable registration evaluated on a large cohort of images in the thorax, abdomen and prostate was similar to the image voxel resolution on average across multiple modalities. Validation of this treatment planning system implementation supports biomechanical deformable registration as a versatile clinical tool to enable accurate target delineation at planning and treatment adaptation.

This study has proposed a novel technique to manufacture deformable volumetric gel dosimeters. By comparing the doses accumulated in MORFEUS and the doses measured with the dosimeters under the condition of deformation, the study has also demonstrated the potential of using deformable gel dosimetry to experimentally validate algorithms that include deformations into dose computation. Since dose less than 200 cGy was not evaluated in this study, future investigations will focus more on low dose regions by either using bigger gel dosimeters or prescribing a lower dose to provide a more complete experimental validation of MORFEUS across a wider dose range.

Purpose:To investigate the effect of the bronchial tree on the accuracy of biomechanical-based deformable image registration of human lungs. Methods: Three dimensional finite element models have been developed using four dimensional computed tomography image data of ten lung cancer patients. Each model is built of a body, left and right lungs, tumor, and bronchial trees. Triangular shell elements are used for the bronchial trees while tetrahedral elements are used for other components. Hyperelastic material properties based on experimental investigation on human lungs are used for the lung parenchyma. Different material properties are assigned for the bronchial tree using five values for the modulus of elasticity of 0.01, 0.12, 0.5, 10, and 18 MPa. Lungs are modeled to slide inside chest cavities by applying frictionless contact surfaces between each lung and corresponding chest cavity. The accuracy of the models is examined using an average of 40 bronchial bifurcation points identified on inhale and exhale images. Relative accuracy is evaluated by comparing the displacement of all nodes within the lungs as well as the dosimetric difference at the exhale position predicted by the model. Results: There is no significant effect of bronchial tree on the model accuracy based on the bifurcation points analysis. However, on the local level, using an average of 38 000 nodes, there is a maximum difference of 8.5 mm in the deformation of the bronchial trees, as the modulus of elasticity of the bronchial trees increases from 0.01 to 18 MPa; however, more than 96% of nodes are within a 2.5 mm difference in each direction. The average dose difference at the predicted exhale position is less than 35 cGy between the models. Conclusions: The bronchial tree has little effect on the global deformation and the accuracy of deformable image registration of lungs. Hence, the homogenous model is a reasonable assumption. Since there are some local deformation differences between nodes as the material properties of the bronchial tree change that may affect the accuracy of dosimetric results, heterogeneity may be required for a smaller scale modeling of lungs.