Four-Dimensional Dose Reconstruction for Evaluation of Pulmonary Fluid–Induced Treatment Replanning
Abstract
Purpose
Pulmonary fluid changes often necessitate treatment replanning; however, associated mass variations compromise deformable dose accumulation (DDA) and complicate its evaluation. Accurate estimation of cumulative dose and its associated uncertainty is therefore critical for replanning assessment.
Methods
Radiation doses and planning CT images were converted into deposited energy and mass distributions and mapped to a reference image using deformation fields generated by the multi-modality (MM) and contour-guided (CG) registration algorithms in MIM Software, as well as two hybrid algorithms (FEM-MM and FEM-CG) implemented in an in-house program. A four-dimensional (4D) dose was then defined voxel-wise as the ratio of total transferred energy to the average mass-transferred on the reference image. For each organ, deposited energy was integrated on each image and summed to establish an organ-specific reference energy. Dose error was quantified using relative energy loss (REL), defined as the difference between transferred and deposited energy within each organ, normalized to the reference energy. The 4D dose framework was applied to a lung cancer patient whose left lung was fluid-filled on the initial planning CT but resolved on the replanning CT, resulting in substantial lung deformation between the two scans.
Results
The MIM-MM, FEM-MM, MIM-CG and FEM-CG registrations show mean distance-to-agreement values of 2.5±3.1, 3.1±2.9, 2.6±1.8 and 2.5±1.8 mm, respectively, with corresponding standard deviations of Jacobian determinants of 0.213, 0.208, 0.558 and 0.465. The resulting 4D dose calculations produced mean ITV doses of 62.6, 62.9, 63.4 and 63.1 Gy, with REL of 1.37, 1.04, 1.57 and 1.38%, respectively. The average REL across other organs (heart, left and right lungs) was 2.68±1.43%, 2.36±2.19%, 3.47±3.61% and 3.72±4.14%.
Conclusion
Four-dimensional dose reconstruction enables simultaneous calculation of cumulative dose and its associated energy loss. The latter may serve as an objective metric for evaluating deformable image registration and dose accumulation errors in treatment replanning.