Is it sensible to "deform" dose? 3D experimental validation of dose-warping

Strategies for dose accumulation in deforming anatomy are of interest in radiotherapy. Algorithms exist for the deformation of dose based on patient image sets, though these are sometimes contentious because not all such image calculations are constrained by physical laws. While tumor and organ motion has been a key area of study for a considerable amount of time, deformation is of increasing interest. In this work, we demonstrate a full 3D experimental validation of results from a range of dose deformation algorithms available in the public domain.

We recently developed the first tissue-equivalent, full 3D deformable dosimetric phantom-"DEFGEL." To assess the accuracy of dose-warping based on deformable image registration (DIR), we have measured doses in undeformed and deformed states of the DEFGEL dosimeter and compared these to planned doses and warped doses. In this way we have directly evaluated the accuracy of dose-warping calculations for 11 different algorithms. We have done this for a range of stereotactic irradiation schemes and types and magnitudes of deformation.

The original Horn and Schunck algorithm is shown to be the best performing of the 11 algorithms trialled. Comparing measured and dose-warped calculations for this method, it is found that for a 10 × 10 mm(2) square field, γ(3%∕3mm) = 99.9%; for a 20 × 20 mm(2) cross-shaped field, γ(3%∕3mm) = 99.1%; and for a multiple dynamic arc (0.413 cm(3) PTV) treatment adapted from a patient treatment plan, γ(3%∕3mm) = 95%. In each case, the agreement is comparable to-but consistently ∼1% less than-comparison between measured and calculated (planned) dose distributions in the absence of deformation. The magnitude of the deformation, as measured by the largest displacement experienced by any voxel in the volume, has the greatest influence on the accuracy of the warped dose distribution. Considering the square field case, the smallest deformation (∼9 mm) yields agreement of γ(3%∕3mm) = 99.9%, while the most significant deformation (∼20 mm) yields agreement of γ(3%∕3mm) = 96.7%.

We have confirmed that, for a range of mass and density conserving deformations representative of those observable in anatomical targets, DIR-based dose-warping can yield accurate predictions of the dose distribution. Substantial differences can be seen between the results of different algorithms indicating that DIR performance should be scrutinized before application todose-warping. We have demonstrated that the DEFGEL deformable dosimeter can be used to evaluate DIR performance and the accuracy of dose-warping results by direct measurement.