Conducting four-dimensional deforming dosimetry with a deformable gel dosimeter presents several challenges for implementation and interpretation of results. In this work, we describe a modification to a respiratory motion phantom to accommodate the DEFGEL dosimeter, and an evaluation of the reproducibility of the setup and deformation cycle. Setup errors are sub-mm while deformation reproducibility is typically less than half the CT voxel size (1 mm AP and LR; 3 mm SI). 4DCT confirms the absence of hysteresis.
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