Purpose: To develop a four-dimensional (4D) dose optimization and calculation algorithm on 4D-CT-number maps with tissue heterogeneity using displacement-vector-fields (DVF) from different deformable-image-registration algorithms (DIR) in order to consider for anatomical variations and respiratory motion to achieve adaptive radiation therapy.
Methods: A simulation study was performed to remap CT-numbers and dose distributions using DVF that were obtained from DIR. The DVF represented a voxel-by-voxel motion of the tumor and surrounding organs-at-risk in the region of interest. These DVF were employed to modify the intensity maps from 3D-conformal and intensity-modulation optimized plans to perform 4D-optimization on remapped CT-numbers to account for heterogeneity to consider anatomical variations and respiratory motion.
Results: The DVF obtained from deformable image registration correlated linearly with the motion amplitudes of the phantom. The dose calculation algorithm produced 4D-optimized plans from the stationary 3D-confromal or intensity-modulated plans using DVF’s extracted from DIR algorithms. The 4D-dose profiles became narrower for the targets moving with larger motion amplitudes. It performed optimization by shrinking and remapping the dose distribution that are obtained from dose optimization on stationary images. The unfolding of the 4D-dose distributions as the patient move during dose delivery produced equivalent dose distributions optimized in 3D for the stationary targets and organs at risk. Heterogeneity correction was also considered by remapping the CT-number values in the CT images using the DVF’s in order to reverse motion artifacts.
Conclusion: This approach provides a novel algorithm to perform adaptive radiation therapy using 4D dose optimization and calculation with heterogeneity correction using remapped CT numbers which is a practical and convenient alternative to manage respiratory motion of cancer patients. The advantages of this approach include achieving equivalent conformal doses without any restriction on anatomical variations and patient breathing during dose delivery, however, it is limited by reproducibility of the DVF from DIR.