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Description
The interplay between the beam delivery time structure and the patient motion makes 4D dose calculation (4DDC) important when treating moving tumors with IMPT. In a conventional 4DDC, each spot is assigned to its nearest phase image in a 4DCT, based on a delivery log and a breathing signal. The doses computed on the phase images are then deformed to a reference phase and accumulated. An apparent source of error in 4DDC is the approximation applied in the nearest-neighbor assignment of spots to phases, due to the coarseness of the temporal image resolution.
We addressed this limitation by applying registration-based interpolation between phase images for increased temporal resolution of the 4DCT. Assuming linear motion between the phases, we deformed each phase image to its neighbor using the deformation vector field that aligns them, scaled by the desired time step. We then applied Monte Carlo-based 4DDC to both the original 4DCT (10 phases), and extended 4DCTs at increasingly fine temporal resolutions (20 - 100 phases).
The method was evaluated on seven NSCLC patients treated with three robustly optimized IMPT beams with spot delivery times of approximately 3 ms and energy switching times of 2 s. Dose differences were compared by computing the volume of the CTV differing by more than 1, 2, and 3 percent of the prescribed dose. The ground truth was taken as the dose computed using 100 phase images, and was justified by considering the diminishing effects of adding more images. For all patients, 20-30 phase images were sufficient to mitigate the errors using 2 and 3 percent thresholds. For three patients, 1 percent errors persisted even at higher resolutions, which was attributed to artefacts and poor image inconsistencies. For the four patients without such problems, 1 percent errors vanished at resolutions beyond 50 images.
To conclude, the inclusion of breathing phases in between the original phases in a 4DCT can remove the approximation errors which arise from the nearest-neighbor assignment in 4DDC.
