Room: Exhibit Hall | Forum 9
Purpose: Our group showed the gating variations of the internal surrogate for respiratory-gated radiotherapy (RGRT) using SyncTraX (Shimadzu corp). The purposes of this study were to evaluate the dose with gating variation using developed four-dimensional (4D) dose calculation for RGRT in lung, and to evaluate the relationship between patient-specific respiratory motion and dose reduction obtained by RGRT.
Methods: Six patients who underwent RGRT using SyncTraX were enrolled in this study. First, the optimal phase of 4DCT which is contained within gating variations were selected using a log file, and the target volume with gating variation was contoured using optimal 4DCT phases. Additionally, internal target volume plan using the all 4DCT phases was created. The planning target volume margin and leaf margin for each plan were defined 5 mm. Created plans were recalculated with irradiation field and optimal phase or all phase using Monte Carlo calculation. The calculated doses for each 4DCT phase were deformed to expiration phase using deformable image registration, and were accumulated. To evaluate dose reduction of RGRT, dose volume histograms were calculated, and V20, V10 and V5 values of ipsilateral lung were compared.
Results: When the respiratory motion was small, the number of optimal phase for the gated plan was increased. The maximum dose differences were 7.8%, 8.2%, 7.0%, and minimum were 0.1%, 0.5%, 0.1%, for V20, V10 and V5. The dose difference also decreased with decreasing respiratory motion. When the respiratory motion and the gating variation are almost the same, almost all of phase is irradiated and dose difference is decreased. If the patient's respiratory motion is small, the burden may be greater than the benefit from gated radiotherapy.
Conclusion: Our 4D dose calculation method for RGRT was possible to calculate realistic gated dose with gating variation. Our evaluation made clear the patient-specific benefit of RGRT.