Is Adaptive 4D SBRT Needed? Evaluating Respiratory, Anatomical, and Combined Effects Using 4DCBCT-Based 4D Dose Accumulation
Abstract
Purpose
In adaptive radiotherapy for static targets, treatment adaptation accounts for interfractional anatomical changes. In 4D SBRT, respiratory motion is superimposed on anatomical variations, such that anatomical, motion-related, and combined effects compromise dosimetric accuracy. This study employs a 4D dose accumulation workflow integrating daily 4DCBCT-based virtual CTs (vCTs) to quantify these effects and to evaluate the need for treatment plan adaptation.
Methods
Seventeen lung SBRT patients (ITV-based gating, VMAT, 5-13 fractions, 3-7.5Gy/fx) with planning 4DCT (pCT), daily 4DCBCT and respiratory signals from dose delivery were analysed. vCTs were generated by rigid registration of pCT/4DCBCT, followed by deformable image registration to warp pCT intensities onto daily 4DCBCT. Dose accumulation was performed by predicting the internal motion (vector fields) based on the external breathing signal. Three scenarios were evaluated: (1) 3D-pCT vs. daily 3D-vCT dose (consideration of only anatomical changes), (2) 3D-pCT/vCT vs. 4D-pCT/vCT dose (only motion effects) and (3) full 4D-vCT dose vs. full 4D-pCT dose (combined effects). Dosimetric metrics included D98 for the GTV and mean lung dose (MLD).
Results
Across all patients and fractions, anatomical changes resulted in a reduction in D98 of up to 28.3% (mean: 5.0+-6.3%), while motion effects led to a maximum reduction of 22.6% (mean: 1.1+-4.8%). Combined effects showed a maximum D98 reduction of 33.8% (mean: 2.8+-7.6%). While the average mean lung dose (MLD) across all patients remained stable (<1%), pronounced outliers were observed, with individual fractions showing increases in MLD of up to 23%.
Conclusion
Daily 4DCBCT-based 4D dose accumulation revealed clinically relevant deviations between planned and delivered dose in lung SBRT that are not accounted for by planning-4DCT-only simulations. While anatomical changes were the dominant contributor, respiratory motion effects remained relevant for individual patients and fractions. These results support the need for daily 4D dose evaluation and 4D adaptive strategies in lung SBRT.