4-D Predicted Accumulated Dose Versus Time Weighted Average 4D-CBCT Recalculated Dose Variations in Liver SABR

Abstract

Quantify the variations to tumor and normal tissues between 4D predicted accumulated dose and doses recalculated on time weighted average 4D CBCT static derived series in liver SABR 25 pts with 38 liver metastatic lesions underwent gold seed fiducial placement, triphasic liver scan, respiratory correlated 4D CT. Lesions were localized on the static Hepatic arterial/portal venous phase CT (as they were well visualized) and fused to corresponding 4D-dataset and ITV thus defined. A dose of 39gy in 6 fractions was planned using step using step and shoot IMRT. The finalized plan Structure were deform registered and dose warped across all the 4D datasets by intensity-based-free-form deformable registration and final 4D predicted accumulated dose thus calculated. Treatment was executed using 4D-CBCT image guidance on a 6 DOF couch. The final planned CT dataset is further dose deformed on to first set-up corrected time weighted Avg. derived static series from 4D-CBCT dataset. Using deformable image registration, subsequent fraction doses on reference geometry (first time weighted average series) were added up to get final delivered doses. The contribution of different geometric errors to changes between the accumulated and delivered dose were quantified and changes greater than 1 gray, reported as percent change (normalized to prescription dose) were considered significant All patients had accumulated dose deviations relative to the planned static prescription dose >5%, ranging 3-18% in tumors and 5-25% in normal tissues. Residual setup errors were most often the largest cause of the deviations, followed by deformations and breathing variations. Gold seed markers, while producing delineation artifacts and deformation errors, were good surrogate for treatment verification when 4D CBCT was used (with acquisition time of 3mts). These deformation errors were minimized with proper deformation QA using registration refining tools and alignment locks Accumulated dose deviations ranging from 5-20% relative to the static plan are observed in all pts undergoing liver SABR. Modalities which impede breathing motion like diaphragmatic compression will produce more deformation errors leading to erroneous prediction of 4D dose and accumulated dose.