SU‐E‐J‐44: A Novel Approach to Quantify Patient Setup and Target Motion for Real‐Time Image‐Guided Radiotherapy (IGRT)

Purpose Isocenter shifts and rotations to correct patient setup errors and organ motion cannot remedy some shape changes of large targets. We are investigating new methods in quantification of target deformation for realtime IGRT of breast and chest wall cancer. Methods Ninety‐five patients of breast or chest wall cancer were accrued in an IRB‐approved clinical trial of IGRT using 3D surface images acquired at daily setup and beam‐on time via an in‐room camera. Shifts and rotations relating to the planned reference surface were determined using iterative‐closest‐point alignment. Local surface displacements and target deformation are measured via a ray‐surface intersection and principal component analysis (PCA) of external surface, respectively. Isocenter shift, upper‐abdominal displacement, and vectors of the surface projected onto the two principal components, PC1 and PC2, were evaluated for sensitivity and accuracy in detection of target deformation. Setup errors for some deformed targets were estimated by superlatively registering target volume, inner surface, or external surface in weekly CBCT or these outlines on weekly EPI. Results Setup difference according to the inner‐surface, external surface, or target volume could be 1.5 cm. Video surface‐guided setup agreed with EPI results to within < 0.5 cm while CBCT results were sometimes (∼20%) different from that of EPI (>0.5 cm) due to target deformation for some large breasts and some chest walls undergoing deep‐breath‐hold irradiation. Square root of PC1 and PC2 is very sensitive to external surface deformation and irregular breathing. Conclusion PCA of external surfaces is quick and simple way to detect target deformation in IGRT of breast and chest wall cancer. Setup corrections based on the target volume, inner surface, and external surface could be significant different. Thus, checking of target shape changes is essential for accurate image‐guided patient setup and motion tracking of large deformable targets. NIH grant for the first author as cionsultant and the last author as the PI