Comparison of carina- versus bony anatomy-based registration for setup verification in esophageal cancer image-guided radiotherapy Melanie Machiels* 1, P. Jin 1, C.H.M. Van Gurp 1, J.E. van Hooft 2, T. Alderliesten 1, M.C.C.M. Hulshof 1 1 Department of Radiation Oncology, Academic Medical Center / University of Amsterdam, The Netherlands 2 Department of Gastroenterology & Hepatology, Academic Medical Center / University of Amsterdam, The Netherlands *m.machiels@amc.uva.nl INTRODUCTION Background: For image-guided radiotherapy (IGRT) in esophageal cancer patients, the optimal structure for image matching has not yet been identified and currently bony anatomy-based registration for setup verification is commonly used. Furthermore, a recent study at our department showed marker-based registration to be infeasible due to tissue deformation.1 The carina, with its close proximity to the esophagus, might be an alternative structure for setup verification in esophageal cancer IGRT. Objectives: Geometric accuracy of carina-based registration for CBCT-guided setup verification compared to standard bony-anatomy based registration. Feasibility of automatic carina-based registration. PTV-CTV coverage using a carina-based registration. 1 2 3 MATERIAL AND METHODS ► Twenty-four esophageal cancer patients with 65 fiducial markers implanted at the tumor borders (surrogate for tumor position). Per patient: 1 planning CT and 7—8 CBCTs (CBCTs in the first 4 consecutive days and weekly thereafter in 23 or 28 fractions). Geometric accuracy investigation by quantification of the marker position variation relative to carina: Markers were classified and analyzed into four subgroups based on their locations in the esophagus (proximal, mid-esophagus, distal, cardia). Rigid registration of all CBCT scans (n=236) to the reference CT with respect to the bony anatomy (BR) or the carina (CR) using XVI software. The mean of the interfractional marker position variation (estimate of the systematic position error (SE)) associated with either bony anatomy- (BR) or carina-based registration (CR) was calculated. Comparison between registration methods was done using a Wilcoxon paired signed-rank test. Automatic registrations were visually checked and manually adjusted when necessary. PTV-CTV coverage was visually evaluated. ► 1 Figure 1. Cone-beam computed tomography images before and after carina-based registration. Overlay of reference CT (purple) and CBCT (green) in XVI with mismatch before registration (a) and match after carina-based registration (b). 2 RESULTS 3 Geometric accuracy : A large systematic position error is associated with the use of both bony anatomy-based and carina-based registration Proximal : no significant advantage, slightly in favor of BR in LR and AP direction Mid-esophagus: no significant advantage, slightly in favor of CR in CC and AP direction Distal : significant advantage of BR in AP direction over CR (p<0.004). Cardia: no significant advantage 1 Figure 3. Scatterplots of the absolute systematic position errors (SE) of the individual markers relative to the bony anatomy versus the carina. Results are given in 3 directions for all 4 subgroups. Above darkred line = preference for bony anatomy-based registration. Underneath darkred line = preference for carina-based registration. The red dot indicates the mean absolute systematic error. Set-up registration feasibility Automatic CR not possible in 7 out of 236 CBCT`s (3%). Target coverage PTV-CTV coverage of 91.3% after CR versus 100% after BR. 2 Figure 2. Distributions of the absolute mean systematic position errors (SE) of the individual markers relative to the carina (blue) and the bony anatomy (darkred). The distribution of absolute mean SE is given for each marker subgroup separately and compared between CR and BR. Results are given in the left-right (LR), cranio-caudal (CC), and anterior-posterior (AP) direction. Boxes: upper and lower quartiles. Whiskers: highest/lowest data within 1.5 interquartile range of the upper/lower quartiles. ** indicates significance. 3 CONCLUSION The mean marker position variation (systematic position error) over the treatment course remains large and is in most directions slightly larger when using a carina-based registration compared with a bony anatomy-based registration. Only for tumors with a small extent located in the mid-esophagus the carina-based registration might be slightly favorable. However, esophageal tumors typically extend across regions and the majority of tumors are located distally. Automatic registration was feasible, however, target coverage was insufficient in 8.7% of cases using a carina-based registration. Therefore, our data endorse the use of bony anatomy-based registration over carina-based registration as a tumor surrogate for setup verification.  1. Jin P, et al. Radiother Oncol 2015;117:412–8.