ESTRO 2025 - Abstract Book

S4121

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

3409

Digital Poster Defining Thresholds For Verification Imaging After 6Dof Setup Correction: A Multi-Anatomical Site Analysis Sjoerd Y. Hoek, Jessica van der Himst, Amy L. de la Fuente, Roy Kossen, Anna M. Dinkla Radiation oncology, Amsterdam UMC, Amsterdam, Netherlands Purpose/Objective: Accurate patient positioning is crucial for effective radiotherapy. A 6DoF couch can be essential for corrections based on target and OAR dimensions and margins. Current literature, mainly in stereotactic treatments, examined correlations between pitch or roll corrections and longitudinal/lateral residual deviations 1 , which may lead to geometric misses and impact treatment outcomes. Verification imaging after 6Dof setup correction can reduce this risk and should be tailored to clinical relevance. Verification imaging after 6DoF corrections takes time and is best applied only when clinically relevant geometric miss is expected. Our clinic considers ≤1mm added geometric uncertainty in 95% of cases an acceptable balance between efficiency and benefit. This study aims to predict geometric uncertainty after 6DoF corrections to define thresholds for efficient, safe protocols across various anatomical sites. Material/Methods: 1,882 fractions from 380 patients across eight anatomical sites (Table 1) were analyzed. SRS, brain and head&neck patients were fixated with DSPS masks 2 or 5-point thermoplastic mask 2 , respectively. All other sites were non-fixated. All patients were treated on a TrueBeam system with a PerfectPitch 6DoF couch3. Initial 6Dof setup correction was followed by a verification CBCT to derive residual deviations for all fractions. Pitch and roll corrections were related to longitudinal and lateral residual deviations through linear regression. When a strong correlation coefficient was observed (Pearson’s r≥0,5), pitch/roll thresholds were derived to achieve 95% of residual deviations ≤0.1 cm.

Results: For SRS patients, weak correlations (r<0.4) were observed between rotations and residual deviations, all 95% ≤0.1 cm. Moderate correlation (r=0.5) was found for brain/head & neck, where pitch ≤2.0° and roll ≤1.5° met this threshold. Strong correlations (r≥0.6) were identified for spine treatments, with pitch ≤1.0° achieving the threshold. For other sites, roll correlated strongly (r≥0.6) with lateral deviations, but pitch showed no correlation to longitudinal

deviations. No useful thresholds were identified for deviations ≤0.1 cm in these sites. In all sites, excluding SRS, roll had significant correlation (R²≥0.5) with lateral deviations.