Evaluation of inter-fractional anatomical variations due to compression belt placement reproducibility using MR-guided radiotherapy treatment fraction images.

backgroundAbdominal compression (AC) using a belt-based system is widely employed in radiation therapy to limit respiration-induced target motion for abdominal and hepatic malignancies. While AC has demonstrated efficacy in reducing intrafraction motion amplitude, the interfractional anatomical consequences of daily compression belt (CB) placement variability remain poorly characterized. A clearer understanding of this variability is necessary to determine whether online adaptive radiotherapy (oART) is required to maintain dosimetric robustness throughout the treatment course. PURPOSE: This study aims to quantify interfractional anatomical variability associated with CB use in a clinical MR-guided radiotherapy cohort and to compare these findings against a matched cohort of non-compressed abdominal patients to determine the clinical significance of CB-induced setup variability.

methodsDaily treatment MR images were retrospectively collected from 17 CB patients and 17 matched non-CB controls treated on an MR-Linac. External contours were delineated at the center of the planning target volume (PTV) and at the region of maximum compression for CB patients. Interfractional variability was assessed using cross-sectional area (CSA), Dice Similarity Coefficient (DSC), Mean Distance to Agreement (MDA), and Hausdorff Distance (HD). Interfractional organ displacement of the liver and kidneys was quantified via rigid registration. Linear mixed-effects models (LMEMs) evaluated the association between patient demographics-including age, BMI, sex, subcutaneous adipose tissue (SAT), visceral adipose tissue (VAT)-and interfractional variability metrics.

resultsInterfractional DSC over the PTV region was significantly lower in the CB cohort compared with non-CB controls (p = 0.01 and p = 0.03 for comparisons to fraction one and sequential fraction comparisons, respectively), and MDA was significantly elevated in the CB cohort (p < 0.001). No significant difference in CSA variability was observed between cohorts. In the maximum compression region, median HD was 1.6 cm relative to fraction one, with no significant correlation between compression-region and PTV-region CSA variability (p = 0.99). Older age, male sex, and higher BMI were associated with improved belt-contact shape reproducibility (all p < 0.05), though these demographic effects did not translate to PTV-region stability. LMEM analysis revealed that fixed demographic effects explained only a small fraction of PTV variability (R

conclusionsCB use introduces measurable but modest increases in interfractional anatomical variability at the PTV level compared with standard abdominal setups. Compression-site variability does not propagate directly to the treatment target, and patient demographics alone are insufficient to predict CB-induced setup variability. Daily MR-based image guidance and selective adaptive replanning remain the appropriate management strategy for patients treated with abdominal compression.