2175 - Surface-Guided Radiotherapy: A Reliable Motion Management Solution for End-Expiration Breath-Hold in Stereotactic Body Radiotherapy to the Liver

Purpose/Objective(s): Stereotactic body radiotherapy (SBRT) for liver tumors presents challenges due to significant tumor motion caused by respiratory movement, which must be accounted for when determining treatment margins¹. Surface-guided radiotherapy (SGRT) is a non-invasive, patient-centered technique that has proven effective in achieving breath-hold (BH) for treatment sites such as breast². However, evidence supporting its use in abdominal SBRT remains limited^3-4. SGRT also eliminates the need for tattoos, enables intrafraction patient monitoring in six degrees of freedom (6DoF), and improves the accuracy of initial patient setup^5-7 . The aim of this study is to demonstrate the intrafraction accuracy of SGRT in reproducing end-expiration breath-hold (EEBH) in liver SBRT patients.

Materials/Methods: 57 kV cone beam CTs (CBCTs) from 19 treatment fractions of four patients receiving SBRT to the liver were analyzed. All patients were treated in EEBH. A minimum of three CBCT scans were obtained per fraction while the patient maintained EEBH as indicated by the SGRT system. CBCT scans were performed before treatment, after applying corrections in 6DoF and post-treatment. Throughout image acquisition, EEBH was monitored via the SGRT terminal, with a motion tolerance of 0.1cm and 1 degree achieved for all patients. In this study, a rigid "bone" registration was performed to the vertebral bodies to assess the superior-inferior (S/I) discrepancy in diaphragm position between CBCT and CT simulation. Each measurement was independently conducted by two radiation therapists using the same predefined windowing level. Interobserver variability was reported as a maximum of 2mm and in these cases an average measurement was used. The S/I diaphragm displacement was then compared across CBCT scans obtained on the same treatment fraction to evaluate the intrafraction reproducibility of EEBH amplitude when using SGRT.

Results: The maximum S/I change in diaphragm position between CBCT scans acquired on the same fraction was 0.6cm. Across all fractions, the mean intrafraction difference in S/I position of the diaphragm was 0.27 cm (95% CI: 0.20, 0.34). The range of displacements was 0.1cm-0.6cm. In 89% of fractions (n=17), the intrafraction difference in diaphragm position was less than 0.5cm.

Conclusion: These findings support the use of SGRT as a reliable method for motion management in EEBH liver SBRT patients. While additional data is needed, SGRT has the potential to eliminate the need for separate breath-control systems, offering a more streamlined, patient centered treatment approach.