P. Qi*1, S. Balik2, T. Zhuang3, L. Lu, G.V. Videtic1, K.L. Stephans1, P. Xia1, 1. The Cleveland Clinic Foundation, Cleveland, OH; 2. The University of Southern California, Los Angeles, CA; (3) The University of Texas at Southwestern, Dallas, TX

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TU-D1000-IePD-F5-3 (Tuesday, 7/12/2022) 10:00 AM - 10:30 AM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 5

Purpose: Randomized studies have shown that single-fraction lung stereotactic body radiotherapy (SF-SBRT) is as safe and effective as fractionated schedules for inoperable early-stage lung cancer. Some clinicians however are reluctant to use SF-SBRT over concerns on reliable verification of delivery during dose administration. We here aim to evaluate whether four-dimensional cone-beam CT (4D-CBCT) provides more benefits than conventional 3D-CBCT for patient setup in SF-SBRT.

Methods: From an IRB-approved registry, all patients treated with 4D-CBCT guided SF-SBRT were selected for analysis. 4D-CBCT was selectively ordered as the guidance imaging modality based on clinician assessment of tumor motion and/or patient factors following CT simulation and planning. During pre-treatment verification, an average-intensity-projected (AIP) CBCT (derived from 4D-CBCT) was registered to the planning AIP-CT by aligning the tumor observed on the AIP-CBCT with the ITV contour on the planning CT. Subsequently, the tumor observed from each phase of 4D-CBCT (10 phases) was reviewed to see whether it remained within the ITV contour from the planning CT. If not, further adjustments were made until the envelope of tumor motion on 4D-CBCT matched or within the ITV contour.

Results: Thirty patients were eligible for this study. Tumor motion labeled “large” was greater than 0.5 cm in one or more directions in 17 (57%) cases (11, 2 and 4 cases in lower, middle, and upper lobes, respectively). Tumors in proximity to chest wall, heart, and diaphragm accounted for other cases. After initial AIP-CBCT based fusion, re-adjustments based on 4D-CBCT were made in about 30% of the cases with large motion. Because the re-adjustment was typically less than 2 mm and we use a 5 mm ITV to PTV margin, the retrospectively calculated ITV dose still received the prescription dose of 34 Gy.

Conclusion: For highly mobile tumors, using 4D-CBCT enhanced pre-treatment alignment when using SF-SBRT.

Funding Support, Disclosures, and Conflict of Interest: This work is partly sponsored by Varian Medical System.

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