3173 - Assessment of Intra-Fraction Motion In Thoracoabdominal DIBH SBRT Using Dual CBCT Verification

Materials/Methods: Four thoracoabdominal SBRT patients (2 lung, 1 pancreas, 1 liver) treated under DIBH to 40–50 Gy in 5 fractions were retrospectively analyzed. Patients were immobilized using a standardized setup and treated with DIBH supported by surface-guided radiation therapy (SGRT, C-RAD Catalyst?) for reproducibility. A dual cone-beam CT (CBCT) verification workflow was employed. An initial CBCT (CBCT1; acquisition time 13.8±3.1 s) was obtained at the start of each fraction for setup correction. A second verification CBCT (CBCT2; acquisition time 13.3±4.1 s) was acquired mid- or end-treatment under DIBH to assess intra-fraction positional stability. Rigid image registration to the planning CT used target-based alignment. Intra-fraction displacement was defined as translational (vertical, longitudinal, lateral) and rotational (pitch, roll, yaw) couch corrections at CBCT2 relative to the CBCT1-corrected position. Fractions lacking CBCT2 were excluded.

Results: Of 20 delivered fractions, 12 with recorded CBCT2 verification were available for intra-fraction displacement analysis. In 7 fractions, CBCT2 was omitted due to the patient’s inability to maintain arm position; 1 fraction was untreated. Among analyzed fractions, 3 demonstrated negligible adjustments, while 5 fractions from 3 patients exhibited notable deviations exceeding 1 cm translationally and/or 1° rotationally (Table 1). Mean±SD translational shifts relative to CBCT1 were –0.05±0.35 cm (vertical), –0.33±0.55 cm (longitudinal), and –0.09±0.29 cm (lateral), with a maximum longitudinal excursion of –1.10 cm. Rotational shifts were –0.08°±1.07° (pitch), –0.12°±0.95° (roll), and 0.15°±0.45° (yaw), with the largest pitch deviation measuring –2.9°. The largest translational and rotational excursions occurred in lung and liver fractions, respectively.

Conclusion: While thoracoabdominal DIBH SBRT setups can remain stable, dual CBCT verification reveals notable intra-fraction variations can unpredictably occur across patients and fractions, emphasizing the need for mid-treatment verification CBCT. Relying strictly on an initial setup scan may be insufficient for highly conformal treatments. Integrating secondary verification imaging or continuous intra-fraction monitoring is crucial to ensure target coverage, minimize interplay effects, and guarantee optimal organ-at-risk sparing.