Presenter(s)
K. Siwakoti1, J. A. Soldner2, R. A. Popple3, R. A. Cardan4, N. Viscariello4, H. Boggs4, C. Dobelbower4, J. A. Pogue4, and A. J. Kole5; 1Department of Medicine, University of Alabama at Birmingham, Birmingham, AL, 2The University of Alabama at Birmingham Heersink School of Medicine, Birmingham, AL, 3University of Alabama at Birmingham Department of Radiation Oncology, Birmingham, AL, 4University of Alabama at Birmingham, Birmingham, AL, 5Department of Radiation Oncology, University of Alabama at Birmingham, Birmingham, AL
Purpose/Objective(s): Accurate target localization is essential in lung stereotactic body radiation therapy (SBRT) due to respiratory motion and potential patient positioning drift during treatment. Fiducial markers placed in or near the tumor can enable real-time tracking during SBRT delivery. However, the clinical value of fiducial-based tracking compared with non-fiducial approaches in routine practice remains unclear. We evaluated the frequency and magnitude of intrafraction positional shifts during lung SBRT and examined treatment delivery characteristics associated with fiducial use.
Materials/Methods: We reviewed consecutive patients treated with definitive lung SBRT with or without fiducial markers at a single institution from 2020-2025. We included plans with BED10 >100 Gy delivered in 3-5 fractions. When available, real-time fiducial tracking was performed during SBRT delivery, with beam interruption and patient repositioning performed when fiducial markers were noted to be misaligned (MD discretion, generally 3-5+ mm). We extracted treatment data including use of fiducial tracking, respiratory gating, and couch shifts occurring after beam-on. The primary endpoint was occurrence and magnitude of intrafraction positional shifts.
Results: We included 490 patients, 521 unique treatment plans and 2,237 fractions. Fiducial-based tracking was used in 65.9% of treatments, and respiratory gating in 37.1%. Intrafraction shifts after beam-on occurred in 11.35% of fractions. Of all detected shifts, 81.1% occurred during treatments using fiducial-based tracking, with the remainder occurring in patients who did not have real-time fiducial tracking. When evaluating over a patient’s entire treatment course (median 5 fractions), intrafraction positional correction was performed in 27.6% of treatments. Among patients requiring repositioning, the median 3D vector shift magnitude was 0.49 cm and the median number of positional shifts per fraction was 1 (range 1-6).
Conclusion: In this large contemporary lung SBRT cohort, intrafraction positional shifts were observed in more than 25% of patients. Median shift magnitude approached 0.5 cm, comparable to our institutional PTV margin, indicating that clinically meaningful misalignments frequently occur during SBRT delivery. Real-time tracking of fiducial markers appears to facilitate identification and correction of intrafraction motion. Ongoing analyses evaluating clinical predictors of intrafraction misalignment and potential associations with treatment outcomes are ongoing.