Purpose: In liver SBRT, high dose gradients and tumor motion make it challenging to accurately deliver dose. Real-time tumor tracking using on-board kV imaging can address intrafraction positional errors online. The purpose of this study was to evaluate the impact of real-time kV imaging on the delivery and accuracy of liver SBRT in a retrospective patient cohort.
Methods: 33 patients treated with 41 courses of liver SBRT on a conventional linac were retrospectively analyzed. Patients were treated in 3-5 fractions with 6-18 Gy/fx, and kV images were acquired every 20° at the start of and during arc-based treatment to visualize the location of implanted fiducial markers as surrogate for tumor position. Corrections were made if markers were >3 mm from the expected reference position. Treatment interruptions and corrections were recorded from each fraction and the average interruption rate was calculated for each patient. Statistical analysis was performed using multiway ANOVA to evaluate interruption rate as a function of gating, compression, number of implanted fiducial markers, treatment time, and PTV volume. The elapsed time of pauses and shifts was recorded as well as the magnitude of shifts.
Results: Treatment interruptions occurred at a rate of 0.69 per fraction. Gating threshold adjustment and patient re-alignment shifts occurred in 61% and 39% of interruptions, respectively. The interruption rate was significantly higher in patients with longer treatment times (p<0.01). Average time per interruption was 1.9 ± 1.6 minutes. No significant difference was observed in the elapsed time for gating adjustment and re-alignment shifts. Median shifts for patient re-alignment were 1.4 mm (AP), 4.0 mm (SI), and 1.2 mm (LR). The median radial (3D) shift was 5.7 mm.
Conclusion: Real-time imaging can correct for significant intrafraction positional errors with relatively minimal clinical impact and ensures accurate dose delivery in liver SBRT.
Funding Support, Disclosures, and Conflict of Interest: Funding in part from the National Institutes of Health under award number K12CA086913, the University of Colorado Cancer Center/ACS IRG #57-001-53 from the American Cancer Society, the Boettcher Foundation, and Varian Medical Systems. Jones/Miften report grants from Varian Medical Systems during the conduct of study, outside of the submitted work.