Room: Exhibit Hall | Forum 5
Purpose: To compare intra-treatment prostate motion during SBRT using an in-house MV-kV fiducial tracking system with the Calypsoâ„¢ localization system.
Methods: Motion information from 127 SBRT prostate patients (619 fractions) treated with VMAT and an in-house MV-kV tracking system was compared to data from 50 patients (213 traces) treated with IMRT and Calypso. The in-house tracking system provides a 3D trace of prostate motion by detecting fiducials on paired MV beams-eye-view images and orthogonal kV-OBI images every 7-10s during treatment. Therapists intervene to correct motion >1.5 mm in any direction. To validate the accuracy of the MV-kV method, the prostate natural motion was found by first removing patient position corrections using couch shift data and then comparing to natural motion from Calypso traces. Scatter plots, mean motion and standard deviation of prostate position at various time points was compared in the X: +Right -Left, Y: +Sup -Inf, and Z: +Ant -Post directions. Additionally, the probability of motion >2mm in any direction was compared at 1, 3 and 5 minutes.
Results: After 5 minutes of tracking, a mean deviation from the initial setup position of 0.07mm, -0.41mm -0.35mm for MV-kV vs. 0.17mm, -0.38mm, -0.21 for Calypso was observed in the X, Y, Z directions, respectively. The standard deviation of positions from all traces at the 5 minute timepoint was 0.9mm, 2.0mm, 1.9mm for MV-kV vs. 0.6mm, 1.3mm, 1.8mm for Calypso. The probability of motion >2mm after 1, 3, and 5 minutes was 9%, 22% and 30% for MV-kV vs. 8%, 17% and 26% for Calypso.
Conclusion: Intra-treatment MV-kV motion monitoring of the prostate provides temporal tracking consistent with Calypso. Post-Inf drift of the prostate position over time is identified with both systems. MV-kV monitoring offers accurate 3D intra-treatment prostate motion monitoring using standard linac imaging systems.
Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by Varian Medical Systems