Room: Room 205
Purpose: Currently there is no reliable strategy to manage intrafractional motion during Tomotherapy treatment. In this project, we studied the effect of motion compensation with realistic frequency on stereotactic body radiotherapy (SBRT) using sequential monoscopic imaging guidance and jaw/MLC tracking.
Methods: Previously recorded actual prostate motions through Calypso tracking were used in this study. Two representative cases, one with large motion (peak-to-peak 15mm) and the other with average motion (6mm), were re-planed on Tomotherapy for SBRT using 35Gy/5 fractions regimen and four different jaw settings: 1cm static, 2.5cm static, 2.5cm and 5cm dynamic. The jaw and MLC was adjusted in our 4D dose calculation software to track the movement of target. In addition to complete compensation at each time point of motion trace, two discrete compensations according to the motion trace update from sequential monoscopic images were explored: sampling motion with real plan gantry rotation period (P-GRP) and with extreme shortest period of 12s (12s-GRP).
Results: For treatment using 1cm jaw, PTV coverage for patient with the large motion was 96.5% for motion-free, 55.0% for motion-included plan, and 91.0%, 71.1% and 70.3% for complete, P-GRP and 12s-GRP compensation strategy, respectively. Rectum 1cc dose was increased 7.3% by the motion and could be reduced within 0.5% from the original by either of compensation methods. When the jaw size was 2.5cm or larger, target coverage dropped 4.7% at most. All three compensation methods achieved acceptable PTV coverage (> 94%) and rectum sparing (<1.6% hotter).
Conclusion: The effect of sequential monoscopic imaging guided motion compensation on Tomotherapy depends on motion amplitude, plan settings and the compensation frequency. For large motion cases, PTV coverage cannot be fully corrected with realistic compensation frequency for the plan of 1cm jaw, but can be largely recovered by plan with 2.5 cm jaw or larger.
Funding Support, Disclosures, and Conflict of Interest: Dr. Lian is in part supported by a research grant (027214) from Accuray Inc.