2280 - Leveraging MV-KV Imaging Based Quantitative Intrafractional Motion Tracking for Safe Paraspinal SBRT: The Impact of Patient Movement on Radiation Dose

Q. Fan¹, H. Pham², P. Zhang², L. Zhang², Y. Fu¹, J. Cuaron³, L. I. Cervino², J. M. Moran², X. Li⁴, and T. Li¹; ¹Memorial Sloan Kettering Cancer Center, NEW YORK, NY, ²Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, NY, ³Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, ⁴Memorial Sloan Kettering Cancer Center, New York, NY

Purpose/Objective(s): A variety of independent studies reported that significant intrafractional patient motion can occur in paraspinal SBRT despite the use of immobilization. However, analysis of the dosimetric impact of such motion on clinical patients is limited in the literature. This work aims to provide the analysis based on our prior work on MV-KV imaging based quantitative intrafractional motion tracking during paraspinal SBRT. Materials/

Methods: Thirty-one patients undergoing paraspinal SBRT at three network clinics were randomly selected for this study. The selected patients had disease sites covering all regions of the spine, and there were no exclusion criteria. SBRT was planned with a 9-field IMRT technique using an in-house automated expedited constrained hierarchical optimization system. During treatment delivery, simultaneous MV-KV image pairs were continuously acquired using the iTools capture software. A proprietary software was used to retrospectively register those image pairs with the corresponding references based on normalized cross-correlation and to triangulate the 3D intrafractional motion throughout the treatment. When applicable, manual registration was performed to verify and correct the software’s automatic registration. To assess motion impact, treatment plans were recalculated with isocenter corrections based on 3D shifts determined from associated MV-KV image pairs. Dosimetric parameters of target volumes and relevant organs at risk (OARs) were analyzed.
Results: The dosimetric impact generally increased with greater 3D motion magnitude. For PTV, V100% was the most affected by intrafractional motion, followed by Dmax and V95%. 16% of patients experienced a reduction in V100% of at least 3.4%, with an average reduction of 6.3% and a maximum reduction of 12.2%. For GTV or CTV, 16% of patients had a change of minimum dose to the coolest 0.035 cc by at least 1.2 Gy with a maximum of 2.4 Gy. Regarding OAR doses, 26% of patients experienced an increase in spinal cord or cauda Dmax of at least 1 Gy, with a maximum increase of 4.8 Gy and 2.7 Gy for the cord and cauda, respectively. The maximum increase in the hottest 0.035 cc of the esophagus was 4.8 Gy. In 42% of all patients, at least one OAR dose for the actual delivery exceeded a dose constraint met during planning.
Conclusion: Intrafractional patient motion during paraspinal SBRT significantly compromises planned dosimetry due to the high conformality in plans, steep dose gradients, and proximity of critical OARs. Continuous monitoring and correcting such motion during treatment delivery support optimal treatment outcomes.