Spine Stereotactic Body Radiation Therapy Without Immobilization: Detailed Analysis of Intrafraction Motion Using High-Frequency kV Imaging During Irradiation

Abstract

Spine SBRT requires high positioning accuracy and a stable patient to maximize target coverage and reduce excessive irradiation to organs at risk (OARs). Positional verification during spine SBRT delivery helps to ensure accurate positioning for all patients. We report our experience with non-invasive 3-dimensional (3D) target position monitoring during volumetric modulated arc therapy (VMAT) of spine metastases, in non-immobilized patients positioned using only a thin mattress and simple arm and knee supports. Fluoroscopic planar kV images were acquired at 7 frames/second using the on-board imaging system during VMAT spine SBRT. Template matching and triangulation were used to track the target in vertical, longitudinal and lateral direction. If the tracking trace deviated >1mm from the planned position in ≥1 direction, treatment was manually interrupted and 6D CBCT-based couch correction was performed. Tracking data was used to retrospectively analyze the target position. Positional data, agreement with CBCT, correlation between position of the couch and direction of any positional correction, and treatment times were analyzed. 175 fractions were analyzed. Delivery was interrupted 83 times in 66 fractions for a deviation >1mm. In 97% of cases the difference between tracking data and subsequent clinical shift performed after the CBCT match was ≤0.5mm. Lateral/longitudinal shift performed after intervention correlated with the couch roll/pitch at the start of treatment (correlation coefficient: -0.63/0.53). Mean (SD; range) time between start of first imaging and end of the last arc was 15.2 minutes (5.1;7.6-36.3). Spine tracking during irradiation can be used to prompt an intervention CBCT scan and repositioning, so that a spine SBRT target deviates by ≤1mm from the planned position, even in non-immobilized patients. kV tracking and CBCT are in good agreement. The data support verification CBCT after all 6 degrees-of-freedom positional corrections in non-immobilized spine SBRT patients.