Purpose: Precision is vital in spine stereotactic radiosurgery(SRS) due to highly-conformal dose distributions and ~1mm margins. However, most image-guided radiotherapy(IGRT) techniques only monitor interfraction motion or require specialized devices to monitor intrafraction motion. Feasibility of utilizing triggered kV imaging with the TrueBeam advanced imaging package to capture intrafraction motion during the treatment of spine SRS patients is investigated.
Methods: Triggered kV imaging for intrafraction motion management was tested with an anthropomorphic phantom and simulated spine SRS treatments to the thoracic and lumbar spine. Various shifts/rotations were introduced in x, y, z, pitch, and yaw. Gantry-angle-based triggering was set to acquire kV images every 45o to investigate how well various shifts could be visualized. The anterior portion of the vertebra(to spinal canal) and spinous process were contoured as IGRT structures. Upon each triggered kV image acquisition, a 2D projection of the IGRT structures was automatically calculated and updated at arbitrary angles for display onto kV images (qualitative visual guide for patient alignment during treatment delivery). This method was implemented clinically for treatment of 32 patients(75 fractions). During treatment, patient positions were monitored with kV images triggered every 15 seconds.
Results: Phantom images revealed that IGRT structure accuracy and therefore utility of projected contour during triggered imaging improved with smaller CT slice thickness. Contouring vertebra superior and inferior to the treated site was necessary to detect clinically relevant phantom rotation. Clinical implementation verified robustness of patient immobilization. Based on visual inspection of projected IGRT contours on planar kV images, appreciable intrafraction motion was detected in only one fraction(1.3%).
Conclusion: Feasibility of triggered imaging for spine SRS intrafraction motion management has been demonstrated in phantom experiments and implementation for patient treatments. This technique allows efficient, non-invasive monitoring of patient position using the TrueBeam OBI and patient anatomy as a direct visual guide.