The purpose of this study was to provide guidance in developing and implementing a process for the accurate delivery of free breathing respiratory amplitude-gated treatments.Methods: A phase-based 4DCT scan is acquired at time of simulation and motion is evaluated to determine the exhale phases that minimize respiratory motion to an acceptable level. A phase subset average CT is then generated for treatment planning and a tracking structure is contoured to indicate the location of the target or a suitable surrogate over the planning phases. Prior to treatment delivery, a 4DCBCT is acquired and a phase subset average is created to coincide with the planning phases for an initial match to the planning CT. Fluoroscopic imaging is then used to set amplitude gate thresholds corresponding to when the target or surrogate is in the tracking structure. The final imaging prior to treatment is an amplitude-gated CBCT to verify both the amplitude gate thresholds and patient positioning. An amplitude-gated treatment is then delivered. This technique was commissioned using an in-house lung motion phantom and film measurements of a simple two-field 3D plan.Results: The accuracy of 4DCBCT motion and target position measurements were validated relative to 4DCT imaging. End to end testing showed strong agreement between planned and film measured dose distributions. Robustness to interuser variability and changes in respiratory motion were demonstrated through film measurements.
Conclusions:The developed workflow utilizes 4DCBCT, respiratory-correlated fluoroscopy, and gated CBCT imaging in an efficient and sequential process to ensure the accurate delivery of free breathing respiratory-gated treatments.
| INTRODUCTIONRespiratory motion management is an important strategy to account for breathing-induced target motion during radiation therapy. 1 If not considered during both the treatment planning and delivery processes, respiratory motion can lead to artifacts and target volumes that do not adequately encompass the target. Active respiratory motion management techniques can be used to minimize target motion during treatment delivery, allowing for a reduction in target volumes and improved normal tissue sparing. Such techniques include abdominal compression, 2 breath hold, 3 and free breathing phase or amplitude gating. 4 Many thoracic and abdominal sites are susceptible to substantial motion, with abdominal and lower lung lesion motion often measured to be greater than 1 cm. 5,6 Inter-and intrafraction changes in target motion magnitude greater than 5 mm have been