Purpose: Airway injury is an important yet poorly understood side-effect in lung stereotactic ablative radiotherapy (SAbR), especially in peripheral airways (<7mm diameter). Previously, we showed that airway radiosensitivity depends on diameter and maximum-point dose. Accurate estimations of airway radiosensitivity are needed for creating SAbR plans that minimize damage to airways, thereby preserving respiratory function. However, respiratory motion presents a barrier to accurate dose-estimations. Here, we develop five methods to account for airway motion and evaluate them using a patient-derived digital motion model.
Methods: A â€œground-truthâ€? digital breathing-model was generated from a 4DCT of a lung SAbR patient (IRB-approved) using b-spline deformable image-registrations (225 breathing-cycles, Â±10% variation in amplitude and respiratory period). A high-resolution breath-hold CT (BHCT) was input into a research virtual bronchoscopy software to auto-segment 239 individual airways. The average-intensity image of the 4DCT-phases (CTavg) was used to manually-contouring the organs-at-risk (OARs) and planning-target-volume (PTV). Five motion-management methods were developed. Methods m1 and m2 used the BHCT to recreate the plan. PTV and OARs were deformed to BHCT in m1. In m2, PTV was transferred undeformed to BHCT. m3 and m4 used the CTavg to recreate the plan. In m3, airways were deformed to CTavg. In m4, the airways were deformed to each 4DCT-phase and union-structures were transferred onto the CTavg. In m5, the plan was recreated at the 10 4DCT-phases, calculating final dose-distributions as the average of the 10 phases deformably-registered to BHCT.
Results: The model simulated ~14 mm max motion for peripheral airways. Airway mean [min, max] dose-errors with respect to ground truth were: m1 (112%, [0.02%, 673%]); m2 (56%, [0.1%, 317%]); m3 (20%, [0.001%, 90%]); m4 (44%, [0.02%, 178%]); m5 (3%, [0.01%, 13%]).
Conclusion: These results indicated that recreating the plan at individual breathing-phases enabled accurate dose-estimations in airways in the presence of significant motion.
Funding Support, Disclosures, and Conflict of Interest: This work is supported by the National Institutes of Health (R01 CA202761).