Daily kV cone-beam CT and deformable image registration as a method for studying dosimetric consequences of anatomic changes in adaptive IMRT of head and neck cancer

Purpose. Evaluating a method for anatomic changes assessment and actually delivered doses during head and neck (H&N) cancer radiotherapy (RT) utilizing volumetric images from cone-beam CT (CBCT) and a commercially available deformable image registration (DIR) software. Material and methods. Thirty-three daily acquired CBCT image sets and the planning CT of one H&N cancer patient were retrospectively transferred from a standard treatment planning system (TPS) to the DIR software. The planning CT was deformed to each CBCT and the contours delineated for planning purposes were propagated. Transfer of each deformed planning CT back into the TPS enabled re-calculation of the actual daily delivered dose distribution based on online image-guidance. For both normal tissues and target volumes the deformed contours were visually evaluated and dose-volume histogram (DVH) parameters were calculated. Results. The workflow of the method took 45 minutes to estimate delivered dose for each treatment fraction. Propagated deformed contours were acceptable for evaluating changes in anatomy. Based on daily DVH parameters the actual delivered dose could be monitored. Conclusion. A proof-of-principle method to quantitatively monitor anatomical changes and delivered dose during the course of fractionated RT for H&N cancer has been demonstrated. This provides a tool for exploring adaptive re-planning strategies.