Dose Accumulation Studies With A Dynamic Physical Anthropomorphic Phantom And An External Surrogate-Based Motion Model

Purpose: To estimate the dose delivered to a physical anthropomorphic phantom based on: 1) CT scans representing each phase of respiration; and 2) 3D images generated from a respiratory motion model and an external surrogate signal. OSLDs are used to measure doses delivered to the phantom. Methods: A commercially available physical phantom was modified, replacing the lung system with foam rubber of lung-equivalent density and placing a tumor made of bolus within the lung. A wooden diaphragm driven by a programmable motor compressed the foam with a realistic breathing pattern based on patient measurements. CT scans of the phantom were taken at several phases of a breathing cycle, and the dose delivered by a nine-field treatment at each phase was calculated with Monte Carlo. Dose distributions at each phase were mapped to a reference phase with vectors from deformable image registration performed on the original CT images. A second estimate of the delivered dose was performed replacing the CT scans and associated vectors with images and deformations generated by a motion model. Finally, with a one-field treatment plan, the estimated delivered dose was compared to measurements with OSLDs placed in the phantom. Results: The estimated dose delivered to the tumor using CT scans agreed with the estimate using model-generated images, and the difference in the D95 for the two approaches was less than 2%. This demonstrates that images generated by the motion model can be used for dose estimates. Dose measured with OSLDs at nine points within the tumor and foam lung of the phantom agreed with predictions within measurement uncertainties. Conclusion: The images generated from a motion model based on an external surrogate trace can be used to estimate dose delivered during treatment. Dose estimates were validated with measurements.