Target and organ dose estimation from intensity modulated head and neck radiation therapy using 3 deformable image registration algorithms

PURPOSE: To estimate the delivered dose to targets and organs at risk for head and neck radiation therapy by accumulating the dose using 3 deformable image registration (DIR) algorithms. METHODS AND MATERIALS: Five head and neck patients, who had daily cone beam computed tomography (CT) images taken during the course of treatment, were retrospectively studied. To overcome the small field of view limitations and Hounsfield unit uncertainties of cone beam CT, a planning CT was deformably registered and resampled onto each cone beam CT image with a 4.2-cm uniform field of view expansion. The dose of the day was calculated on these resampled CT images, warped, and accumulated to the planning CT using 3 different DIR algorithms. Dosimetric indices for targets and organs at risk were determined from dose-volume histograms and compared with corresponding planned quantities. RESULTS: There were no significant differences among the cumulative dose-volume histograms estimated by the 3 DIR algorithms. The cumulative mean dose deviation was less than 2% from the corresponding plan dose in general for the planning and gross tumor volumes. However, the parotid gland mean dose showed a large variation, with a maximum 33% deviation. This was due in part to considerable patient weight loss during the first 3 weeks of treatment. The corresponding target and organ mean dose deviations ± standard deviation, estimated as an average of the 3 DIR algorithms, were 1.0 ± 1.6% for planning target volumes, 1.6 ± 2.3% for gross tumor volumes, 7.3 ± 9.6% for left parotid, 10.3 ± 11.9% for right parotid, and 3.3 ± 4.7% for mucosa. The target coverage deviation (dose to 95% of the volume) was -2.8 ± 1.8% for planning target volumes and 0.2 ± 2.8% for gross tumor volumes. The deviation of the maximum dose to the spinal cord was 2.1 ± 2.4%. CONCLUSIONS: Results of this study indicate that the estimated target dose generally remains within 2% of the intended dose for plans with a 5-mm planning margin. More frequent plan adaptation might be beneficial to avoid unintended excessive dose to parotid glands