| Objective:To analyze changes in parotid glands dose resulting from anatomic changes throughout a course of image-guided radiotherapy in a cohort of head-and-neck patients.Methods:The study population consisited of 5 nasopharyngeal carcinoma patients treated definitively with intensity-modulated radiotherapy.A total of 35 weekly kilovoltage computed tomography images acquired by Varian On-board Imaging system were retrospectively processed through a deformable image registration algorithm to be registered to the planning kilovoltage computed tomography images.The dynamic changing contouring of parotid glands were manually delineated on each axial slice to investigate the anatomic changes. The original treatment plans were overlaid onto each registered CT images,and each hybrid plan was evaluated.The weekly and cumulative dose deviation from the original treatment plan were analyzed.Corrections between dosimetric and anatomic changes were investigated.Results:Parotid glands decreased in volume to 88.7%~64.4% from the original volume.The lateral and medial aspects of both parotid glands showed a medial translation of 0.4mm/wk and 0.6mm/wk. At the end of the treatment,the cumulative parotid mean dose of the 5 patients differed from the plan dose by an average of 15%.Compared with original treatment plans,the weekly volume planned receiving 30Gy and the volume of cumulative mean dose at treatment completion increased by 13.4% and 15.9%,respectively.The dose difference was correlated with weight loss and a migration of parotids toward the high-dose region.Conclusion:The use of weekly kilovoltage computed tomography imaging and deformable image registration techniques make it possible to investigate the dynamic changes in parotid glands and calculate the dose deviations resulting from anatomic changes.Significant dose variations were observed as result of interfraction anatomic changes.These techniques enable the implementation of dose-adaptive radiotherapy.
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