Dosimetric evaluation of four adaptive IMRT strategies for head-neck cancer

Objective: To investigate the effects of numerous re-planning strategies on the anatomic and dosimetric outcomes of target volume and organs at risk (OARs) in patients with head and neck cancer receiving fractionated radiotherapy. Methods: From 2015 to 2016, 28 patients with head and neck cancer were enrolled in this study with Shandong Cancer Hospital, consisting of 19 patients with nasopharyngeal carcinoma, 4 patients with laryngocarcinoma, and 5 patients with carcinoma of the maxillary sinus. All of them received conventionally fractionated radiotherapy. Each patient had six weekly cone-beam CT (CBCT) scans, which were performed on the first day of every week, to obtain reference images. A virtual CT image was generated by registration of planning CT and each weekly CBCT image. The four re-planning strategies were used for the reconstruction of re-planned dose, while the initial planning was used as a reference. The weekly doses calculated using virtual CT were summed together to obtain the actual dose. The actual and initial planned doses were evaluated. The nonparametric Friedman test was used to evaluate the differences between multiple groups, and the differences between any two groups were analyzed by paired t test. Results: The sizes of planning target volume, clinical target volume, and left/right parotid glands (PGs) changed significantly within the six weeks (P = 0.041, 0.046, 0.024, and 0.017, respectively). For these four re-planning strategies, there were significant differences between the actual dose and the initial planned dose to the PGs (all P< 0.05), with average values decreased by 5.02%, 11.17%, 12.08%, and 13.19%, respectively, compared with that in the reference strategy. Conclusions: Re-planning during treatment course could ensure the sparing of OARs and allow for sufficient dose to the target volume. The higher the number of re-planning strategies, the more the actual dose is close to the initial planed dose; the efficiency of two re-planning strategies is the highest. (authors)