Parotid gland radiation dose-xerostomia relationships based on actual delivered dose for nasopharyngeal carcinoma

Xerostomia induced by radiotherapy is a common toxicity for head and neck carcinoma patients. In this study, the deformable image registration of planning computed tomography (CT) and weekly cone-beam CT (CBCT) was used to override the Hounsfield unit value of CBCT, and the modified CBCT was introduced to estimate the radiation dose delivered during the course of treatment. Herein, the beams from each patient's treatment plan were applied to the modified CBCT to construct the weekly delivered dose. Then, weekly doses were summed together to obtain the accumulated dose. A total of 42 parotid glands (PGs) of 21 nasopharyngeal carcinoma patients were analyzed. Doses delivered to the parotid glands significantly increased compared with the planning doses. V-20, V-30, V-40, D-mean, and D-50 increased by 11.3%, 28.6%, 44.4%, 9.5%, and 8.4% respectively. Of the 21 patients included in the study, eight developed xerostomia and the remaining 13 did not. Both planning and delivered PG D-mean for all patients exceeded tolerance (26 Gy). Among the 21 patients, the planning dose and delivered dose of D-mean were 30.6 Gy and 33.6 Gy, respectively, for patients with xerostomia, and 26.3 Gy and 28.0 Gy, respectively, for patients without xerostomia. The D-50 of the planning and delivered dose for patients was below tolerance (30 Gy). The results demonstrated that the p-value of V-20, V-30, D-50, and D-mean difference of the delivery dose between patients with xerostomia and patients without xerostomia was less than 0.05. However, for the planning dose, the significant dosimetric difference between the two groups only existed in D-50 and D-mean. Xerostomia is closely related to V-20, V-30, D-50, and D-mean.