Factors influencing the parotid function in nasopharyngeal carcinoma treated with parotid-sparing radiotherapy

Background: To evaluate the factors influencing post-irradiation parotid gland function in nasopharyngeal cancer treated with parotid-sparing radiotherapy. Methods: This study consisted of 45 patients with nasopharyngeal cancer treated with radiotherapy including 3D conformal radiotherapy, intensity-modulated radiotherapy and high-dose-rate brachytherapy. The mean follow-up time was 37.5 months (range: 15-50 months). Objective parotid gland function was assessed by series sialoscintigraphy pre-irradiation and post-irradiation at 1, 6, 12 and 18 months. Subjective salivary function was recorded by the LENT/SOMA system. Wilcoxon signed-rank test was used to compare the secretion ratio (SR) of parotid gland before and after treatment. Mann-Whitney rank-sum test was used to determine the factors influencing the post-irradiated SR. Chi-square test was used to compare the correlation between subjective grading of xerostomia and objective grading of parotid glands. Results: There was a significant difference between the pre-irradiation and post-irradiation parotid gland's SR at 1 (0.30 versus 0.01, P <0.05) and 6 months (0.03 versus 0.08, P <0.05). However, there was no significant difference compared with the pre-irradiation SR at 12 (0.30 versus 0.20, P > 0.05) and 18 months (0.30 versus 0.18, P > 0.05). There was significant correlation between subjective and objective salivary function (P = 0.024) at 12 months after radiotherapy. The factor that impacted the preservation of parotid function was mean dose to the parotid gland >38.0 Gy (P <0.05). Conclusions: Our results demonstrated that parotid function could recover 1 year after treatment with parotid-sparing radiotherapy in patients with nasopharyngeal cancer. The most important factor that influenced parotid function was the mean dose to the parotid gland.