Influence of Radiation Dose on Changes in Parotid Gland Function

Objectives: Xerostomia is one side effect after radiotherapy of malignant tumors in the head and neck region. Therefore, the aim of this study was to determine the influence of parotid-sparing radiotherapy on salivary flow rate and quality in irradiated patients. Methods: 32 patients receiving parotid-sparing radiotherapy were included. The patients received either a mean dose of 20-35 Gy (11 patients, group A), 35-50 Gy (11 patients, group B) or 50-65 Gy (10 patients, group C). Stimulated saliva was evaluated in regard to salivary flow, buffer capacity, colonisation with Streptococcus mutans and Lactobacillus before, three weeks after beginning and four weeks after the end of radiotherapy. Results: The mean stimulated salivary flow rate of all patients was 0.78 ml/min before radiotherapy. After three weeks the stimulated saliva flow was decreased in all groups. At this point no significant differences between the three groups could be observed. Patients receiving a mean dose below 35 Gy showed a salivary flow of 0.47 ml/min (60.3% of baseline) 4 weeks after irradiation. Increasing the mean parotid dose to 35-50 Gy and 50-65 Gy resulted in a significantly higher decreased flow rate of 0.17 ml/min (21.8%) and 0.15 ml/min (19.2%) compared to group A. Between group B and C no significant differences could be detected. The buffer capacity in group A retained at baseline level, while in group B and C a reduction could be observed. The colonisation with Streptococcus mutans and Lactobacillus varied little in all groups over the whole period. Conclusions: The results demonstrate that parotid-sparing radiotherapy has a significant influence on salivary flow. Patients treated with mean parotid gland doses lower than 35 Gy showed significant higher salivary flow compared to both other groups. Therefore, a mean parotid gland dose of at least less than 35 Gy might be desirable in radiotherapy planning. Keywords: radiation, parotid gland, salivary flow rate