Quantitative dose-volume response analysis of changes in parotid gland function after radiotherapy in the head-and-neck region

Purpose: To study the radiation tolerance of the parotid glands as a functi on of dose and volume irradiated. Methods and Materials: One hundred eight patients treated with primary or p ostoperative radiotherapy for various malignancies in the head-and-neck reg ion were prospectively evaluated. Stimulated parotid flow rate was measured before radiotherapy and 6 weeks, 6 months, and I year after radiotherapy. Parotid gland dose-volume histograms were derived from CT-based treatment p lanning. The normal tissue complication probability model proposed by Lyman was fit to the data. A complication was defined as stimulated parotid flow rate < 25% of the preradiotherapy flow rate. Results: The mean stimulated preradiotherapy flow rate of 174 parotid gland s was 0.34 mL/min. The mean flow rate reduced to 0.12 mL/min 6 weeks postra diotherapy, but recovered to a mean flow rate of 0.20 mL/min at I year afte r radiotherapy. Reduction in postradiotherapy flow rate correlated signific antly with mean parotid dose. No threshold dose was found. Increasing the i rradiated volume of parotid glands from 0%-40% to 90-100% in patients with a mean parotid dose of 35-45 Gy resulted in a decrease in flow ratio from, respectively, approximately 100% to less than 10% 6 weeks after radiation. The flow ratio of the 90%-100% group partially recovered to 15% at 6 months and to 30% at I year after radiotherapy. The normal tissue complication pr obability model parameter TD50 (the dose to the whole organ leading to a co mplication probability of 50%) was found to be 31, 35, and 39 Gy at 6 weeks , 6 months, and I year postradiotherapy, respectively. The volume dependenc y parameter n was around 1, which means that the mean parotid dose correlat es best with the observed complications. There was no steep dose-response c urve (m = 0.45 at I year post radiotherapy). Conclusions: This study on dose/volume/parotid gland function relationships revealed a linear correlation between postradiotherapy flow ratio and paro tid gland dose and a strong volume dependency. No threshold dose was found. Recovery of parotid gland function was shown at 6 months and I year after radiotherapy. In radiation planning, attempts should be made to achieve a m ean parotid gland dose at least below 39 Gy (leading to a complication prob ability of 50%). (C) 2001 Elsevier Science Inc.