O-040. Intensity modulated radiotherapy: A valuable tool to preserve parotid function

Aims: In a prospective study of 108 patients, direct measurement of parotid salivary flow was correlated with dose-volume histograms (DVHs). One year after radiotherapy, a flow rate of less than 25% was seen for a TD₅₀ (the dose at which 50% complications is seen) of 39 Gy. Since with conventional radiotherapy for oral cavity/oropharynx tumours the dose to the parotid glands in general is 50 Gy, preservation of parotid gland function by intensity modulated radiotherapy (IMRT) was evaluated. Method: Various IMRT plans have been generated for a number of patients, using the same gross target volume (GTV) and clinical target volume (CTV) as in conventional radiotherapy plans, providing a margin of 5 mm for setup errors (PTV). The mean parotid gland dose was calculated for each IMRT design. Results: Dose to the PTV was, as in conventional radiotherapy, 70 Gy to the primary tumour and positive neck node(s) and 50 Gy to the area at risk for subclinical disease. The mean dose of parotid glands using an IMRT plan with five beams for an oropharyngeal case was 26 Gy, resulting in a normal tissue complication rate (NTCP) of 0.2. In the conventional treatment, the calculated NTCP was 0.6. The DVHs for the PTV were equal for both cases. Conclusion: IMRT for oropharyngeal/oral cavity tumours may result in a significant increase in mean parotid gland function after radiotherapy with comparable dose to the PTV.