Scintigraphic evaluation of long-term salivary function after bilateral whole parotid gland irradiation in radiotherapy for head and neck tumour.
In 25 patients with head and neck tumour, in whom bilateral whole parotid glands were totally included in the irradiation volume, parotid gland function was estimated and compared with that of 58 non-irradiated parotid glands in 29 patients, by symptomatic grading and radioisotope (RI) sialography. Sequential follow-up by RI sialography has suggested that if the total dose to the parotid gland is less than 52 Gy or if the time-dose fractionation (TDF) value is less than 80, then partial recovery of salivary function can be expected 1 or 2 years after irradiation, even if patients show severe xerostomia during the first 6 months. Long-term recovery rarely occurs in parotid glands which have been irradiated at doses exceeding 55 Gy or with TDF values of 88 or higher. The 50% complication probability (ED50) was less than 33 Gy for subacute xerostomia during the first 6 months and 52.5 Gy for chronic xerostomia after 12 months, respectively. Although a prospective study will be required for confirmation, differences between the dose-response curves of xerostomia at subacute and chronic phases suggest a basis in different biological mechanisms.