title: Dose-response relationships within the parotid gland after radiotherapy for head and neck cancer.
abstract: OBJECTIVE
  To determine the salivary function, after parotid-sparing radiotherapy (RT), of different regions within the parotid gland and to evaluate dose-function relationships within the parotid glands and between patients.
  
  
  METHODS
  Sixteen head and neck cancer patients, irradiated between September 1999 and November 2000 using a conformal parotid-sparing technique, were included in this study. Before RT and 7 months after RT (range 6-10 months), a salivary gland scintigraphy was performed in all patients combined with a single photon emission computed tomography (SPECT). The salivary excretion fraction (SEF) was measured, after stimulation, in 8-12 transverse 5mm SPECT slices of each parotid. Loss of salivary excretion fraction (dSEF %) of these slices was calculated as the proportion of SEF after RT as compared to SEF before RT. Since the planning CT-scan and the SPECT-scintigraphy were performed in the same treatment position, the dose to a transverse slice within the parotid gland could be matched to the loss of salivary excretion fraction of that respective slice. A non-linear model was fitted to the dose-loss of function data and the dose resulting in 50% loss of salivary excretion fraction (D50) was calculated.
  
  
  RESULTS
  Before RT, all but one patient presented with normal salivary excretion fractions (SEF) of both parotid glands. Within the same parotid gland, the SEF's of the different slices were almost equal. Seven months after RT, the reduction in SEF was statistically significant (P-value<0.0001). A significant difference in loss of salivary excretion fraction (dSEF) was also observed between both parotid glands (P<0.0001) as a result of the parotid-sparing technique. When plotting the dSEF of a slice versus the dose given to that slice, doses as low as 10-15 Gy could result in a serious loss of function (dSEF>50%). After fitting a non-linear model to these plots, the mean dose resulting in 50% loss of salivary excretion fraction (D50) 7 months after RT was 22.5 Gy. A large inter-patient variability was found in D50.
  
  
  CONCLUSIONS
  Salivary SPECT is a useful tool for the evaluation of the salivary function of different slices within the parotid gland. Before irradiation, the different slices within one parotid gland act as functional sub-units contributing equally to the function of the entire gland. Seven months after an average dose of 22.5 Gy (D50) the functional sub-unit has lost 50% of its excretion fraction. The high inter-patient variability in D50 and the observation that low doses (10-15 Gy) can induce serious loss of function should prompt us in the clinic to reduce the dose to the parotids even lower than the threshold of 22.5 Gy.
authors.affiliation: Department of Radiation Oncology, University Hospital Gasthuisberg, Herestraat 49, B-3000 Leuven, Belgium.
authors.forename: Barbara
authors.initials: B
authors.lastname: Bussels
authors.forename: Annelies
authors.initials: A
authors.lastname: Maes
authors.forename: Patrick
authors.initials: P
authors.lastname: Flamen
authors.forename: Philippe
authors.initials: P
authors.lastname: Lambin
authors.forename: Katrien
authors.initials: K
authors.lastname: Erven
authors.forename: Robert
authors.initials: R
authors.lastname: Hermans
authors.forename: Sandra
authors.initials: S
authors.lastname: Nuyts
authors.forename: Caroline
authors.initials: C
authors.lastname: Weltens
authors.forename: Silvia
authors.initials: S
authors.lastname: Cecere
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Lesaffre
authors.forename: Walter
authors.initials: W
authors.lastname: Van den Bogaert
mesh_terms: D000328:Adult; D000368:Aged; D004307:Dose-Response Relationship, Radiation; D005260:Female; D005500:Follow-Up Studies; D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D008297:Male; D008875:Middle Aged; D008962:Models, Theoretical; D010305:Parotid Diseases / Q000000981:diagnostic imaging / Q000209:etiology; D010306:Parotid Gland / Q000000981:diagnostic imaging* / Q000502:physiology / Q000528:radiation effects*; D011832:Radiation Injuries; D020266:Radiotherapy, Conformal; D015203:Reproducibility of Results; D015899:Tomography, Emission-Computed, Single-Photon