Microstructural and physiological changes of parotid glands after RT for head and neck cancer

Purpose/Objective:
Salivary gland hypofunction and xerostomia are prominent complications of conventional radiotherapy (RT) of head and neck cancer (HNC), having considerable negative impact on the patients’ quality of life. It is therefore important to assess the salivary gland function in these patients for diagnostic and management purposes.
The aim of this study is to investigate use of diffusion-weighted magnetic resonance imaging (DWI) in the assessment of radiation-induced microstructural changes of the parotid glands in terms of apparent diffusion coefficients values (ADC), compared to physiological changes in terms of measurements of whole salivary flow rates. It is hypothesized that use of combined methods for assessment of parotid gland function, provides further insight into the factors that can predict xerostomia.
Material/Methods:
Five patients with HNC who have completed their RT course were assessed in this pilot study. The patients were examined before the first RT fraction (scan 1), after completion of the RT course (scan 2) and one year after completion of the RT course (scan 3). Unstimulated (at rest) and gustatory stimulated DWI examinations were performed on a 1T MRI system using the DWIBS sequence. Whole-volume region of interest (ROI) method was used for delineation of parotid glands. To suppress perfusion and salivary flow effects high b-values (400, 600 and 800 s/mm²) were used for ADC calculation using a mono exponential fit. Unstimulated and chewing-stimulated whole saliva were collected and the flow rates were calculated. The degree of xerostomia was evaluated using the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer (RTOG/EORTC) morbidity score. Mean radiation dose (Gy) was registered for both contra- and ipsilateral parotid glands.
Results:
Mean ADC (ADCMean) values and salivary flow rates were higher in the stimulated than in the unstimulated state (Figure 1a). An increase in ADCmean from scan 1 to scan 3 both in unstimulated and stimulated state is observed. Further, the unstimulated salivary flow tended to decrease, while chewing-stimulated salivary flow decreased between scan 1 and2 and increased from scan 2 to scan 3. The ADCMean tended to increase (R^2 = 0.29-0.37), and the salivary flow rates to decrease (R^2 = 0.01-0.12), with increasing mean radiation dose to the glands (Figures 1b). Mean radiation dose to the ipsilateral parotid gland was used for the correlation between saliva flow rates and mean dose. Figure 1c shows a strong correlation between both ADCMean and saliva flow, and the degree of xerostomia (R^2 = 0.66-0.97).
Conclusion:
The results of this pilot study indicate that the severity of microstructural and physiological changes of the parotid glands after RT is dependent on the mean radiation dose to the glands. It also suggests that DWI and salivary flow measurements can be used for assessing parotid gland function after RT and are associated with degree of xerostomia.