Diffusion-weighted MRI of non-irradiated parotid glands and parotid glands exposed to radiotherapy

Diffusion-weighted MRI of non-irradiated parotid glands and parotid glands exposed to radiotherapy Purpose/Objective Xerostomia is a prominent complication of conventional radiotherapy (RT) of head and neck cancer (HNC), with a considerable impact on quality of life. Therefore, it is important to assess the salivary gland function in these patients for diagnostic and management purposes. Diffusion-weighted magnetic resonance imaging (DWI) has previously been used to evaluate salivary gland function after radiotherapy. The purpose of this study is to compare ADC values of parotid glands in one healthy volunteer and one HNC patient who underwent volumetric modulated arc therapy (VMAT). The hypothesis is that DW-MRI can be used to detect changes in parotid glands during and after head and neck RT and that those changes are more prominent than day to day changes in healthy glands. Material/Methods One healthy volunteer (woman, 37 years old) and one HNC patient (man, 46 years old, carcinoma of the oropharynx, left tonsil) who underwent radiotherapy (2Gy x 34, mean dose to right parotid gland of 8Gy, and 48Gy to the left parotid gland) at Herlev Hospital, Copenhagen University, Denmark were assessed in this study. DWI examinations were performed on a 1T MRI system using the following sequences: T1W-3D GRE, T2W SE and a DWIBS sequence with b-values of 50, 150, 400, 600 and 800 s/mm2 and a voxel size of 2mm x 2mm x 3mm. Only the high b-values (400, 600 and 800 s/mm²) were used in calculating the apparent diffusion coefficients (ADC). The healthy volunteer was scanned on three consecutive days and at the same time each day. The HNC patient was scanned 3 days before the first RT fraction, once during RT (after the 24th fraction), and once 6 weeks after completion of the RT course. In each scan, three slices were selected among the DW images and regions of interest (ROIs) were delineated in the parotid glands for each b-value. ADC values were compared using analysis of variance (ANOVA: Single Factor) and Student's t-test. Results Results of the statistical analysis for the healthy volunteer showed no significant differences in ADC values for the three scans or between the glands (ANOVA p>0.05; t-test p>0.05). For the HNC patient, the statistical analysis showed no significant difference between the right end left parotid gland (t-test p>0.05), when the analysis included all three scans. Mean ADC values for the left, most irradiated gland, in the first and the third scans are very similar, whereas the values in the second scan (during RT) differs a lot. ANOVA analysis confirms this finding, showing a significant difference between the three scans of the irradiated left gland (p<0.05). Conclusion DW-MRI can be used to detect changes in irradiated parotid glands and that changes are much more prominent than day to day variations in non-irradiated parotid glands. This method can be used to assess parotid gland function during and after radiotherapy, which might lead to earlier diagnosis of permanent xerostomia.