Longitudinal multi-b-value diffusion MRI for the early assessment of radiation-induced parotid injury in nasopharyngeal carcinoma.

OBJECTIVE: Complications associated with nasopharyngeal carcinoma (NPC) treatment significantly impair patients' quality of life. Radiotherapy-induced xerostomia is one of the most common complications of NPC treatment. This study aimed to evaluate the utility of quantitative parameters derived from DKI, CTRW, and FROC models for the early assessment of parotid gland damage following intensity-modulated radiotherapy (IMRT) in NPC. The findings may provide theoretical support for early identification of parotid injury and timely therapeutic intervention. METHODS: A total of 54 patients with newly diagnosed NPC underwent multi-b-value DWI and routine MRI before radiotherapy, after 15 fractions, and at the end of IMRT. Parameters from the single-exponential DWI model, as well as from the DKI, CTRW, and FROC models, were calculated. Correlations among these parameters, IMRT dose, and xerostomia scores at the end of radiotherapy and 3 months after treatment were analyzed. RESULTS: Most diffusion parameters derived from multi-b-value diffusion MRI of the parotid glands in patients with NPC gradually increased over the treatment period, whereas DKI_K values progressively decreased, with significant differences among time points (P < 0.05). The magnitude of parameter changes gradually decreased as treatment progressed. ΔDKI_K_Post-15RT and ΔDKI_K_Post-Pre were positively correlated with xerostomia scores at 3 months after IMRT (r=0.216 and 0.277; P = 0.012 and 0.002, respectively), whereas ΔCTRW_α_15RT-Pre and ΔCTRW_α_Post-Pre were negatively correlated (r=-0.258 and -0.232; P = 0.003 and 0.008, respectively). Statistically significant but weak correlations were found between several parameters and the planning target volume of nasopharyngeal carcinoma, minimum radiation dose (Dmin), and mean radiation dose (Dmean) (0.206≤|r|≤0.311, P ≤ 0.05). CONCLUSION: Quantitative parameters derived from the single-exponential model, DKI, CTRW, and FROC enable dynamic detection of early microstructural alterations in the parotid glands after IMRT. The DKI_K and CTRW_α showed statistically significant but weak correlations with xerostomia scores in patients with nasopharyngeal carcinoma. These preliminary associations indicate that the two indices may serve as exploratory imaging biomarkers for evaluating radiation-induced parotid injury; however, their reliable predictive value requires further validation. In addition, dynamic changes in ADC, DKI_D, CTRW_α, FROC_β, FROC_D, and FROC_μ were correlated with radiation dose. These findings may provide preliminary evidence to support future investigations of individualized radiotherapy regimens.