Early Changes in Apparent Diffusion Coefficient for Salivary Glands during Radiotherapy for Nasopharyngeal Carcinoma Associated with Xerostomia

Objective: To evaluate the early changes in the apparent diffusion coefficient (ADC) of the salivary glands during radiotherapy (RT) and their association with the degree of xerostomia at 6 months after RT in patients with nasopharyngeal carcinoma (NPC). Materials and Methods: We enrolled 26 patients with NPC who underwent RT. Each patient underwent diffusion-weighted MRI of the salivary glands at rest and with gustatory stimulation within 1 week before RT and 2 weeks after the beginning of RT. The ADC at rest ($ADC_R$) and increase and increase rate with stimulation ($ADC_I$, $ADC_{IR}$) of the submandibular and parotid glands were calculated. The differences in the variables' values between 2 weeks after the beginning of RT and baseline (${\Delta}ADC_R$, ${\Delta}ADC_I$, and ${\Delta}ADC_{IR}$) were compared to the degree of xerostomia at 6 months after RT. Results: The $ADC_R$ of the submandibular and parotid glands were both significantly higher at 2 weeks after the beginning of RT than found at baseline (both p < 0.01). The $ADC_I$ and $ADC_{IR}$ for the parotid glands were both significantly lower at 2 weeks after the beginning of RT than found at baseline (both p < 0.01). ${\Delta}ADC_I$ and ${\Delta}ADC_{IR}$ of the parotid glands were associated with the degree of xerostomia at 6 months after RT (r = -0.61 and -0.72, both p < 0.01). Conclusion: The ADCs of the salivary glands change early during RT. The differences in the ADC increase and increase rate of the parotid glands between 2 weeks after the beginning of RT and baseline were associated with the degree of xerostomia at 6 months after RT.