Inclusion Criteria:

1. Provide informed written consent.
2. Age ≥ 18 years.
3. Histologically confirmed as nasopharyngeal carcinoma, and currently undergoing radical radiotherapy or chemoradiotherapy.
4. Complete and continuous records of oral / oropharyngeal mucositis grading, and self-reported swallowing-induced breakthrough pain.

Exclusion Criteria:

1. Existence of poor oral hygiene, untreated dental or periodontal diseases, and metal dental restorations.
2. Current untreated or unresolved conditions like unstable heart diseases requiring treatment, and poorly controlled diabetes mellitus.
3. Body mass index (BMI) <18.5.
4. Unsuitable to participate in current study, according to researchers' assessment.