To evaluate the efficacy of sodium fluoride to preventing radiation caries for patients with nasopharyngeal carcinoma receiving radiotherapy.

Inclusion Criteria:

• a. Patients with histologically confirmed non-keratinizing NPC (including differentiated type and undifferentiated type, world health organization [WHO] II or III).

  b. Original clinical staged as II-IVa (according to the 8th AJCC edition) c. Male or female who are not pregnant. d. No evidence of distant metastasis (M0). e. Age between 18-60 years. f. WBC≥4×109/L, PLT≥100×109/L, HGB≥90g/L. g. Normal liver function test: Alanine Aminotransferase (ALT), Aspartate Aminotransferase (AST) <1.5×upper limit of normal (ULN) concomitant with alkaline phosphatase (ALP) ≤2.5×ULN, and bilirubin ≤ULN.

  i. With normal renal function test (creatinine clearance rate≥60 ml/min or creatinine< 1.5×ULN.

  j. The Eastern Cooperative Oncology Group (ECOG) Performance status less or equal to 1.

  k. Patients must be informed of the investigational nature of this study and give written informed consent.

  l. Patients receive nasopharyngeal and neck radiotherapy. m. After pre-radiotherapy oral examination, no less than 2 functional teeth in any 1/6 segment of the mouth.

Exclusion Criteria:

• a. With clinical stage of I and IVb (according to the 8 th AJCC edition). b. Patients with tumor recurrence or distant metastasis. c. Prior malignancy except adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer.

  d. History of previous radiotherapy (except for non-melanomatous skin cancers outside intended RT treatment volume).

  e. Prior chemotherapy or surgery (except diagnostic) to primary tumor or nodes. f. Pregnancy or lactation (consider pregnancy test in women of child-bearing age and emphasize effective contraception during the treatment period).

  g. Fewer than 2 functional teeth in any 1/6 segment of the mouth after pre-radiotherapy scaling.

  h. Lesions of infection in the oral cavity at baseline, severe gingivitis (gingival index = 3, i.e., severe inflammation of the gums, marked redness, swelling or ulceration of the gums, and a tendency to bleed automatically).

  i. Age < 18 or age > 60. j. Any severe intercurrent disease, which may bring unacceptable risk or affect the compliance of the trial, for example, unstable cardiac disease requiring treatment, acute exacerbation of chronic obstructive pulmonary disease or other respiratory illness requiring admission to hospital, renal disease, chronic hepatitis, diabetes with poor control (fasting plasma glucose >1.5×ULN), and mental disturbance.