The purpose of this study is to examine the efficacy of maintenance chemotherapy for the treatment of metastatic nasopharyngeal carcinoma. Participants will be randomized to 3 arms. Arm 1, control group, participants will receive only 4-8 cycles of conventional chemotherapy; arm 2, participants will receive 6 cycles of maintenance chemotherapy after conventional chemotherapy; arm 3, experiment arm, after conventional chemotherapy, participants will receive maintenance chemotherapy until disease progression or intolerable toxicities.

This is a phase 3 randomized clinical trial to determine the efficacy of maintenance chemotherapy for the treatment of metastatic chemotherapy and how it should be delivered. Participants will be randomized to 3 arms. Arm 1 (control group), participants will receive only conventional chemotherapy; arm 2 (experimental group), participants will receive up to 6 cycles of maintenance chemotherapy after conventional chemotherapy; arm 3 (experimental group), after conventional chemotherapy, participants will receive maintenance chemotherapy until disease progression or intolerable toxicities. Participants in all 3 arms will receive similar conventional chemotherapy. Four to eight cycles of conventional chemotherapy with platinum-based regimen is preferred. Maintenance chemotherapy should use one of the regimens: S1 (40mg, twice a day for 14 days, every 3 weeks), capecitabine (1000mg/m^2, twice a day for 14 days, every 3 weeks) or Tegafur-uracil (100mg/m^2, q8h, for 14 days, every 3 weeks). The response of treatment will be evaluated according to RECIST criteria. Adverse events will be documented according to CTCAE v4.03.

Inclusion Criteria:

• Age ≥ 18 and ≤ 70 years
• Histologically confirmed nasopharyngeal carcinoma, WHO (World Health Organization) classification II/III
• With distant metastasis
• With measurable lesions that can be detected by imaging studies
• Achieving PR (partial response) after 4 cycles of conventional chemotherapy
• Life expectancy ≥ 6 months
• ECOG (Eastern Cooperative Oncology Group): 0-1, no significant active concurrent medical illnesses
• Adequate laboratory values within 30 dyas of enrollment to study defined as follows: N > 2000/mm^3; PLT (platelet count) > 100,000/mm^3; total bilirubin < 1.5mg/dl; AST (aspartate aminotransferase)/ALT (alanine aminotransferase) < 1.5 ULN (upper limit of normal); SCr (serum creatinine) < 1.5mg/dl; CCR (creatinine clearance rate) > 60ml/min
• Willing to accept adequate contraception for women with childbearing potential
• Ability to understand character and individual consequences of the clinical trial
• Willing to sign the written informed consent; Informed consent must be signed before the enrollment in the trial

Exclusion Criteria:

• Received previous treatment for metastatic disease
• Pregnant or lactating women
• A diagnosis of malignancy other than CIS (carcinoma in situ) of the cervix, BCC (basal cell carcinoma) and SCC (squamous cell carcinoma) of the skin within the past 5 years
• Refusal of the patient to participate into the study