This study investigates whether administering reduced dose is non-inferior to standard dose radiotherapy in terms of 3-year locoregional relapse-free survival (LRRFS) rate for stage I nasopharyngeal carcinoma patients who are sensitive to radiotherapy.

All the patients receive intensity-modulated radiotherapy. If patients receive CR and EBV DNA undetectable when completing 50.88Gy radiation, they will continue to receive radiotherapy until 61.48Gy, then the patients would be randomised assigned ( 1:1 ) to reduced dose group (observation) or standard dose group (continue to receive radiotherapy until 69.96Gy). Patients with stage IA will receive radiotherapy alone, and patients with stage IB will receive concurrent chemoradiotherapy. For patients with stage IB, those assigned to reduced dose group will receive a total of 2 cycles cisplatin concurrent chemotherapy, with 100mg/m2 per cycle; those assigned to standard dose group will receive a total of 3 cycles cisplatin concurrent chemotherapy, with 100mg/m2 per cycle. This study investigates whether administering reduced dose is non-inferior to standard dose radiotherapy in terms of 3-year locoregional relapse-free survival (LRRFS) rate for stage I nasopharyngeal carcinoma patients who are sensitive to radiotherapy. The primary endpoint is 3-year locoregional relapse-free survival.

Inclusion Criteria:

1. Age 18-70, regardless of sex.
2. Patients with newly histologically confirmed non-keratinizing nasopharyngeal carcinoma, type of WHO II or III, clinical stage I (according to the 9th American Joint Committee on Cancer[AJCC] edition).
3. Patients with CR according to RECIST and EBV DNA undetectable after received 50.88Gy radiation.
4. ECOG (Eastern Cooperative Oncology Group) score: 0-1.
5. Women in their reproductive years should ensure that they use contraception during the study period.
6. Hemoglobin (HGB) ≥90 g/L, white blood cell (WBC) ≥4×109 /L, platelet (PLT) ≥100×109 /L.
7. Liver function: Alanine transaminase(ALT), Aspartate aminotransferase(AST)< 1.5 times the upper limit of normal value (ULN), total bilirubin <1.5×ULN.
8. Renal function: serum creatinine <1.5×ULN or creatinine clearance rate≥60mL/min.
9. Patients must sign informed consent and be willing and able to comply with the requirements of visits, treatment, laboratory tests and other research requirements stipulated in the research schedule.

Exclusion Criteria:

1. Histologically confirmed keratinizing squamous cell carcinoma (WHO I).
2. Patients with PR/SD/PD according to RECIST and/or EBV DNA detectable after received 50.88Gy radiation.
3. Receiving radiotherapy or chemotherapy or targeted therapy previously.
4. Women of child-bearing potential who are pregnant or breastfeeding because of the potentially dangerous effects of the preparative chemotherapy on the fetus or infant.
5. Suffered from other malignant tumors (except the cure of basal cell carcinoma or uterine cervical carcinoma in situ) previously.
6. Patients with significantly lower heart, liver, lung, kidney and bone marrow function.
7. Severe, uncontrolled medical conditions and infections.
8. At the same time using other test drugs or in other clinical trials.
9. Refusal or inability to sign informed consent to participate in the trial.
10. Other treatment contraindications.
11. Emotional disturbance or mental illness, no civil capacity or limited capacity for civil conduct.