All patients had routine 6-site nasopharyngeal biopsies at 8 weeks after IMRT to confirm local clinical remission. Repeated nasopharyngeal biopsies were performed at 10 weeks and 12 weeks after IMRT if residual tumour cells were still observed in previous post-IMRT nasopharyngeal biopsies. Additional radiation therapy in the form of intracavitary brachytherapy, stereotactic radiation therapy or IMRT would be given if patients developed local persistence at 12 weeks after IMRT. For patients confirmed to be local clinical remission, they received regular clinical follow up and imaging surveillance every 3 to 4 months for the 1st year after IMRT, then every 4 to 6 months for the 2nd and 3rd year and yearly afterwards to monitor for any treatment-related chronic complications and relapse. Blood tests for thyroid function tests including free T4 and thyroid stimulating hormone (TSH) will be arranged at least once yearly to monitor post-IMRT hypothyroidism. Biochemical hypothyroidism is defined as either elevation of TSH above the upper normal limit or reduced free T4 below the lower normal limit or both, without the presence of clinical symptoms of hypothyroidism. Clinical hypothyroidism is defined as the presence of biochemical abnormalities of thyroid function same as that defined for biochemical hypothyroidism together with the presence of clinical symptoms of hypothyroidism. Those who are found to have clinical hypothyroidism will receive thyroxine supplement starting with 50 micrograms daily with repeated serum thyroid function tests 2 to 3 months later followed by dose titration if necessary, then every 6 to 12 months thereafter when therapeutic dosage was reached.