The primary objective of this study is to estimate the 5 year locoregional control rate after External Beam Radiotherapy (EBRT) in the patient with locally advanced differentiated thyroid cancer. In a prior retrospective trial for this same group of patients, the 5 year locoregional control rate was 85% in EBRT group and 70% in no EBRT group. For the total number of patients to detect a minimum of 20% improvement in 5 year locoregional control rate with 80% of statistical power and the 5% significance level, the table for sample size planning given by Makuch and Simon was chosen. Forty-three patients are required for this regimen to regard as worthy of further investigation. Considering 15% follow up loss, 50 eligible patients will be enrolled.