The trial examines the clinical benefit of adjuvant external beam radiotherapy (RTx) for locally invasive differentiated carcinoma (TNM stages pT4 pN0/1/x M0/x; 5th ed. 1997) of the thyroid gland (DTC). Patients are treated with surgery (thyroidectomy and lymphadenectomy), radioiodine therapy (RIT) to ablate the thyroid remnant tissue, and TSH-suppressive L-thyroxine therapy with or without RTx after documented elimination of cervical I-131 uptake.