['To determine whether inactivation of epidermal growth factor receptor (EGFR) kinase activity will sensitize thyroid cancer cell lines to ionizing radiation-induced death.', 'Established human thyroid cancer cells lines were studied.', 'Colony formation assay was used to determine the effect of Gefitinib, a small molecule inhibitor of EGFR, on anaplastic (ARO) and follicular (WRO) thyroid cancer cell lines. In addition, colony formation assay was used to determine the effect of ionizing radiation in the presence or absence of Gefitinib. EGFR protein expression on the cell lines and inactivation of EGFR kinase by Gefitinib was analyzed by Western blot. Immunohistochemistry was performed on archived thyroid cancer tissue to demonstrate expression of EGFR.', 'Incubation with Gefitinib caused decreased phosphorylation of EGFR protein in established thyroid cancer cell lines as measured by Western blot. Inhibition of EGFR kinase activity by Gefitinib resulted in a dose-dependent decrease in colony formation in both ARO and WRO thyroid cancer cell lines. Addition of Gefitinib in combination with ionizing radiation reduced cell proliferation in ARO (P = .0084) and WRO (P = .0252) as measured by colony formation assay.'] Does gefitinib ( Iressa ) potentiate the effect of ionizing radiation in thyroid cancer cell lines?