Gleevec, a selective tyrosine kinase inhibitor, retarded the growth of anaplastic thyroid cancer cell lines in vitro and in vivo through selective inhibition of ABL tyrosine kinase activity. In the present study, we investigated the ability of Gleevec to modulate the in vitro and in vivo radiation response of anaplastic thyroid cancer cells. Cell growth assays, colony formation assays and xenograft models were used to quantify the radiosensitizing effect of Gleevec in cells of the anaplastic thyroid cancer cell lines ARO and FRO. FACS, Western blotting and histochemical techniques were employed to study the mechanisms of radiation response after exposure to Gleevec. Gleevec (7.0 microM) increased the anti-proliferative effect of radiation on the growth ARO and FRO cells in vitro. Clonogenic analysis demonstrated that Gleevec reduced cell survival after irradiation. Gleevec combined with radiation produced an increase in tumor growth inhibition compared to treatment with either modality alone in mice bearing anaplastic thyroid cancer xenografts. The drug suppressed radiation-induced ABL activation and promoted CDKN1A (p21(cip1)) accumulation in irradiated anaplastic thyroid cancer cells. Gleevec had an additional effect on radiation-induced apoptosis in cells of both cell lines and potentiated the induction of terminal growth arrest accompanied by the expression of senescence-associated beta-galactosidase. The antitumor effect of Gleevec is potentiated in adjunctive therapy with radiation not only due to inhibition of proliferative cell growth with transient cell cycle arrest and apoptosis, but also due to the terminal growth arrest associated with senescence, suggesting that tumor cell senescence is a mechanism for tumor targeting therapy in combination with ionizing radiation.