Aim. To determine the inner shell ionization model can describe and predict the radiosensitization (RS) induced by iododeoxyuridine---IDU- and bromodeoxyuridine---BRDU.Material and methods. CHOK1 cells exposed to 4muM IDU or BRDU were irradiated using 60Co (5cm, 10cm and 20cm depth) and 192Iridium (2cm and 5cm depth). Clonogenic assay was used to assess the extent of RS. Theoretical and experimental sensitization enhancement ratios (SER) were compared.Conclusions. The inner shell ionization model described well the radiobiologic data, suggesting a different mechanism of radiosensitization for BRDU (electron knock-on interactions) compared with IDU (photoelectric events). IDU may provide increased RS compared with BRDU in a brachytherapy setting.Results. Both IDU and BRDU induced RS increased with increasing depth of irradiation. For the same depth of irradiation, RS was higher when lower energy primary photons were used. The RS induced by BRDU was less dependent on beam energy and depth than IDU. The theoretical and experimental SER were highly correlated (p=0.02)