Objective: Cervical cancer is the fourth common malignancy in women for which combinational therapy is the main treatment regimen. This study aimed to determine the effects of 60Co γ-ray high dose rate brachytherapy on the survival of HeLa cells with respect to X-ray of 6 MV linear accelerator (EBRT, external beam radiation therapy) as a comprehensive radiobiological concept. Furthermore, alterations in relative biological effectiveness (RBE) values were examined in combined treatments of cisplatin and radiation. Methods: Cell survival was evaluated following different treatments by clonogenic assay. The synergistic effects of cisplatin and X-ray were evaluated in terms of the combination index (CI). Moreover, fold change reduction of each treatment in combination therapy was determined by evaluating the dose reduction index (DRI). RBE values were also determined after different treatments. Results: Maximum values of DRI for cisplatin and radiation were 128 and 2.2-fold reduction when used in combination, respectively. As a result of the CI and DRI values, pretreatment with 2 μg/ml cisplatin for 2 h was selected as the most effective concentration and time, and it was used in RBE evaluation. The RBE values at 50% cell survival after treatment with radiation alone and also combined with cisplatin were measured as 1.308 and 1.553, respectively. These RBE values indicate a more destructive effect of γ-radiation brachytherapy than X-ray EBRT. Conclusion: The results of combined treatment indicate a remarkable synergism. Furthermore, our data determined that 60Co γ-ray is more effective than EBRT X-ray to induce cell death in HeLa cells.

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