Introduction: The aim of the present study was to assess the radiosensitivity parameters for SK-BR-3 (SKBR3) breast cancer cells that could be implemented in the cutting-edge treatment planning systems (TPS) for accelerated partial-breast irradiation (APBI). Materials and Methods: The cell survival fraction and its relevant radiosensitivity coefficients, namely α and β, in linear-quadratic (LQ) formalism were evaluated for 6 MV X-rays and 60Co γ-rays using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. During the irradiation time, the medium temperature was kept at 4°C to prevent the repair of sublethal radiation damages over the exposure time and keep the survival fractions independent of the dose rate. Results: Fitting the LQ model to experimental data, α, β, and α/β radiosensitivity parameters were obtained as 0.156±0.027 Gy-1, 0.026±0.007 Gy-2,and 6.0 Gy for 6 MV X-rays and 0.162±0.028 Gy-1, 0.028±0.007 Gy-2, and 5.8 Gy for 60Co gamma radiation, respectively. The average relative biological effectiveness (RBE) values were 0.91 and 0.96 for 6 MV X-rays and 60Co γ-rays, respectively. The derived LQ parameters were also compared with those previously obtained from in vitro studies for different breast cancer cell lines using various regimes, such as radiotherapy modality with different dose rates and delivered doses. Conclusion: The results of this study provided essential constant values for α and β parameters. The data could be useful for the improvement of TPS to include the effect of different biological responses to radiation in APBI treatment plans.

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