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About half of all cancer patients receive radiotherapy (RT), which, despite its effectiveness, can damage healthy tissue and cause late side‑effects such as breast fibrosis; because individual genetic variability strongly influences risk, reliable biomarkers of radiosensitivity are urgently needed. We evaluated four candidate assays γ‑H2AX foci kinetics (DNA‑damage induction and repair), radiation‑induced apoptosis, the diepoxybutane (DEB) chromosome instability test and cytogenetic analysis of radiation induced chromosomal aberrations after G₀ and G₂ irradiation as predictors of late RT induced breast fibrosis. Peripheral blood from 22 matched breast‑cancer patient pairs in remission (one with and one without documented fibrosis) was processed: whole blood cultures served for cytogenetic assays, while peripheral blood mononuclear cells were isolated for functional tests. Samples were irradiated with 1 Gy for the G₂ chromosomal assay, 2 Gy for γ‑H2AX kinetics measured at 1, 2, 4 and 24 h (plus additional G₂ cultures) and 8 Gy for the apoptosis assay assessed after 48 h; γ‑H2AX foci and apoptosis were quantified by flow cytometry, whereas chromosomal aberrations were scored microscopically, and linear regression adjusted for total RT dose, adjuvant therapies as well as clinical features. The G₂ chromosomal assay showed significantly higher aberration yields in fibrosis versus non‑fibrosis patients, and the DEB chromosome instability test revealed elevated breakage scores in the fibrosis group, while radiation induced apoptosis trended toward significance but lost power after covariate adjustment, indicating that the G₂ chromosomal assay and DEB instability test most robustly discriminate patients susceptible to late RT induced breast fibrosis and merit further development as predictive clinical biomarkers. The values of the files correspond to: DNA damage and its repair, cell death, clinical covariates and chromosomal aberration frequencys for every culture studied.