Comparison of three different cell viability assays. Surviving fractions of PC-3 (a), DU145 (b), A549 (c) and U2OS (d) cells upon 0–8 Gy IR, as measured by CFA (open circles), CellTiter-Blue assay (closed circles) or Acumen automated cell counting (triangles). In CellTiter-Blue and Acumen assays, 500 cells were seeded per well. Data points are means from three independent experiments. Graphs were drawn after fitting the data to the linear quadratic equation. The α and β values calculated with this equation and correlation coefficients are given in Table 1.