Figure 2  Viability of HeLa cells treated with increasing PpIX concentration and X-ray exposure. The data correspond to the mean ± SD at 1 h post-irradiation (a) (n = 6), at 24 h post-irradiation (b) (n = 4) and 72 h post-irradiation (c) (n = 4). The cell proliferation reagent 4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate (WST-1) was used to quantify cell viability. The percent survival is shown relative to untreated control cells. No change was observed in cell viability upon increasing the X-ray dose and PpIX concentration in control cells. Statistical significance (p < 0.01) relative to the experiment performed without PpIX treatment at the same irradiation dose is indicated by (TT). Statistical significance (p < 0.01) relative to the experiment performed without irradiation at the same PpIX concentration is indicated by (+).