Photodynamic treatment and radiotherapy: combined effect on the colony-forming ability of V79 Chinese hamster fibroblasts.

The effect of photodynamic tumor therapy with Photosan 3 (PS3) combined with gamma-radiation on the colony-forming ability of Chinese hamster fibroblasts was studied using a modified clonogenic assay. Only the combination of the lowest doses (5 micrograms/ml PS3 + 100 mJ/cm2 red light + 1 Gy gamma-radiation) shows a more than additive effect. Combinations of higher doses resulted in survival levels which are not significantly different from calculations based on single treatments. Additionally, it could be shown that the application of PS3 in the dark impairs the colony-forming ability of V79 cells already at sensitizer concentrations of 2 1g/ml in a concentration-independent manner.