Photodynamic therapy and γ-irradiation of tumours: Effect of tumour-cell reoxygenation

The presence of adequate oxygen appears to be essential for photodynamic therapy (PDT) of tumours (1, 2). We used the mouse sarcoma 180 tumour model to investigate how the reoxygenation of the tumour cell population after a single exposure to γ-irradiation influenced the effect of photodynamic therapy. The combination of γ-irradiation with PDT leads to a significant enhancement of the therapeutic effect. The best effect is observed when the γ-irradiation precedes the PDT by 24 h, at which time reoxygenation of the tumour is greatest. Also, there is some enhancement of the effect when PDT is given before γ-irradiation, although the mechanism of this is not yet clear.