The effect of combined modality treatment with ionising radiation and TPPS-mediated photodynamic therapy on murine tail skin.

The effect on normal skin of combined modality treatment with 300 kV X-rays and photodynamic therapy (PDT) using the photosensitising drug meso-tetra (sulphonatophenyl) porphine (TPPS) was studied using the mouse tail necrosis assay. Prior treatment with a tolerance dose of PDT produced a significant increase in the probability of necrosis following graded doses of ionising radiation. A tolerance dose of X-rays administered prior to graded doses of PDT also produced a significant rise in the necrosis rate. TPPS appeared to have a radiosensitising effect but, as the animals were kept in subdued light, the low dose of PDT they therefore received may provide an alternative explanation. The effect of prolonging the interval between the modalities on the necrosis rate did not appear to be related to the time course of either the changes in blood flow produced by each modality, measured by xenon clearance studies or the development of the skin reaction following X-ray irradiation.