Light dose fractionation versus single-dose irradiation in photodynamic therapy of tumours

The effect of photoirradiation fractionation on photodynamic therapy of Sarcoma 180 in mice was investigated. Photofrin II (10 mg/kg bodyweight) and different schedules of light dose fractionation were used. The inhibition of tumour growth was comparable when a dose of 1×300 J/cm² or 2×150 J/cm² was applied. However, the skin photoreaction and the normal tissue reactions in the irradiated area were weaker with the second schedule of irradiation. The best effect was reached with 6×50 J/cm², doses being given at 5-hour intervals between the 24th and 48th hours after drug administration. Despite the small total dose in the case of irradiation with 6×35 J/cm², the effect on the tumour was comparable with that reached using the first two schedules of treatment (total irradiation dose 300 J/cm²). With multiple photoirradiation fractionation the photodynamic therapeutic effect is considerably greater than with a single or double fractionation of irradiation using the same total dose. With the multiple light dose fractionation the normal tissue reactions in the irradiated areas are more weakly expressed than with a single or double fractionation of irradiation using the same total dose. The multiple photoirradiation fractionation allows a reduction in the total light dose necessary to achieve the same biological effect as with single or double irradiation.