Dosimetry study of PHOTOFRIN-mediated photodynamic therapy in a mouse tumor model

It is well known in photodynamic therapy (PDT) that there is a large variability between PDT light dose and therapeutic outcomes. An explicit dosimetry model using apparent reacted ¹O₂ concentration ([¹O₂]_rx) has been developed as a PDT dosimetric quantity to improve the accuracy of the predicted ability of therapeutic efficacy. In this study, this explicit macroscopic singlet oxygen model was adopted to establish the correlation between calculated reacted [¹O₂]_rx and the tumor growth using Photofrin-mediated PDT in a mouse tumor model. Mice with radiation-induced fibrosarcoma (RIF) tumors were injected with Photofrin at a dose of 5 mg/kg. PDT was performed 24h later with different fluence rates (50, 75 and 150 mW/cm²) and different fluences (50 and 135 J/cm²) using a collimated light applicator coupled to a 630nm laser. The tumor volume was monitored daily after PDT and correlated with the total light fluence and [¹O₂]_rx. Photophysical parameters as well as the singlet oxygen threshold dose for this sensitizer and the RIF tumor model were determined previously. The result showed that tumor growth rate varied greatly with light fluence for different fluence rates while [¹O₂]_rx had a good correlation with the PDT-induced tumor growth rate. This preliminary study indicated that [¹O₂]_rx could serve as a better dosimetric predictor for predicting PDT outcome than PDT light dose.