Combination of Photodynamic Therapy with Radiotherapy for Cancer Treatment

Photodynamic therapy (PDT) is a promising treatment modality for the management of malignant diseases. However, general acceptance of PDT has been hampered due to the limited tissue penetration of light and unavailability of suitable photosensitizers (PSs). The innovative combination of the conventional radiotherapy (RT) with PDT might reduce the unacceptable normal tissue toxicity while maintaining the desired tumor suppression effect. There have been a number of attempts to examine the interaction of PDT and RT; however, the previous results presented are ambiguous. The exact mechanisms for the variable responses of diverse panel of cell lines to the combination therapeutic regimen are still unclear. Novel ways are being explored to overcome the weaknesses of the conventional PDT and RT treatment regimen. The novel application to enable PDT of deep cancers is the utilization of scintillating nanoparticles as an intracellular light source for PDT activation. Upon simulation by X-rays, the nanoparticles emit scintillation or persistent luminescence to activate the PS to generate singlet oxygen. For future clinical applications, several questions are worthy of further elucidation, including the specific light dose, PS dose, radiation dose, the risk of complications, and the accurate time interval between administration of PDT and administration of RT.