Sing role of UCNP to excite PS for PDT in deep tissue. More generally, rare earth materials used in upconversion or similar nanoparticles are also promising candidates for deep-buy SIS3 tissue PDT based theranostics and medical imaging due to versatile synthesis, modification chemistries, photostability and relative safety. However, their in vivo bioclearance and toxicity still need thorough investigation to bolster their clinical applicability.ether-PDT [97] or indocyanine green-PDT [98] and RT on Eherlich ascites carcinoma cells and prostate carcinoma cell lines (PC3) respectively. More recently, Montazerabadi et al. demonstrated in vitro a synergistic effect on breast cancer cells (MCF-7) treated with both indocyanine green-PDT and RT [99]. Several methods of combining PDT and RT are illustrated in Figure 5B and will be discussed in the following sections.PS that act as radiosensitizersPSs that double as radiosensitizers (RSs) have been developed by several groups to combine the SIS3 site effects of PDT with RT and impart greater cytotoxicity in deep tissues. Luksiene et al. reported that Haematoporphyrin dimethyl ether (HPde) acts as a RS when injected at a concentration higher than 30 mg/kg of body weight in mice with transplanted tumors. The combination of PDT and RT for these mice led to a 4x reduction in relative tumor growth compared to radiation only [97]. In a subsequent study, the RS properties of three different PS (HPde, Photofrin II (PII) and hematopoprhyrin derivative (HPD)) were compared [100]. In this in vivo study, it was demonstrated that the HPde was the most efficient RS, followed by the PII and HPD. Furthermore, this study also showed that the RS effect of these PSs was cell line dependent. The interest in the development of dual PS and RS agents has diminished due to the low efficiency of PSs that act as RSs under direct excitation. Alternatively, various approaches were developed to locally generate visible light using X-ray irradiation that could excite the PS in deep tissue.Ionizing radiation for PDT in deep tissuesThough better than visible radiation, NIR radiation still has a limited penetration depth of approximately 1 cm. Treatment of large superficial tumors may be possible with NIR light but tumors residing in deeper tissues remain unreachable without the secondary light delivery strategies discussed in section 2. To substantially improve the the penetration depth of excitation photons, X-rays that are already used for radiation therapy (RT) with little tissue penetration limitations, are interesting candidates even though they are known to cause intrinsic toxicities [95]. That being said, taking advantage of X-rays used during RT to simultaneously activate PSs, thereby potentiating the localized cytotoxic effect in deep tumors, may improve the overall treatment efficacy by affording PDT-RT combination effects and by decreasing the dose required for RT. This is because combining PDT and RT, which imparts cytotoxicity by mechanistically distinct avenues, may lead to treatment synergism. Although several in vitro studies have been published on the combination of PDT and RT, there is no consensus on the overall utility of this combination therapy. Berg et al. demonstrated that the time interval between RT and ALA-PDT is crucial and the treatment combination may lead either to an antagonistic or synergistic effect in adenocarcinoma cells (WiDr) [96]. Other studies report an additive effect between haematoporphyrin dimethylCere.Sing role of UCNP to excite PS for PDT in deep tissue. More generally, rare earth materials used in upconversion or similar nanoparticles are also promising candidates for deep-tissue PDT based theranostics and medical imaging due to versatile synthesis, modification chemistries, photostability and relative safety. However, their in vivo bioclearance and toxicity still need thorough investigation to bolster their clinical applicability.ether-PDT [97] or indocyanine green-PDT [98] and RT on Eherlich ascites carcinoma cells and prostate carcinoma cell lines (PC3) respectively. More recently, Montazerabadi et al. demonstrated in vitro a synergistic effect on breast cancer cells (MCF-7) treated with both indocyanine green-PDT and RT [99]. Several methods of combining PDT and RT are illustrated in Figure 5B and will be discussed in the following sections.PS that act as radiosensitizersPSs that double as radiosensitizers (RSs) have been developed by several groups to combine the effects of PDT with RT and impart greater cytotoxicity in deep tissues. Luksiene et al. reported that Haematoporphyrin dimethyl ether (HPde) acts as a RS when injected at a concentration higher than 30 mg/kg of body weight in mice with transplanted tumors. The combination of PDT and RT for these mice led to a 4x reduction in relative tumor growth compared to radiation only [97]. In a subsequent study, the RS properties of three different PS (HPde, Photofrin II (PII) and hematopoprhyrin derivative (HPD)) were compared [100]. In this in vivo study, it was demonstrated that the HPde was the most efficient RS, followed by the PII and HPD. Furthermore, this study also showed that the RS effect of these PSs was cell line dependent. The interest in the development of dual PS and RS agents has diminished due to the low efficiency of PSs that act as RSs under direct excitation. Alternatively, various approaches were developed to locally generate visible light using X-ray irradiation that could excite the PS in deep tissue.Ionizing radiation for PDT in deep tissuesThough better than visible radiation, NIR radiation still has a limited penetration depth of approximately 1 cm. Treatment of large superficial tumors may be possible with NIR light but tumors residing in deeper tissues remain unreachable without the secondary light delivery strategies discussed in section 2. To substantially improve the the penetration depth of excitation photons, X-rays that are already used for radiation therapy (RT) with little tissue penetration limitations, are interesting candidates even though they are known to cause intrinsic toxicities [95]. That being said, taking advantage of X-rays used during RT to simultaneously activate PSs, thereby potentiating the localized cytotoxic effect in deep tumors, may improve the overall treatment efficacy by affording PDT-RT combination effects and by decreasing the dose required for RT. This is because combining PDT and RT, which imparts cytotoxicity by mechanistically distinct avenues, may lead to treatment synergism. Although several in vitro studies have been published on the combination of PDT and RT, there is no consensus on the overall utility of this combination therapy. Berg et al. demonstrated that the time interval between RT and ALA-PDT is crucial and the treatment combination may lead either to an antagonistic or synergistic effect in adenocarcinoma cells (WiDr) [96]. Other studies report an additive effect between haematoporphyrin dimethylCere.