PDT is based on the generation of singlet oxygen (
1O
2) in a photochemical reaction among a tumor-accumulated photosensitizer, irradiation light, and oxygen in a tumor, and the detection of
1O
2 is very important for understanding the mechanism of PDT. We used a near-infrared detection system to detect a 1270nm emission that was derived from the
1O
2. Utilizing this system, we detected the generation of
1O
2 from Photofrin dissolved in water or injected in a HeLa tumor-bearing mouse. We showed the irradiation power dependence of the generation of
1O
2 in 5-ALA-mediated PDT with 9L cells and 9L tumor-bearing rats. High-power irradiation reduced the generation of
1O
2 more than that with low power, and an MTT assay revealed that more 9L cells were killed by low-powered irradiation. In tumors, low-powered irradiation resulted in efficient tumor necrosis. Generations of
1O
2 in ICG and TiO
2 irradiation are also described.
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