Photothermal therapy (PTT) using a photo-absorbent in the near-infrared (NIR) region is an effective methodology for local cancer treatment. Before PTT using a NIR absorbent is executed, the operator generally determines the two parameters of fluence rate and irradiation time. However, even if the irradiation parameters are unchanged, the therapeutic effect of PTT is often different for individual tumors. Hence, we examined the therapeutic effect of PTT using a NIR absorbent (ICG lactosome) while changing two parameters (fluence rate and irradiation time) in various combinations. As a result, there was no robust correlation between those parameters and the therapeutic effect. Compared to those parameters, we found that a more reliable determinant was maintenance of the tumor temperature above 43 °C during NIR irradiation. To reconfirm the significance of the determinant, we developed a new system that can regulate the temperature at the NIR irradiation site at a constant level. By using the new system, we verified the treatment outcomes for tumors in which the NIR absorbent had accumulated. All of the tumors that had been kept at 43 °C during NIR irradiation were cured, while none of the tumors that had been kept at a temperature below 41 °C were cured. In conclusion, PTT using a NIR absorbent with thermal dosimetry is a highly reliable treatment for cancer.
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