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A key factor that influences the optical penetration depth is the amount of blood perfusion in a tissue. A highly inflamed tumor site has a high blood content which strongly absorbs the treatment light used to activate photosensitizer.

The figure shows the absorption coefficient [cm^-1] as a function of wavelength in a typical tissue. The figure indicates the contributions from water, dry material (protein, DNA, etc.), and typical variable blood contents from 1% to 12%. The treatment light in our clinical work is red light at a wavelength of 630 nm.

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