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The optical penetration depth (delta) is the tissue thickness that causes light to attenuate to 37% its initial value. delta is calculated:

where mua = absorption coefficient [cm^-1] and mus' = reduced scattering coefficient [cm^-1], and mua << mus'. Hence, light penetration depends on the geometric mean of absorption and scattering. Regardless of scattering, a 4-fold increase in blood, typical of inflammation, causes a 2-fold decrease in delta, and consequently a 2-fold decrease in the zone of PDT treatment.

Figure 1 shows the calculation of delta for several tissue types based on in vivo optical measurements. Figure 2 shows the histogram of delta for this same data. There is only a slight decrease in the delta for tumor due to increased blood content, relative to normal tissue. However, the site-to-site variation in delta is quite large.

Figure 1: delta for various tissues. ("lung" means bronchial tumor.)

Figure 2: Histogram of delta for normal and tumor tissues.

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