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The fluorescence you observe with an optical fiber collector (F [W]) is proportional to the concentration of photosensitizer (C [mg/ml tissue]) that has accumulated in the tissue. Inversely, C is proportional to observed F, but scaled by all the other factors. Quantitative fluorescence determination of photosensitizer concentration requires knowledge of all the parameters of these equations.

A key factor is the tissue optics factor [1/cm] which is the integral of the transport of excitation light to a location in the tissue times the transport of emission light from that location to the collection fiber, integrated over the entire tissue volume. This integral summarizes all the dependence of PDT dosimetry on tissue optical properties. The integral is called chi (X [1/cm]) for convenience.

The other parameters are

• P_ex [W] = power of excitation laser
• epsilon [cm^-1 (mg/ml)^-1] = extinction coefficient of photosensitizer at excitation wavelength (here defined using base e: T = exp(-epsilon C L).
• C [mg/ml = concentration of photosensitizer in tissue
• phi_f [J/J] = energy efficiency of fluorescence
• A [cm²] = area of collection fiber in contact with tissue
• f [dimensionless] = fraction of fluence rate at fiber which couples into optical fiber and reaches detector.
• calib [counts/W] = the calibration relating detector counts to watts of power detected.

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