Estimation of chlorin-based photosensitizer penetration depth prior to photodynamic therapy procedure with dual-wavelength fluorescence imaging

Photodynamic therapy (PDT) is a modern treatment technique employed as an antitumor, antibacterial, or rejuvenation aid in superficial tissues. The use of fluorescent photosensitizers (PSs) implements the principles of theranostics when the applied medical agents serve both for diagnostic and treatment purposes. For efficient PDT performance it is important to evaluate the in-depth distribution of PSs in tissues prior to irradiation. Fluorescence imaging is a common technique to monitor the distribution of PSs in tissue. However, in-depth resolution is challenging. Chlorin-based PSs reveal two narrow fluorescence excitation peaks at 405 and 660 nm providing additional diagnostic opportunities. We demonstrate that the ratio of the fluorescence signals upon excitation at these wavelengths provides the evaluation of the PS penetration depth after topical application. The study is based on Monte Carlo simulations that are in agreement with phantom experiments. The effect of medium optical properties on the depth-dependent fluorescence signal ratio is analyzed.