Effective fluence and dose at skin depth of daylight and lamp sources for PpIX-based photodynamic therapy.

Significance: Skin-based photodynamic therapy (PDT) is used for the clinical treatment of actinic keratosis (AKs) and other skin lesions with continued expansion into the standard of care. Due to the spectral dependency of photosensitizer activation and skin optical fluence, there is a need for more accurate methods to estimate the delivered dose at depth from different PDT light sources and treatment regimens.

Aim: Develop radiometric methods for calculating photosensitizer-effective fluence and dose at depth and determine differences between red-lamp, blue-lamp, and daylight-based PDT treatments.

Methods: Radiometric measurements of FDA-approved PDT lamp sources, outdoor daylight, and indoor daylight were performed for clinically relevant AK treatments. The protoporphyrin IX (PpIX) equivalent irradiance, fluence, and dose for each light source were calculated from the PpIX absorption spectrum and a 7-layer skin fluence model. The effective fluence and dose at depth was estimated by combining the spectral attenuation predicted at each wavelength and depth with the source fluence at each wavelength.

Results: The red-lamp source had the highest illuminance (112,000 lumen/m), but lowest PpIX-effective irradiance (9.6 W/m), and highest effective fluence at depth (10.8 W/m at 500 µm). In contrast, the blue light source had the lowest illuminance (2300 lumen/m), but highest PpIX effective irradiance (37.0 W/m), and ultimately the lowest effective fluence at depth (0.18 W/cm at 500 µm). The daylight source had values of (outdoor | indoor) illuminance of (49,200 | 37,800 lumen/m), effective irradiance of (19.2 | 10.7 W/m), and effective fluence of (1.50 | 1.08 W/m at 500 µm). The effective fluence and dose at depth facilitated the comparison of treatment regimens, for example, calculating an equivalent dose for a 2 hr indoor daylight treatment and a 10 min red-light treatment for the 300-1000 μm depth range.

Conclusions: The consideration of PpIX-effective fluence at varying depths is necessary to provide adequate comparisons of the delivered dose from PDT light sources. Methods for calculating radiometric fluence and delivered dose at depth were introduced, with open source MATLAB code, to help overcome the limitations of commonly used photometric and irradiance-based reporting.