Assessment of the actual light dose in photodynamic therapy

•Photodynamic therapy assessment depends on the number of photons absorbed.•Spectral overlap between the light source emission and photosensitizer absorption.•Light dose can be corrected by the number of photons absorbed resulting in the actual light dose.•The actual light dose allows for a correct assessment of the PDT effect for all light sources and photosensitizers. Photodynamic therapy (PDT) initiates with the absorption of light, which depend on the spectral overlap between the light source emission and the photosensitizer absorption, resulting in the number of photons absorbed, the key parameter starting PDT processes. Most papers report light doses regardless if the light is only partially absorbed or shifted relatively to the absorption peak, misleading the actual light dose value and not allowing quantitative comparisons between photosensitizers and light sources. In this manuscript a method is presented to calculate the actual light dose delivered by any light source for a given photosensitizer. This method allows comparing light doses delivered for any combination of light source (broad or narrow band or daylight) and photosensitizer.