**4. Dosimetry**

Distinct light sources can be used for PDT. For therapy, the tissue must be irradiated with light at appropriate wavelengths (within the absorption spectrum of porphyrins). The porphyrins exhibit a very typical absorption spectrum with the highest peak at approximately 405 nm, called the Soret-band. Other lower absorption peaks, the Q-bands, are centered at 510, 545, 580 and 630 nm. The absorption band at 630 nm is preferentially used for irradiation since light at the red spectrum results in a higher skin penetration. Lasers and incoherent light sources (lamps, light-emitting-diode – LED – lamps and, intense pulsed light – IPL) have been used. When endoscopic applications are necessary, the activating light has to be delivered through optical fibers, and laser systems are the best option for this purpose. For dermatological application, incoherent light sources are more attractive, due to the possibility of distinct emission geometries and comparable lower cost [18-20].

The therapeutic efficacy of PDT involves administration to the patient of a photosensitizer or a pro-drug, a waiting time to allow adequate concentration of the sensitizer molecules in the tumor, and irradiation of the target tissue with a proper wavelength to activate the photosen‐ sitizer generating cytotoxic products, mainly the singlet-oxygen. To trigger cell death, a minimum number of singlet-oxygen molecules have to be produced. The minimal energy dose required to achieve the irreversible tissue damage, resulting in tumor necrosis, is called the threshold dose.

The energy dose is given in Joules per centimeter square (J/cm2 ), that is the amount of energy delivered to the tissue per unit area. Light intensity is measured in Watts (W) and corresponds to the energy per unit of time. One W corresponds to 1 J per 1 second. Irradiance is measured in Watts per centimeter square (W/cm2 ), representing the light power delivered to the tissue surface [19, 21-22]. A simple relationship between light dose (D), irradiance (I) and time (t) is: D=I.t (1)

Energy doses delivered for the treatment of basal cell carcinoma and other dermal conditions are in the range of 40-150 J/cm2 and with irradiances of 40-150 mW/cm2 . The PDT illumination of a BCC lesion of 2 cm of diameter, for example, may be of 8 to 20 minutes, depending on the chosen irradiation parameters.
