Photodynamic therapy (
PDT) has evolved since its inception at the beginning of the 20th century, when it was defined as an
oxygen-dependent reaction between a photosensitizing
dye and light.
Photosensitizers and light sources have since been continually optimized for distinct applications and tissues. Systemic
porphyrins, such as
hematoporphyrin, were the first
photosensitizers to be used, mostly to treat
tumors. The first light sources used were broad-band, noncoherent lights, such as
quartz,
xenon,
tungsten, or
halogen lamps. The wavelengths of light chosen were based upon the absorption spectrum of
porphyrins: blue because the largest peak is at 400 nm (the Soret band) and red because of its greater penetration depth but lesser absorption at 650 nm (a Q band). Systemic
photosensitizers caused prolonged photosensitivity, and broad-band light sources had limitations and side effects. The development of topical
photosensitizers, such as 5-aminolevulinic
acid, and the advent of
lasers in recent years have advanced
PDT for cutaneous use. In the 1990s, red
lasers were applied to
PDT because of their increased skin penetration despite lesser absorption by
porphyrins. Broad-band blue light and red light have been studied extensively, the former achieving Food and Drug Administration approval in combination with topical
aminolevulinic acid for the treatment of
actinic keratosis in 1997. These
lasers and light sources caused significant side effects, such as discomfort,
erythema, crusting, blistering, and dyspigmentation. The recent application of the long-pulsed
pulsed dye laser (595 nm) after topical
aminolevulinic acid greatly minimized side effects without compromising efficacy. Long-pulsed
pulsed dye laser-mediated
PDT has since been shown to be effective in treatment of
actinic keratosis,
actinic cheilitis, sebaceous
hyperplasia,
lichen sclerosus, and, most recently,
acne vulgaris. Finally, intense pulsed light sources have been introduced to
PDT for the treatment of photodamage and
acne, offering advantages of versatility in wavelengths and applications.