For information on photodynamic therapy at SOMA Skin & Laser click here. For general PDT information read below.
About Photodynamic Therapy
Photodynamic therapy (PDT) is a procedure involving the use of a photosensitizing agent plus light to treat various skin conditions, including skin cancer, acne, and photoaging. As used by dermatologists photodynamic therapy is a two step process. In the first step, a photosensitizer is applied to the patient’s skin (it can be topical, oral, or intravenous, but this post will focus on topical), where it is taken up by the target cells. In the second step the photosensitizer in the presence of oxygen is activated with a specific wavelength of light. Because the photosensitizer is preferentially absorbed by certain types of cells, such as skin cancer cells, and the activating light source is targeted only to certain areas, photodynamic therapy is selective, targeting the diseased tissue and minimizing damage to surrounding skin. The sensitizing agent is typically either aminolevulinic acid (ALA) or methylaminolevulinate (MAL). In the United States, the trade names are Levulan and Metvixia, respectively. In most practices, ALA is applied for a 30-90 minute incubation time for the treatment of actinic keratoses, though the product label calls for longer incubation.
Photodynamic therapy for acne
ALA and MAL when applied to the skin accumulates preferentially in sebaceous glands and the epidermis, as well as in P. acnes bacteria, allowing PDT to target acne. The use of PDT for acne is off-label but widely used by dermatologists. After a short incubation time (30-60 minutes) ALA is activated with a light source, either a laser or a blue light source. The resulting decrease in sebaceous glands and P. acnes bacteria can lead to significant acne improvement.
Photodynamic therapy for skin cancer and precancers
Cancer cells accumulate more porphyrins than normal cells, allowing photodynamic therapy (PDT) for the treatment of actinic keratoses, Bowen’s disease, and basal cell carcinoma. Treatment of actinic keratosis is the only FDA-approved indication. A phototoxic reaction with erythema, edema, crusting, and erosion occurs in most patients treated for these conditions, which is considered necessary to clear these lesions.
Light sources for PDT
The light used must be at a wavelength near a peak of porphyrin absorption in tissue. The Soret band at 405-420 nm is the key peak of protoporphyrin IX absorption. This wavelength is included in the output of the Blu-U or ClearLight devices used to activate ALA. However, there is also a red peak at around 635 nm, which can be targeted by other light sources, especially the Pulsed Dye Laser, with an output at 595 nm.
How does PDT work?
Following light activation, porphyrins are excited to a higher energy state, which can result in generation of reactive oxygen species, such as singlet oxygen or free radicals. Porphyrins from ALA are concentrated near mitochondria, leading to cell death of malignant or pre-malignant cells upon light exposure. For the treatment of acne, preferential accumulation of ALA in sebaceous glands as well as reduction in Propionibacterium acnes, the bacteria implicated in acne, is thought to be the responsible mechanisms. For treatment of photoaging, increased collagen synthesis following ALA photodynamic therapy may play a role.
During and after PDT
During PDT patients may feel a burning or stinging sensation. A fan or misting of liquid nitrogen can be used to keep patients comfortable. After PDT patients must entirely avoid the sun for several days, as they can experience severe reactions since there is still photosensitizer on the skin. Erythema and edema often occur, followed by peeling. There can be a burning sensation, pain, crusting, and blister formation. Hyperpigmentation is sometimes seen after PDT, usually resolving over several months. Hypopigmentation has also been reported.