Photodynamic Therapy in Dermatology
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Direct answer
Photodynamic therapy in dermatology combines a topical photosensitiser with light of a matching wavelength to generate reactive oxygen species that destroy abnormal cells. Aminolevulinic acid and methyl aminolevulinate are converted to protoporphyrin IX within the lesion, and light near 630 nm or 415 nm activates it. Its commonest uses are actinic keratosis, superficial basal cell carcinoma, Bowen disease and inflammatory acne.
What you must remember
- Photosensitiser: topical 5-aminolevulinic acid (ALA) or methyl aminolevulinate (MAL), which the cells convert to protoporphyrin IX.
- Red light: about 630 nm, used with MAL for actinic keratosis and superficial basal cell carcinoma.
- Blue light: about 415 nm, used with ALA for inflammatory acne and for some actinic keratoses.
- Actinic keratosis: the commonest indication, with good response in thin lesions and when treated as a field on sun-damaged skin.
- Superficial basal cell carcinoma: responds well, whereas thick nodular tumours respond poorly and usually need surgery.
- Acne: blue light photodynamic therapy is thought to act on sebaceous glands and Cutibacterium acnes, and it is used for resistant inflammatory acne.
- Cutaneous T-cell lymphoma: topical photodynamic therapy has been tried for early patch-stage mycosis fungoides, with limited evidence.
- Contraindications: porphyrias and photosensitising disorders are contraindications, because the drug and light cause harm there.
A six-step PDT session
A standard session follows a fixed sequence, and each stage affects how well the photosensitiser is absorbed. Lesion preparation and timing matter as much as the light dose.
- Prepare the lesion: scale and crust are removed, since they block penetration of the photosensitiser.
- Apply the photosensitiser: ALA or MAL cream is applied to the lesion and a margin of surrounding skin, then covered with an occlusive dressing.
- Incubate: the drug is left under occlusion for a fixed period, usually a few hours, so protoporphyrin IX accumulates in abnormal cells.
- Irradiate: the lesion is exposed to light of the matched wavelength at a set dose, with stray light blocked.
- Manage discomfort: burning and pain are common during irradiation, so cooling or analgesics are offered.
- Protect and review: sun exposure is avoided for about 48 hours, and the response is assessed after several weeks.
The sequence is short, but the incubation and light steps must match the protocol for the chosen photosensitiser. Deviations are a common reason for a poor response.
Where the exam tries to catch you
Stems on photodynamic therapy test whether you match the light and the photosensitiser to the lesion, and whether you protect the patient afterwards.
- Trap: expecting the same cure rate for a thick nodular basal cell carcinoma as for a superficial one. Correct: photodynamic therapy works best on thin superficial lesions, while thick nodular tumours usually need surgery.
- Trap: sending the patient into strong sunlight soon after treatment. Correct: photosensitivity persists for about 48 hours, so strict sun protection is essential.
- Trap: using any light source with a photosensitiser. Correct: the wavelength must match the photosensitiser, so red light is paired with MAL and blue light is paired with ALA for acne.
Frequently asked questions
Which wavelength is used for photodynamic therapy of actinic keratosis?
Red light at about 630 nm is commonly used with methyl aminolevulinate for actinic keratosis, while blue light at about 415 nm is also used in some protocols. Photodynamic therapy of actinic keratosis is often delivered as field treatment on sun-damaged skin.
Why is photodynamic therapy used for acne?
Photodynamic therapy with aminolevulinic acid and blue light is thought to act on sebaceous glands and Cutibacterium acnes, which lowers inflammatory lesion counts. It is used for resistant acne and for patients who cannot take systemic therapy.
Which lesions respond poorly to photodynamic therapy?
Thick, nodular basal cell carcinomas respond poorly to photodynamic therapy, because the photosensitiser and light penetrate them incompletely. Such tumours are usually treated with surgery or Mohs micrographic surgery.
What is the main adverse effect of photodynamic therapy?
Burning, pain and erythema during light exposure are the main immediate adverse effects of photodynamic therapy. Photosensitivity to sunlight persists for about 48 hours after the treatment, so patients must avoid direct sun exposure.