D2.55 - Allergic contact dermatitis and cross-sensitization after laser-assisted 5-aminolevulinic acid PDT

Photodynamic therapy (PDT) is an established and effective treatment for actinic keratosis, with adverse effects usually limited to transient and self-limiting phototoxic reactions. Allergic contact dermatitis (ACD) to topical photosensitizers is rare and has mainly been described in association with methyl aminolevulinate.

We report the case of a 67-year-old male patient with a history of prior well-tolerated PDT, who developed a severe eczematous skin reaction following laser-assisted PDT with topical 5-aminolevulinic acid for multiple actinic keratoses of the scalp and arms. Fractional ablative laser pretreatment was performed to enhance photosensitizer penetration, followed by topical administration of 5-aminolevulinic acid and activation by daylight illumination. Within 24 hours, the patient developed pronounced erythema, vesiculation, and pruritus, extending beyond treated areas and persisting longer than typical phototoxic reactions.

After complete clinical resolution, patch testing was performed using the European baseline series, 5-aminolevulinic acid and methyl aminolevulinate. Readings demonstrated positive reactions to both active compounds, while the vehicle and baseline allergens remained negative, supporting a diagnosis of allergic contact dermatitis. Clinical photographs documenting the initial reaction and the patch test are available.

This case highlights ACD as a rare but clinically relevant adverse effect of PDT and demonstrates cross-sensitization between aminolevulinic acid derivatives. Enhanced percutaneous penetration during laser-assisted PDT may facilitate sensitization in susceptible individuals. Patch testing is essential for diagnostic clarification and to guide future treatment decisions. 

Written informed consent for publication was obtained.