Triggering of Koebner Phenomenon by Picosecond Laser in a Psoriatic Patient: Clinical and Immunopathologic Insights

ABSTRACT Objectives To report the first case of Koebner phenomenon induced by picosecond laser treatment in a patient with psoriasis. We analyzed the clinical and histopathological features and discussed the possible mechanism by which the laser's photomechanical effect triggered the reaction. Methods A 58‐year‐old woman with well‐controlled plaque psoriasis underwent 755‐nm picosecond alexandrite laser treatment (pulse duration: 750 ps; fluence: 3.77 J/cm 2 ; spot size: 3 mm) for seborrheic keratoses on the back and lower waist. Clinical, dermoscopic, and histopathological assessments were performed. Results One month after treatment, erythematous, scaly plaques emerged at the laser‐irradiated sites. Clinical and dermoscopic findings were consistent with psoriasis. Histology confirmed psoriasiform changes, and immunohistochemistry demonstrated a T‐cell–predominant infiltrate (CD3 + ) with dermal CD4 + predominance and epidermal CD8 + enrichment. Notably, no Koebner phenomenon was seen in coexisting melanocytic nevi. Lesions resolved after 4 weeks of topical corticosteroids, with no recurrence up to 4 months. Conclusions To our knowledge, this is the first report of the Koebner phenomenon induced by picosecond laser therapy in a patient with psoriasis. Although picosecond lasers are known for minimal thermal damage, their photomechanical effects may activate immune responses and trigger new psoriatic lesions. Therefore, caution and careful monitoring are advised when using picosecond lasers in patients with psoriasis.