Histopathological distinction between ICI-induced psoriasis vulgaris and psoriasiform dermatitis in patients with immune-related cutaneous adverse events (irDAEs): A multicenter prospective observational study.

e24191 Background: Immune checkpoint inhibitors (ICI) significantly improve survival in cancer patients but are frequently associated with dermatologic immune-related adverse events (irDAEs) that exhibit clinical heterogeneity and may mimic psoriasis. The histopathological distinctions between ICI-induced psoriasis vulgaris and psoriasiform irDAEs remain insufficiently characterized despite their critical importance for accurate diagnosis and treatment decisions. Study Design: Multicenter prospective study including patients who developed irDAEs during ICI therapy and underwent clinical evaluation with or without skin biopsy. Retrospective data collection included demographics, oncologic characteristics, ICI regimens, cutaneous manifestations, CTCAE severity grading, and available histopathological findings, with histologic analysis focused on psoriasiform cases. Methods: The study included 50 patients who developed immune-related cutaneous adverse events during ICI therapy (mean age 69 years; 56% male). The most common underlying malignancies were skin and soft tissue cancers (38%), genitourinary cancers (28%), and lung/bronchial cancers (20%). Results: Isolated pruritus without visible rash was observed in 20 patients (40%). The remaining 30 patients (60%) developed irDAEs of varying severity: Grade 1 in 19 patients (38%), most commonly maculopapular rash (26%), and Grade 2–3 in 11 patients (22%), including psoriasis vulgaris (8%), psoriasiform eruptions (6%), lichenoid eruptions (6%), and bullous pemphigoid (2%). Histopathological evaluation was performed in 7 patients with Grade 2–3 psoriasis vulgaris (n = 4) or psoriasiform eruptions (n = 3). Psoriasiform eruptions showed irregular acanthosis with prominent parakeratosis, preserved granular layer, and mild dermal vascular changes with moderate perivascular lymphocytic infiltrates, whereas psoriasis vulgaris demonstrated regular acanthosis, diffuse parakeratosis, complete loss of the granular layer, and pronounced papillary dermal vascular dilatation with prominent perivascular inflammation. Conclusions: Routine histopathological assessment of the skin is essential for the accurate confirmation of irDAEs. Morphological findings enable the differentiation of these events from other dermatoses and provide a rationale for selecting supportive treatment without unjustified modification of antitumor therapy.