Histopathological Aspects of Common Immune Checkpoint Inhibitors Induced Skin Toxicities

ABSTRACT Background The histological characteristics of checkpoint inhibitors‐related adverse events (cirAEs) are not well studied and there are conflicting data regarding their similarities and differences with their idiopathic counterparts. Objectives To investigate the histological characteristics of the most common cirAEs. Methods We reviewed the pathology database of our hospital towards identification of eczematoid, psoriasiform, lichenoid and bullous pemphigoid (BP)‐like rashes as a result of cirAEs. We recorded their clinical, epidemiological and histological characteristics. Results We retrieved 35 oncologic patients who were histologically diagnosed with eczematoid (10/35 [28.6%]), psoriasiform (10/35 [28.6%]), lichenoid (10/35 [28.6%]) and BP‐like (5/10 [14.8%]) rashes, mostly of grade 3 severity (58.8%). Acanthosis was present in high rates (60%) in lichenoid rashes, whilst in psoriasiform rashes, Kogoj pustules and spongiosis were found in a comparatively high percentage (80% and 50%, respectively). We also observed a tendency towards poor‐in‐cells variants of BP‐like rashes. Histology of eczema‐like rash was identical to the idiopathic form. Conclusions cirAEs share common clinical and histological features with their corresponding idiopathic counterparts and should be treated accordingly. Clinicopathologic correlation is needed in ambiguous cases.