Erythematous, pruritic and indurated plaques following treatment for melanoma

A 35-year-old man presented with two itchy, nontender, indurated erythematous plaques on his forehead and upper back. Three months prior, he had completed a 12-month course of adjuvant pembrolizumab for stage IIIC melanoma. Clinical and imaging surveillance revealed no evidence of melanoma recurrence of his left cheek. Incisional biopsies of both plaques with immunohistochemistry favoured a CD4⁺ reactive lymphoid process in keeping with cutaneous pseudolymphoma (CPL), secondary to pembrolizumab, an immune checkpoint inhibitor (ICI). The conspicuous, pruritic forehead plaque was unresponsive to clobetasol proprionate under occlusion. A single course of intralesional triamcinolone acetonide 10 mg L^-1 resulted in significant improvement within days, with subsequent complete resolution in a month. The plaque on the back resolved spontaneously after 6 months. Drug-induced CPL describes an adverse cutaneous drug reaction mimicking B- or T-cell lymphomas clinically and/or histologically. It has been described with anticonvulsants, antidepressants and biologic agents, often with resolution on cessation of the responsible drug. It has been proposed that the drugs affect immune surveillance, leading to an abnormal cutaneous lymphocyte response. As with the other adverse effects of immunotherapy, it is likely that CPL with ICIs is due to an iatrogenic immune dysregulation leading to T-cell overactivity. We highlight the successful use of intralesional steroid as treatment for drug-induced CPL with an ICI. It is important to recognize CPL as an adverse cutaneous effect of immunotherapy, and that it can present after completing treatment due to an ongoing immune response.