Abstract We present the case of a 73-year-old male patient with a history of tobacco use who presented with a central nervous system mass that was confirmed to be a lung adenocarcinoma metastasis. High PD-L1 expression as well as negativity to other targetable drivers led to initiation of pembrolizumab monotherapy and ablative stereotactic radiation therapy on oligo-residual disease, achieving a complete response after 2 years of therapy. Following discontinuation of systemic treatment, the patient developed widespread desquamative plaques. A skin biopsy revealed subepidermal blistering and eosinophilic infiltration in conjunction with C3 and IgG depositions on the basement membrane, detected by immunofluorescence. A diagnosis of bullous pemphigoid was obtained, and systemic corticosteroids were administered with lesion progression. Infliximab was also administered without meaningful clinical improvement. Metronomic cyclophosphamide achieved a complete resolution of skin lesions and up to this day the patient continues with tumor control and is free of dermatological findings. In conclusion, bullous pemphigoid is a very rare dermatological adverse effect related with pembrolizumab treatment. Only two cases, including this one, have been reported, especially with this medication for the treatment of non-small cell lung cancer. With more reported cases, management strategies can be optimized even in the steroid refractory setting.

Conclusion In conclusion, pembrolizumab associated BP in lung cancer is a very rare adverse event, which has been documented in only two occasions, including the presented case. Pathophysiology as well as management are similar to primary or idiopathic BP, with steroid refractoriness being a relevant and considerable clinical course, requiring immunomodulatory medications. Widespread knowledge, especially in recognizing and treating these rare manifestations, is important for their adequate management.

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