Supplementary Material for: Pembrolizumab-Associated Severe Radiation Recall Dermatitis in a Patient with Metastatic Head and Neck Carcinoma: A Case Report

Introduction
Radiation recall dermatitis is defined as an acute skin reaction confined to a previously irradiated area that occurs following the administration of systemic therapy.
Pembrolizumab is a standard of care for metastatic or recurrent head and neck squamous cell carcinoma, particularly in patients priviously treated with radiotherapy. Its use is expanding into resectable settings, both in combination with and following radiotherapy.
Case Presentation and Conclusion
We present the case of a 65- year old man diagnosed with p16 negative stage IVB squamous cell carcinoma of the oropharynx treated with definitive chemoradiotherapy.
Five months later, disease progression was observed. He received a first cycle of pembrolizumab, seven months and 20 days after his last radiation dose. Ten days later, he developed burn-like pain, erythema and edema of the left side of the face and neck, progressing to epidermal detachment, with a positive Nikolsky sign involving the entire neck and left supraclavicular area, along with grade 4 mucositis with purulent exudate. Skin biopsy findings were consistent with drug hypersensitivity and chronic radiation changes. No improvement was observed with antibiotic therapy, and stabilization was achieved on the second day of 30 mg of prednisolone. Unfortunately, the patient experienced disease progression after discharge from hospitalization.
This case further contributes to the description of a dermatosis that remains insufficiently understood and whose incidence may rise with the broader use of immunoradiotherapy.