Skin metastasis of a p16-positive squamous cell carcinoma mimicking radiation recall dermatitis

Radiation recall dermatitis is an acute inflammatory reaction confined to previously irradiated skin that occurs after the administration of certain drugs. Herein, we report the case of a 48-year-old man irradiated for bilateral supraclavicular and right axillary lymph nodal metastases from p16-positive esophageal or occult head-and-neck squamous cell carcinoma (SCC). Several months after the completion of radiotherapy, systemic therapy with a combination of methotrexate and pembrolizumab was commenced. The patient developed increased skin pigmentation and inflammation generally consistent with the region that had previously been irradiated. The skin reaction progressed with a protruding mass after prednisolone treatment. A biopsy confirmed p16-positive SCC. Systemic therapy was given, but the patient died 2 months after the confirmation of skin metastasis. We believe that ionizing radiation can modulate the tissue microenvironment of skin and subsequently promote carcinogenesis. It may also alter the tissue response to anticancer therapy, including anti-programmed death-1/PD-ligand 1. Corticosteroids may worsen the skin lesions and conflict with immunotherapy.