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This case discusses a 62-year-old woman with de novo metastatic lung adenocarcinoma (PD-L1 >50% with a KRAS G12C mutation, ALK and EGFR negative) who was on pembrolizumab for 1 year without any significant toxicity, only low-grade dermatitis and hypothyroidism. She was transitioned to pembrolizumab every 6 weeks at 4 mg/kg and began to develop oral sores shortly thereafter. The sores proved refractory to nystatin and mouth rinses containing corticosteroids, and the patient was ultimately diagnosed with autoimmune-triggered lichen planus. Unfortunately, her symptoms also proved refractory to typical treatments for lichen planus and worsened to the point where she began to develop cutaneous lesions and difficulty swallowing. Unfortunately, she also developed a keratoacanthoma that required excision. The pembrolizumab was stopped, and the patient’s symptoms improved with 5 days of systemic prednisone, metronidazole, and triamcinolone oral paste. Her NSCLC remains stable off active treatment for 6 months. This case study is on rare auto-immune toxicity as well as a keratoacanthoma from anti-PD-(L) 1 blockade, accompanied by sustained treatment response after cessation of the offending drug.

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