Pembrolizumab-induced gastritis in a NSCLC patient: Insights from PET-CT on an uncommon immune-related adverse event

Immune checkpoint inhibitors (ICIs) have emerged as primary therapeutic agents for the treatment of malignant tumors. In clinical practice, Programmed Cell Death Protein 1 (PD-1), Programmed Cell Death Protein-ligand 1 (PD-L1), and Cytotoxic T-lymphocyte-associated Antigen 4 (CTLA-4) are common targets of ICIs, and the attention towards immune-related adverse events (irAEs) is increasing. However, there have been relatively few reports on immune-related gastritis. In this article, we present a case of gastritis induced by pembrolizumab. The patient was diagnosed with lung adenocarcinoma Stage IV (cT1cN2M1c), and started as first-line chemotherapy including pembrolizumab. After 5cycles, the patient experienced persistent grade 4 nausea and vomiting. Severe nausea and vomiting, initially diagnosed as drug reactions, immune-related gastritis, and post-cholecystectomy bile reflux gastritis, exhibited poor response to treatment. Gastroscopy revealed only mild superficial gastritis without indications for biopsy. In this condition, PET-CT played a crucial role, providing essential evidence for the diagnosis of immune-related gastritis. Subsequent treatment outcomes further affirmed the accuracy of the immune-related gastritis diagnosis. While it is well-known that corticosteroid therapy is relative contraindicated for common gastritis, it is the most effective treatment for Grade 2 or higher immune-related gastritis. Following corticosteroid treatment, the patient experienced significant symptom relief. Hence, this article provides a clinical approach, suggesting that PET-CT may assist in the diagnosis of immune-related gastritis unreachable for diagnosis by gastroscopy, ensuring timely intervention to alleviate the distress of nausea and vomiting for patients.