Colitis presenting 5 months after the final dose of anti-PD-1: long-term monitoring is warranted after adjuvant therapy.

Immune checkpoint inhibitors have been approved as adjuvant therapy. Adverse immune events occurred during the administration of treatment, and delayed immune-related events have low incidence. A 66-year-old man was treated for hypopharynx cancer in 2012. In 2019, he was treated for a new oropharynx cancer. After undergoing surgery and complete response, the patient received nivolumab as adjuvant treatment. 5 months after the last dose of nivolumab, he presented with grade III diarrhea and abdominal pain for 3 weeks. Rectoscopy showed infiltration of mucous by lymphocytes. Corticosteroid was started resulting in a rapid decrease in symptom severity. With the increasing immune checkpoint inhibitors in adjuvant therapy, strict surveillance and education of patient in remission is necessary.