Bronchoesophageal fistula formation after three courses of nivolumab for carcinoma of unknown primary with a subgroup of lung squamous cell carcinoma.

Immune checkpoint inhibitors (ICIs) are widely used in both monotherapy and combination chemotherapy for various types of cancers. Nivolumab is the most popular among ICIs, and the number of adapted malignant diseases for nivolumab is increasing. Bronchoesophageal fistula formation is a serious complication of the treatment for esophageal or lung cancer. However, the development of bronchoesophageal fistula as a complication of ICIs is obscure. A 59-year-old man who was diagnosed with carcinoma of unknown primary with a subgroup of lung squamous cell carcinoma had bronchoesophageal fistula formation after three cycles of nivolumab as the fourth line treatment. Before the initiation of nivolumab, he had received two esophageal stents and an angiogenesis inhibitor. These are known risk factors for fistula formation. This is a rare case showing that nivolumab monotherapy might induce bronchoesophageal fistulae. Therefore, clinicians should be aware of the factors related to fistula formation when using ICIs.