Two Cases of Retroperitoneal Fibrosis During Anti-programmed Cell Death 1 Antibody Treatment

━━ Background. Retroperitoneal fibrosis is characterized by fibrosis around the abdominal aorta and iliac arteries. Two-thirds of reported cases had idiopathic causes. Idiopathic retroperitoneal fibrosis is thought to result from a local inflammatory reaction to antigens in the atherosclerotic plaques of the abdominal aorta. Therefore, it is thought that idiopathic retroperitoneal fibrosis may be a manifestation of a systemic autoimmune or inflammatory disease. In contrast, antiprogrammed cell death 1 ( anti-PD-1 ) antibodies may overexpress innate immunity. There have been only two case reports on retroperitoneal fibrosis during antiPD-1 antibody treatment. We encountered two cases of retroperitoneal fibrosis during anti-PD-1 antibody treatment. Cases. Case 1 involved a 57-year-old man who experienced recurrence of squamous cell lung carcinoma after resection. More than 75% of his cancer cells were positive for programmed cell death ligand 1 (PD-L1). He experienced urinary urgency after one year and six months of nivolumab treatment. Computed tomography revealed a homogeneous plaque that was isodense with muscle surrounding the abdominal aorta and iliac arteries. Positron emission tomography revealed the accumulation of fluorodeoxyglucose at the same site. The serum IgG4 level was normal. Although a biopsy specimen was not obtained, the patient was diagnosed with retroperitoneal fibrosis. The patient was treated with steroids. Case 2 involved a 64-year-old man with pulmonary large cell carcinoma. An analysis of the cancer cells revealed that 50-74% of the cells were positive for PD-L1. Lower back pain appeared after one year and 11 months of pembrolizumab treatment. Computed tomography revealed right hydronephrosis and homogeneous plaque on the right side of the abdominal aorta. Although a biopsy specimen was not obtained, the patient was diagnosed with retroperitoneal fibrosis. The patient improved once with steroids but relapsed. Conclusion. We hypothesized that the administration of anti-PD-1 antibodies may have affected the onset of retroperitoneal fibrosis. (JJLC. 2022;62:1044-1047)