When Immunotherapy Hits the Joints: A Case of Late-Onset Polyarthritis Induced by an Immune Checkpoint Blockade and Literature Review.

Immune checkpoint inhibitors (ICPis) are increasingly used across a wide range of malignancies, underscoring the growing need for awareness of ICPi-related adverse events (irAEs), especially outside cancer centers. We report a case of polyarthritis developing over 15 months after the initiation of nivolumab in a patient with advanced urothelial carcinoma of the prostate. The patient presented with symmetrical polyarthritis characterized by inflammatory features, which responded well to corticosteroid therapy. Given the delayed onset of symptoms following ICPi initiation, an extensive infectious and autoimmune workup was conducted, all of which returned negative. This case underscores the importance of continued vigilance for irAEs even during long-term therapy. Timely recognition of ICPi-induced arthritis is crucial, as early initiation of corticosteroids can lead to rapid symptom relief and significant improvement in quality of life.