Pembrolizumab-Induced Retroperitoneal Fibrosis Detected on 68 Ga-FAPi PET/CT.

Hepatocellular carcinoma is one of the leading causes of cancer-related death worldwide. Immune checkpoint inhibitors (ICI) have improved progression and overall survival in patients progressing on sorafenib therapy. But activation of the immune system can lead to numerous immune-related adverse events. Here we present a case of hepatocellular carcinoma with abdominal pain after initiation of pembrolizumab. 68 Ga-FAPi PET/CT highlighted FAPi avid soft tissue density in the paraaortic and presacral regions with diffusely increased FAPi uptake in the thyroid, suggestive of ICI-induced retroperitoneal fibrosis and thyroiditis.