18F-FDG PET/CT Imaging of Immune Checkpoint Inhibitor-Related "Retroperitoneal Panniculitis".

A 71-year-old woman was operated on for undifferentiated lung adenocarcinoma. Four months after surgery, she developed bone and adrenal metastases. She underwent palliative radiation therapy of left scapula and right iliac bone. Thereafter, she started immune checkpoint inhibitor (ICI) therapy with anti-PD-1 antibodies achieving complete tumor response. Twenty months later, a follow-up 18F-FDG PET/CT confirmed tumor response and revealed high radiotracer accumulation in correspondence of retroperitoneal and subcutaneous fat opacities. The contiguous fasciae were mildly thickened. The temporal relation with ICI therapy together with tumor response and corticosteroids therapy effectiveness led to conclude for ICI-related adverse events.