Rectosigmoid Intussusception Secondary to Colorectal Carcinoma Imaged With 18F-FDG PET/CT.

Intussusception of bowel in adults is relatively rare and most commonly secondary to gastrointestinal malignancies. We present the case of an 80-year-old man with rectosigmoid adenocarcinoma evaluated via 18F-FDG PET/CT. Imaging revealed rectosigmoid intussusception with a characteristic "target sign" on both PET and CT images involving the invaginated bowel, alongside synchronous FDG-avid lung metastases. Subsequent MRI confirmed the anatomical findings. This case highlights the unique utility of hybrid 18F-FDG PET/CT in simultaneously delineating both malignant and nonmalignant metabolic and anatomical changes and providing critical whole-body staging to guide clinical management.