Macrophagic Reaction to Sodium Polystyrene Sulfonate Crystals Mimicking Peritoneal Carcinomatosis on 18F-FDG PET/CT in Rectal Cancer

A 68-year-old man presented with rectal bleeding 1 year after a highly complicated surgical course for upper rectal cancer. Endoscopic biopsy confirmed a local recurrence at the rectal stump. 18 F-FDG PET/CT showed intense uptake in peritoneal nodules, suggesting peritoneal carcinomatosis. However, exploratory laparoscopy and pathologic analysis identified basophilic crystals consistent with sodium polystyrene sulfonate (Kayexalate) as the cause of a foreign-body granulomatous reaction, with no evidence of tumor recurrence. This case underscores the importance of considering rare causes of peritoneal false-positive PET/CT findings in patients with a history of severe abdominal complications, ensuring accurate diagnosis and appropriate management.