Peritoneal loose body: an incidental finding, a challenging diagnosis and multimodal assessment to avoid resections

A man in his early 40s presented with urinary symptoms and was diagnosed with a right ureteric calculus, hydronephrosis and urinary tract infection. A non-contrast CT scan of the kidneys, ureters and bladder revealed an incidental pelvic mass in the rectovesical pouch, initially suspected as a gastrointestinal stromal tumour/fibroma. Multidisciplinary management prioritised cystoscopy-guided double-J stenting for obstruction and infection. In a second admission, ureteroscopic lithotripsy with stenting and a laparoscopic low anterior resection were planned simultaneously. Following ureteroscopic lithotripsy, diagnostic laparoscopy identified a large peritoneal loose body (PLB), which was retrieved laparoscopically, avoiding resection. Peritoneal loose bodies are extremely rare, benign entities prone to misdiagnosis as malignancy on imaging, leading to unnecessary major surgery and psychological distress. This case highlights the pathogenesis, clinical features, differential diagnoses and characteristic imaging findings of PLB, emphasising the value of preoperative recognition and diagnostic laparoscopy to prevent unnecessary major surgery.