Of all the odd things you’d expect to find in a human body, this is definitely one of the top spots.

Image credits: Rachael Sussman et. al

A 62-year-old man knew something was wrong; he had been complaining of irregular urination frequency; but he wasn’t expecting this. Described in The New England Journal of Medicine as a “free-floating, smooth, firm, rubbery mass measuring 10 cm by 9.5 cm by 7.5 cm and weighing 220 g,” this boiled egg-like mass is actually a peritoneal loose body – a phenomenon described only a handful of times in the literature.

“Histologically, the mass contained predominantly acellular, laminated, fibrous tissue; centrally, the specimen contained proteinaceous material with fibrinoid necrosis, surrounded by a ring of calcification,” the study writes. “The findings were consistent with a peritoneal loose body, a formation that is thought to result from torsed, infarcted, and detached epiploic appendages that transform into fibrotic masses.”

Image credits: Rachel Sussman et al.

Peritoneal loose bodies (or peritoneal mice, for some reason) are rare asymptomatic lesions usually only discovered through incidental surgery in the abdomen. These bodies are usually infarcted appendices epiploicae, which become detached and appear as a peritoneal loose body in the abdominal cavity according to Medscape. This particular body consisted of acellular, fibrous tissue, surrounded by a ring of calcification – not really stuff you want floating inside of you.

But the story has a happy ending. After it was removed, the patient reported normal urination frequency and no other discomfort. However, the pathology of these bodies is not really that well understood.