Teaching Case

A 70-year-old male patient was investigated for the cause of his 5 kg weight loss and anorexia over 12 months. His past medical history includes type 2 diabetes mellitus, chronic iron deficiency anaemia, ischaemic heart disease, chronic kidney disease and pleural plaques. Recent upper-GI endoscopy only showed hiatus hernia.

Figure 1: An ovoid 4 cm soft tissue mass with a fatty calcified core is seen posterior to the bladder, left of the rectosigmoid, separate from both structures.

Figure 2: On MRI pelvis, the mass is seen at the right side of the rectum. The lesion has a low peripheral signal and a high core signal in both T1 and T2, with no contrast enhancement.

Figure 3: The CT scan from 21 months before for investigation of anaemia showed the same mass, located more superiorly in the left iliac fossa along the left proximal wall of the urinary bladder with identical size and morphology. As before, it does not arise from any pelvic organs and has a distinct fat plane separating it from adjacent organs.

Figure 4: MR enterography from 17 months before showed the same lesion with the same signal morphology to be located in the right iliac fossa.

Peritoneal loose bodies (PLBs) are benign intra-abdominal masses that arise from torsed, infarcted epiploic appendages. [1] An infarcted epiploic appendage can become calcified and detached from the colonic serosa when its pedicle atrophies. [1] These can then move freely within the peritoneal cavity and form a fibrous shell due to repeated exposure to peritoneal serum. [2] PLBs are classically <2.5cm in diameter and macroscopically appear like yellow-white peas or beans. [3] They have a smooth, firm surface texture and a fatty/proteinaceous core which may be calcified. [4-7]

PLBs rarely cause symptoms and are usually found incidentally in autopsies or laparotomies. [1] However, if they grow to a larger size (>3cm), they can cause chronic abdominal pain, intestinal obstruction, urinary retention or urinary frequency [7] and cause diagnostic confusion on imaging.

On CTs and MRIs, large PLBs usually have a smooth ovoid outline with a soft-tissue shell and calcified and/or fatty core. [4, 6-8] The key features of large PLBs on imaging are:

- Mobile nature: PLBs change position on scans that were taken on separate occasions, or with different patient positioning.
- Separation from surrounding organs: PLBs move freely within the peritoneal cavity and are not attached to visceral organs. A distinct fat plane can sometimes be seen separating PLBs from surrounding structures.
- Slow change in size: PLBs only gradually increase in size over years.
- Non-enhancing: PLBs do not enhance with IV contrast.
- No diffusion restriction: On MR diffusion-weighted imaging, PLBs show no restriction.


If above radiological criteria are met and there is no clinical suspicion about a more sinister pathology, a radiological diagnosis of PLB can be made without histology. [8]

As PLBs are benign, a correct radiological diagnosis is central in forming a management plan. If the patient is asymptomatic, no surgical intervention is required. However, if urinary or gastrointestinal symptoms are present, PLBs can be removed laparoscopically. [4, 6, 7]

In conclusion, radiologists should consider PLBs as a differential diagnosis when faced with a mobile intra-abdominal mass that does not arise from any of the surrounding organs. This will avoid unnecessary surgical interventions and patient distress.