When rads encounter symmetric macroscopic fat in the perirenal space, the initial instinct is often to fear well-differentiated liposarcoma or lymphoma. Distinguishing these from benign extra-adrenal perirenal myelipoma is critical to avoiding unnecessary clinical anxiety and aggressive surgical intervention.
Impressive Abs! In this excerpt from the ARRS 2026 Challenging Abdominal Cases Instructional Course, Cooky Menias, MD, presents a patient with abdominal bloating who revealed a symmetric retroperitoneal process pressing on the kidneys. Despite the lack of subcutaneous fat, the patient exhibited significant intra-abdominal fat that really impressed on the renal parenchyma.
The Diagnosis:
- Extra-adrenal myelipoma is a rare, benign mesenchymal tumor composed of fat and hematopoietic cells.
- They typically appear in the 5th to 6th decades of life.
- The most common extra-adrenal locations are the perirenal and presacral spaces.
The Pearls:
- Fat Chance: Unlike perirenal fibrosis (IgG4-related disease) or Erdheim-Chester, which encase the kidneys and lack fat, myelipomas contain macroscopic fat and embed (i.e., “impress”) upon the renal tissue.
- PET-CT Utility? While not diagnostic, PET-CT can be reassuring by showing a lack of significant uptake, helping to rule out high-grade lymphoma or aggressive malignancy.
- Stress Test: These tumors may arise from ectopic adrenal tissue or reticular endothelial cells that become exuberant during chronic stress, endocrine dysfunction, or inflammation.
Bottom Line: If you see symmetric macroscopic fat embedding the kidneys in an older patient with low metabolic activity on PET, think extra-adrenal myelipoma. While biopsy (even open biopsy) may be required for definitive comfort, recognizing this embedding fat pattern can narrow the differential significantly.
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