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Abdomen » Retroperitoneum
Adrenal myelolipoma
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Author(s) :
Pamela T. Johnson, MD
Presentation This 55 year old woman was found to have microscopic hematuria on a routine physical exam. Her physician referred her for a renal sonogram. She has no flank pain.
Caption: Sagittal sonogram, superior to left kidney.
Description: Hyperechoic, solid mass which attenuates sound. Located superior and slightly medial to the left kidney.
Caption: Sagittal image, superior to left kidney.
Description: Hyperechoic, solid mass (white arrows), which attenuates sound (black arrows), located superior and slightly medial to left kidney.
Differential Diagnosis Adrenal myelolipoma, retroperitoneal lipoma, exophytic renal angiomyolipoma, retroperitoneal liposarcoma
Final Diagnosis Left adrenal myelolipoma
Discussion An adrenal myelolipoma is a benign mass comprised of fat and hematopoetic tissues. They can be seen in patients from 12 – 90 years of age, but are typically identified incidentally on imaging in the 5th to 7th decade. While some patients have pain, most are asymptomatic; Cushing’s disease has been reported in one case.

In one series, the masses ranged in size from 2-12 cm, measuring 5 cm in diameter on average. Unlike renal angiomyolipomas, they do not carry a high risk of hemorrhage, despite their size. In cases where the patient does experience pain, the symptom has been attributed to compression by the mass, internal hemorrhage or necrosis. However, in one series where patients were followed over time, the masses which were not surgically resected did not go on to hemorrhage. In fact, while some increased in size, a number of the myelolipomas stayed the same, and a few even decreased in size at follow up.

Imaging with CT, US or MRI will reveal an adrenal mass comprised of fat and soft tissue components. On ultrasound, the mass will be hyperechoic and attenuate sound, as shown here. However, the hematopoeitic components have a soft tissue appearance, resulting in some heterogeneity. Calcification can be seen.

CT and MRI will provide a specific diagnosis, and are useful to distinguish the two most significant differential considerations: renal angiomyolipoma and retroperitoneal liposarcoma. On CT, the renal angiomyolipoma usually has a small renal parenchymal defect and enlarged vessels within the mass. It may contain secondary hemorrhage and additional angiomyolipomas can sometimes be identified. Retroperitoneal liposarcomas tend to be large (well differentiated measured 18 x 12 cm on average in one series). They may appear infiltrative, and the appearance depends on the degree of differentiation. The distinction may be difficult and may require biopsy for definitive diagnosis.
Case References 1.  Han M, Burnett AL, Fishman EK and Marshall FF. The natural history and treatment of adrenal myelolipoma. J Urology 157(4):1213-16, April 1997.

2.  Israel GM, Bosniak MA, Slywotzky CM and Rosen RJ. CT differentiation of large exophytic renal angiomyolipomas and perirenal liposarcomas. AJR 2002; 179: 769-73.
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