Extra-adrenal myelolipoma and extramedullary hematopoiesis: Imaging features of two similar benign fat-containing presacral masses that may mimic liposarcoma.

Objective: To describe the imaging characteristics of extra-adrenal myelolipoma and mass-forming extramedullary hematopoiesis (EMH) occurring in the presacral region.
Materials and Methods: Retrospective review between 1980 and 2015 revealed 11 patients with biopsy-proven presacral extra-adrenal myelolipoma and 11 patients with presacral EMH and radiologic imaging studies.
Results: All cases of both myelolipoma and EMH directly abutted the anterior sacrum and were centered on the midline. Myelolipomas were round or bilobed (8/11), or were multilobulated in appearance (3/11). On CT, margins could be smooth or ill defined. On MRI, all masses were well-marginated with evidence of a "capsule". All myelolipomas showed evidence of macroscopic fat on both CT and MRI with scattered hazy and nodular soft tissue components. On MRI, all cases showed areas of high T1 signal intermixed with areas of intermediate T1 signal. On T2, all cases showed areas of low and high T2 signal. Enhancement was usually minimal or mild. Most EMH cases had a well-defined round, bilobed or macro-lobulated presacral mass (9/11); 2/11 cases were ill-defined. On CT, when a dominant fatty component was present, the soft tissue components had hazy and nodular features, similar to myelolipoma. On MRI, the masses demonstrated intermediate T1 signal, isointense to skeletal muscle with varying amounts of hyperintense T1 signal. On T2-weighted images, 3 of 4 cases showed areas of mildly hyperintense T2 signal with varying amounts of dark signal. All patients with clinically or biopsy-proven EMH had a hematologic or myeloid disorder.
Conclusion: Presacral myelolipoma and presacral EMH are two benign fat-containing masses that can present as discrete, often encapsulated, lipomatous masses with varying amounts of non-lipomatous soft tissue components and should be considered in the differential diagnosis of a well-defined fat-containing presacral mass.
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