Presacral myelolipomas

Presacral myelolipomas classically occur in older patients, with a female predominance of approximately 2:1 (4). Usually, they are asymptomatic and incidentally discovered; however, they may cause symptoms from mass effect on adjacent structures, including the bladder, ureters, sacral nerve plexus, and rectum (2,4,5). They are not associated with hematologic disturbances; however, they have been associated with Cushing syndrome, Addison disease, adrenal hyperplasia, and chronic exogenous steroid use (4).

Presacral myelolipomas are typically well-encapsulated round or oval masses that can vary in size (4). (Masses up to 26 cm in diameter have been described.) They may contain varying amounts of fat and interspersed soft-tissue elements that may enhance after contrast material administration. Small areas of hemorrhage within the mass can calcify, and they can adhere to the sacrum without bone invasion (1,2,6).

The characteristic finding of a presacral myelolipoma (besides its location) is the presence of fat within the mass, which would appear lucent on conventional radiographs, hyperechoic on ultrasonographic images (7), and hypovascular on conventional angiograms (8). However, the fatty tissue within a myelolipoma can be definitively diagnosed with only CT or magnetic resonance (MR) imaging. At CT, this tissue would be of low attenuation (–20 HU), while at MR imaging, it would have increased signal intensity at T1-weighted sequences and decreased signal intensity at fat-suppressed T1-weighted sequences (Fig 2) (1,9).

It may be difficult to distinguish presacral myelolipoma from liposarcoma, teratoma, extramedullary hematopoiesis, or neurogenic tumor (1,2,5,10,11). A combination of the imaging findings and the clinical scenario might be helpful in this differentiation. The most common fat-containing retroperitoneal tumor is well-differentiated liposarcoma (5,11), which typically does not have a capsule, is poorly marginated with irregular contours, and exhibits an infiltrative growth pattern (2). Extramedullary hematopoietic tissue is usually multifocal, occurs in patients with underlying hematologic disease, and typically does not contain fat (1,10). Some neurogenic tumors—such as sacrococcygeal chordomas—typically cause aggressive bone destruction and symptoms that include pain, swelling, and neurologic defects (12), while other tumors—such as neurofibromas—slowly remodel the bone and may extend into the sacral foramina. Sacrococcygeal teratomas are classically noted at birth and can be associated with vertebral anomalies.
However, there is overlap in the appearance of these different entities; therefore, they cannot be distinguished with imaging alone. Even so, myelolipomas are histologically distinct and sometimes can be differentiated from other fat-containing lesions with needle biopsy (1,4,6,10,11). When compared with myelolipomas, liposarcomas lack areas of hemorrhage and contain lipoblasts and zones of cellular atypia (11). Neurogenic tumors contain neural elements that are not present in myelolipomas. Teratomas and mesenchymal tumors may contain hematopoietic tissue and fat (similar to myelolipomas); however, they also contain other tissue subtypes (1).

Since presacral myelolipoma is benign and does not necessarily warrant surgical resection, percutaneous biopsy can be helpful in differentiating it from other presacral masses. However, if the patient is symptomatic or if the mass cannot be definitively diagnosed at needle biopsy, surgical resection may be necessary.

This patient had classic features of presacral myelolipoma. She was an older woman who had a well-defined and encapsulated presacral soft-tissue mass that contained macroscopic fatty components. Although the mass was in direct contiguity with the sacrum, it did not destroy or invade the sacrum. Even though the mass was thought to represent a presacral myelolipoma, a liposarcoma could not be excluded on the basis of the imaging findings alone. It was believed that the mass was contributing to the patient's symptoms; therefore, percutaneous biopsy was not requested and surgical resection was performed.




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