|ECR 2018 / C-1457|
|Imaging Features of Extra-adrenal Myelolipoma|
Findings and procedure details
Myelolipoma is a mesenchymal tumor that is composed of a mixture of adipose and hematopoietic cells. This type of tumor is most commonly localized to the adrenal gland; however, there are rare but well-documented cases of extra-adrenal involvement. The presacral region is the most frequent extra-adrenal location. Other reported locations include the pelvic retroperitoneum, musculofascial tissue,mediastinum, kidney, stomach, and liver.
Extra-adrenal myelolipomas exhibit a slight female predominance and are typically discovered between the ages of 50 to 70 years old. Most tumors are unilateral and have been found to range from 2 to 26 cm in size at the time of diagnosis. The etiology of extra-adrenal mielolipomas is still to be established.
The majority of patients are asymptomatic at the time of diagnosis, and lesions are discovered incidentally on imaging for alternative medical problems.
Depending on the size and location of the lesion, some patients may present with vague flank or abdominal pain due to hemorrhage, mechanical compression, or tumor infarction.
The imaging characteristics of extra-adrenal myelolipomas are similar to their adrenal counterparts. Myelolipoma appearance on CT depends on the composition of the lesión. Presacral myelolipoma contains macroscopic fatty tissue and hematopoietic softtissue density elements on CT. It appears as a heterogeneous mass that is well encapsulated and is closely attached to the anterior aspect of the sacrum. The mass has a low attenuation value consistent with fat (less than −20 HU) but may have soft-tissue density components representing the hematopoietic tissue. Hemorrhage and calcifications can also be seen on CT. The hematopoietic soft tissue elements may enhance after injection of intravenous contrast.
On MR imaging, the fatty components of a myelolipoma yield high signal intensity on T1-weighted images, which demonstrate fat suppression when applied. Hematopoietic elements will have lower signal intensity on T1-weighted images and intermediate signal intensity on T2-weighted images. Intra-tumoral hemorrhage may also be seen, and its resultant appearance and intensity characteristics will vary depending on the age of the blood . In-phase and out-of-phase gradient recalled echo (GRE) sequences may also be useful, as areas of a lesion with intracellular fat will lose signal on out-of-phase images, while areas of macroscopic fat (more typical of myelolipoma) will maintain signal intensity. The hematopoietic soft tissue elements may enhance after administration of intravenous gadolinium contrast. Myelolipoma may show restricted diffusion.
Differentiating presacral myelolipoma from other fat containing retroperitoneal tumors (namely liposarcoma, teratoma, and extramedullary hematopoiesis) may be difficult depending on the amount of visible fat and clinical history, as there is overlap in their imaging and microscopic appearances. Well-differentiated liposarcoma is the most common fat-containing retroperitoneal tumor. The malignant nature of the tumor causes it to exhibit more infiltrative growth without the well-defined borders and circumscription that myelolipoma tends to exhibit. In addition, liposarcoma can present with metastasis, whereas presacral myelolipoma will never present with metastasis.
Sulfur colloid scan using 99mTc can be used for imaging the reticuloendothelial system. In a normal study, 80–90% of uptake is seen in the liver, 5–10% is seen in the spleen, and the remainder is seen in the bone marrow. By taking advantage of this bone marrow distribution uptake, one can look for the distribution of myeloid elements within a presacral myelolipoma that would not be seen in liposarcoma. Technetium-99m sulfur colloid scintigraphy will confirm the presence of erythroid cells in presacral mielolipoma. Sulfur colloid imaging might be considered when a well-encapsulated mixed fatty and soft-tissue mass is seen. These features would more favor a diagnosis of presacral myelolipoma over liposarcoma. When a more aggressive-appearing fatty presacral mass is encountered with ill-defined margins, then biopsy as a next step would be warranted to rule out liposarcoma.
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