You are in: eMedicine Specialties > Radiology > GENITOURINARY Adrenal MyelolipomaArticle Last Updated: Feb 15, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 12
  Author: Parvati Ramchandani, MD, Associate Professor of Radiology, University of Pennsylvania School of Medicine; Consulting Staff, Section of Genitourinary Radiology, Department of Radiology, Hospital of the University of Pennsylvania at Philadelphia Parvati Ramchandani is a member of the following medical societies: American Association for Women Radiologists, American College of Radiology, American Roentgen Ray Society, Radiological Society of North America, and Society of Uroradiology Editors: Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Arnold C Friedman, MD, FACR, Associate Chairman, Department of Radiology, University of Florida Health Science Center; Chief, Department of Radiology, Shands-Jacksonville Hospital; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Eugene C Lin, MD, Consulting Staff, Department of Radiology, Virginia Mason Medical Center Author and Editor Disclosure Synonyms and related keywords: myelolipoma, adrenal gland myelolipoma, adrenal tumors, adrenal gland tumors, adrenal neoplasms, adrenal gland neoplasms INTRODUCTIONSection 2 of 12
  BackgroundAdrenal myelolipoma is a rare benign neoplasm composed of mature adipose tissue and a variable amount of hematopoietic elements. Most lesions are small and asymptomatic, discovered incidentally at autopsy or on imaging studies performed for other reasons. Most tumors are unilateral but show no predilection to one particular side. Tumor size varies from several millimeters to more than 30 cm. PathophysiologySeveral explanations of this neoplasm's development have been offered. One theory by D. C. Collins suggests that a myelolipoma represents a site of extramedullary hematopoiesis. The most widely accepted theory, as cited by Meaglia and Schmidt in a 1992 study of the natural history of adrenal myelolipoma, is the existence of metaplasia of the reticuloendothelial cells of blood capillaries in the adrenal gland in response to stimuli such as necrosis, infection, or stress. FrequencyUnited StatesIncidence varies from 0.08-0.4% at autopsy. Mortality/MorbidityNo death rate has been reported in the literature because of the rarity of this lesion. Myelolipomas do not undergo malignant transformation. The primary complication, which is uncommon, is spontaneous rupture of the mass, resulting in retroperitoneal hemorrhage. Rupture and hemorrhage also can occur following blunt trauma. RaceAs reported by Han et al in 1997 in a series of 20 patients, 85% of patients with adrenal myelolipoma were white. No other generalizations have been reported in the literature. SexMostly, the male-to-female ratio is 1:1; however, a study conducted by Han et al in 1997 reported a male-to-female ratio of 2:3. AgeLesions most commonly occur in patients in their fifth to seventh decades, although they have been seen in patients aged 12-93 years. AnatomyMost myelolipoma lesions occur within the adrenal gland. Rarely, myelolipomas can exist in extraadrenal locations, such as the presacral retroperitoneum, perirenal and pararenal retroperitoneum, mediastinum, liver, and muscle fascia. Kammen et al reported a rare case of an extra-adrenal myelolipoma that presented as a fat-containing retroperitoneal mass not contiguous with the adrenal gland. Clinical DetailsMost myelolipomas are asymptomatic and hormonally nonfunctional. Occasionally, patients present with nonspecific abdominal or flank pain secondary to intratumoral or peritumoral hemorrhage, tumor necrosis, or mechanical compression from tumor bulk. Other rare presenting symptoms include hematuria and abdominal mass. In a review of the literature, Hisamatsu et al documented 25 cases of endocrine dysfunction associated with adrenal myelolipomas, including conditions such as Cushing syndrome, Conn syndrome, and congenital adrenal hyperplasia. In 85% of these patients, the abnormality involves the pituitary adrenal axis. In patients with associated endocrine dysfunction, stimulation with cortisol or adrenocorticotropin hormone has been suggested to be involved in the pathogenesis of myelolipomas. Patients with small asymptomatic myelolipomas are monitored clinically for symptoms, and routine follow-up radiologic tests appear unnecessary for such lesions. Symptomatic tumors are treated by adrenalectomy. Transcatheter embolization prior to surgical resection has been used successfully to achieve hemostasis in cases of ruptured myelolipomas leading to retroperitoneal hemorrhage. Occasionally, large silent tumors are excised to prevent the occurrence of rupture. Preferred ExaminationThe fatty component of a myelolipoma is macroscopic in most patients and is diagnostic when discovered on cross-sectional imaging. The preferred imaging modality is CT, which shows focal fatty density within the mass. MRI also accurately depicts both microscopic and macroscopic fat using chemical shift imaging and explicit fat saturation technique, respectively. Myelolipomas may be discovered incidentally on ultrasound (US), which otherwise is not used routinely to characterize adrenal neoplasms. Limitations of TechniquesOccasionally, myelolipomas can enlarge enough to make the organ of origin difficult to discern on CT, resulting in a differential diagnosis that includes renal angiomyolipoma and retroperitoneal liposarcoma or lipoma. In these patients, the multiplanar capability of MRI can help define the tissue planes and confirm that the mass is adrenal in origin. Some myelolipomas may have a larger amount of hematopoietic tissue and no recognizable fat, making them impossible to distinguish from well-differentiated retroperitoneal malignancies or other adrenal tumors on CT or MRI. Percutaneous biopsy may be necessary to establish a diagnosis. DIFFERENTIALSSection 3 of 12
Adrenal Adenoma Adrenal Carcinoma Angiomyolipoma, Kidney Liposarcoma, Soft Tissue RADIOGRAPHSection 4 of 12
FindingsIf large enough, a suprarenal mass may cause inferior displacement of the kidney. The mass may be radiolucent if it is predominantly fatty and, occasionally, can contain calcifications. Degree of ConfidencePlain radiographs are nonspecific and not part of the imaging workup of these lesions. Perform cross-sectional imaging using CT or MRI to document the presence of fat. False Positives/NegativesSmaller myelolipomas are unlikely to exert enough mass effect to be visible on conventional radiographs, thereby constituting false-negative results. Similarly, inferior displacement of the kidney on radiographs can be a result of other adrenal or extra-adrenal lesions. CT SCANSection 5 of 12
FindingsCT appearance of myelolipomas depends on their histologic composition. Large amounts of fat often are seen interspersed with higher-attenuation myeloid tissue. The attenuation values are low (-20 to -30 Hounsfield units [HU]), reflecting the mixture of adipose and myeloid elements. The density of the mass may be slightly higher than that of the adjacent retroperitoneal fat secondary to the presence of the myeloid tissue. The masses usually have a recognizable capsule and may contain calcification in as many as 20% of patients. Calcification may be related to previous hemorrhage. After contrast administration, the mass enhances. If intratumoral or peritumoral hemorrhage has occurred, high-attenuation or low-attenuation fluid may be present, depending on the age of the blood. Degree of ConfidenceCT is sensitive in depicting macroscopic fat in myelolipomas. The key to a CT diagnosis is to find a true focal fat collection in the adrenal mass. This modality is specific and diagnostic if the focal fat density is revealed. CT can be problematic if the mass is large and the organ of origin is not obvious using axial imaging. MRI may be warranted for further evaluation as a result of its multiplanar capability. Additionally, if little or no macroscopic fat is present, the lesion has soft tissue attenuation. False Positives/NegativesAdrenal adenomas may have low-attenuation values because of a large amount of intracellular lipid, but the density usually is not less than –20 HU. In almost all myelolipomas, some regions have densities of less than –30 HU and as low as –100 HU. If the mass contains more hematopoietic tissue, it may appear more heterogeneous and mimic a retroperitoneal liposarcoma. A definitive diagnosis requires percutaneous-guided cytologic or histologic sampling. MRISection 6 of 12
FindingsThe fat in an adrenal myelolipoma is of high signal intensity on T1- and T2-weighted sequences and is key to diagnosis. The hematopoietic tissue has low signal intensity on T1-weighted images and moderate signal intensity on T2-weighted images. The nonuniform admixture of fat and marrow elements also may result in a heterogeneous appearance on T2-weighted sequences. Explicit (chemical) fat-suppressed T1-weighted sequences show a focal loss of signal intensity in the fatty part of the mass, which confirms the diagnosis. The presence of myeloid tissue or hemorrhage results in persistent areas of increased signal intensity on fat-suppressed images. A water-saturated T1-weighted sequence shows the fat as high signal intensity on a background of low signal intensity. Chemical shift gradient-echo imaging depicts microscopic (intracellular) fat that is contained in the same voxel as a water proton. Two scans are obtained through the mass using parameters that are identical except for the echo time (TE), which is selected so that the fat and water protons are either in phase or out of phase. Out-of-phase images show signal loss, since the fat and water protons in the same voxel cancel each other out. This decrease in signal intensity indicates the presence of microscopic fat. Adrenal myelolipomas enhance after gadolinium administration as a result of the presence of hematopoietic tissue. Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. Elsayes et al described 3 categories of myelolipomas based on their MRI features. These categories are (1) homogeneous, hyperintense masses on T1-weighted imaging indicating that they are composed mainly of fat; (2) heterogeneous masses composed of an admixture of fatty and myeloid elements; and (3) focal enhancing masslike areas composed mainly of myeloid cells. Degree of ConfidenceMRI is as sensitive and specific as CT in depicting macroscopic fat and confirming the diagnosis. In evaluating an adrenal myelolipoma, explicit (chemically selective) fat saturation sequences show more signal loss than chemical shift imaging, since the presence of macroscopic fat is typical of a myelolipoma. In contrast, a lipid-rich adrenal adenoma would reveal greater loss of signal intensity on chemical shift imaging as it contains intracellular (microscopic) fat. An added advantage of MRI is the ability to image directly in the coronal and sagittal planes, confirming that the origin of a large mass is adrenal and not hepatic or renal. False Positives/NegativesLesions, such as adrenal adenomas, metastases, and primary cortical carcinomas, rarely have been reported to contain focal fat and can be misdiagnosed as myelolipoma. However, these lesions are rare enough that the detection of macroscopic fat should not prevent the diagnosis in almost all patients. Conversely, some adrenal myelolipomas (eg, giant myelolipomas) have been shown to contain minimal fat, resulting in difficulty in making a definitive diagnosis by MRI. As with CT, markedly heterogeneous masses may not be distinguished easily from well-differentiated retroperitoneal malignancies, such as liposarcoma, and require tissue sampling for definitive diagnosis. ULTRASOUNDSection 7 of 12
FindingsSonographic appearance varies with the composition of the neoplasm. If the tumor contains predominantly fatty components, it appears uniformly hyperechoic. If the tumor contains a predominance of myeloid cells, it may appear heterogeneous or hypoechoic. The classic appearance is an echogenic suprarenal mass, which suggests the diagnosis. The ipsilateral hemidiaphragm artifactually appears as a disrupted line, a finding unique to fat-containing masses. The margins of the lesion are difficult to define because of the lack of contrast with the adjacent retroperitoneal fat. Degree of ConfidenceAlthough a predominantly hyperechoic adrenal mass strongly suggests the diagnosis of myelolipoma, US is not as specific as CT or MRI, either of which should be obtained for further evaluation. US is more nonspecific if the tumor contains only a small quantity of fat and appears hypoechoic, thus mimicking other benign or malignant adrenal neoplasms. False Positives/NegativesSmall lesions are more difficult to diagnose by US, since they may be masked by adjacent echogenic retroperitoneal fat. The finding of apparent disruption of the diaphragmatic echoes is appreciated only for tumors larger than 4 cm; however, the absence of this finding does not mean that a hyperechoic mass does not contain fat. Lesions containing a predominance of myeloid cells appear more hypoechoic and mimic an adenoma, hematoma, metastasis, or primary cortical carcinoma. The presence of hemorrhage or calcification within the tumor alters the sonographic appearance and further confounds the issue. ANGIOGRAPHYSection 8 of 12
FindingsAs a result of vascular specificity, an angiogram can confirm that a mass is adrenal in origin rather than arising from the liver or kidney. The mass is predominantly avascular, with a peripheral rim of vascularity from branching vessels. Degree of ConfidenceThe procedure is fairly specific in determining the organ of mass's origin but is otherwise nonspecific regarding its tissue characteristic. With the advent of cross-sectional imaging modalities, little indication exists for routine angiography. False Positives/NegativesOther adrenal masses, such as adenomas, lipomas, cysts, and metastases, can have a similar angiographic appearance. INTERVENTIONSection 9 of 12
Since myelolipomas contain different proportions of fat and myeloid tissue, a definitive diagnosis using CT or MRI may be difficult, although rarely so, if only a small amount of fat is present. To distinguish the mass from a well-differentiated liposarcoma, a percutaneous fine needle aspiration via US or CT guidance can confirm the diagnosis. The presence of mature adipose tissue intermixed with hematopoietic elements, including megakaryocytes on cytology or histology, is diagnostic of myelolipoma. ACKNOWLEDGMENTSSection 10 of 12
The authors and editors of eMedicine gratefully acknowledge the contributions of previous author Saroja Adusumilli, MD, to the development and writing of this article. MULTIMEDIASection 11 of 12
REFERENCESSection 12 of 12
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