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Genitourinary Imaging |
1 Both authors: Radiology Service (114), VA North Texas Health Care System, 4500 S Lancaster Rd., Dallas, TX 75216.
Received April 21, 2003; accepted after revision December 1, 2003.
Address correspondence to M. Kumar
(mkumar2001{at}aol.com).
Case Report
A 77-year-old man was admitted to our hospital with a history of acute renal failure, lower extremity edema, and abdominal distention as well as diabetes and hypertension. The patient's creatinine level at the time of admission was 5.6 mg/dL. Imaging was performed using multiple techniques, including conventional radiography, sonography, CT, and MRI. Finally, CT-guided fine-needle aspiration and core biopsy of both perirenal masses were performed. Biopsies of the kidneys and bone marrow were also performed as part of the diagnostic workup.
On the conventional abdominal radiograph, the kidneys appeared enlarged because of the fat-tissue masses surrounding them (Fig. 1A). An unenhanced abdominal CT scan (Fig. 1B) revealed large bilateral fat-containing masses that were strictly confined to perirenal spaces. These masses did not communicate with each other. The kidneys were entirely embedded in fat masses interspersed with small nonfatty soft-tissue densities. The adrenal glands and kidneys appeared normal, and the fascial planes of the kidneys were preserved. In retrospect, renal sonography (Fig. 1C) revealed echogenic perinephric tissue representing fat. Multiple MRI sequences were used, including T1-weighted spin-echo (Fig. 1D), T2-weighted HASTE (Fig. 1E), and gadolinium-enhanced dynamic imaging (Fig. 1F). The MRI examination showed large amounts of well-encapsulated fat in the perirenal spaces bilaterally. Typical fat characteristics were displayed: high signal intensity on T1-weighted images and intermediate signal intensity on T2-weighted images. No significant contrast enhancement after gadolinium administration was observed.
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The core biopsy of both perirenal masses revealed adipose tissue with interspersed hemopoietic precursors such as scattered megakaryocytes, normoblasts, and a few myeloid precursors. Several mature plasma cells and occasional hemosiderin-containing macrophages were found. The core biopsy results for both kidneys were normal. A bone marrow biopsy revealed megakaryocytic hyperplasia with occasional atypia and normocytic normochromic anemia with increased iron stores. A diagnosis of bilateral perirenal myelolipomas was made. The patient was treated conservatively for acute renal failure and improved clinically. His creatinine level returned to normal. CT performed at 3-month follow-up showed stable perirenal masses with no change evident.
Discussion
Myelolipomas are rare benign nonfunctioning neoplasms and are found in less than 1% of the population at autopsy [2]. They manifest in four distinct clinicopathologic patterns: isolated adrenal myelolipoma, adrenal myelolipoma with hemorrhage, extraadrenal myelolipoma, and myelolipoma associated with other adrenal disease [3]. Although the cause of these tumors is not well documented, hypotheses about their origin include metaplasia of adrenal glands, myeloid cells misplaced during embryogenesis, embolization of bone marrow, and a response to trigger stimulus [1]. Likewise, their natural history is not well understood, but it is known that they may enlarge over time and may result in bleeding. Often these tumors are detected incidentally. As the name suggests, myelolipomas consist of hemopoietic precursors and variable amounts of mature adipose tissue. The tumors are well encapsulated and are symptomatic when small. A large tumor may displace or compress surrounding structures.
In our patient, the masses were bilateral and large enough to possibly cause acute renal failure by compressing the kidneys. Adrenal myelolipomas are simple to identify because of their location and predominantly fatty content. Differentiation of myelolipomas from other soft-tissue masses when myelolipomas are in extraadrenal locations is difficult. Extraadrenal myelolipomas have been reported in various sites, including the mediastinum, liver, stomach, lungs, pelvis, spleen, retroperitoneum, presacral region, and mesentery [1]. None of the approximately 45 reported cases presented in the manner of the case we describe. A bilateral perirenal occurrence is a rare presentation of a rare tumor. Completely embedded kidneys make this case unique.
Several types of tumors can involve retroperitoneal structures. The location, size, vascularity, and local invasion by such tumors may give some clues to the correct diagnosis. Radiologic features that can assist one in diagnosing retroperitoneal masses are the beak sign, a prominent feeding artery, or fatembedded or fat-obscured organs. The patient whom we describe displayed kidneys embedded in fat. The benign nature of the lesion was suggested by the fact that all adjacent structures and fascia were intact.
Perirenal masses can be classified according to their major tissue component. Because fat is a major constituent of perirenal space, almost all tumors in that region have some amount of fat. The imaging characteristics of tumors vary according to the major component of the tumor. On sonography, fat is hyperechoic, whereas cellular components are hypoechoic. On CT, fat is displayed with low attenuation, and on T1-weighted MRI, it is displayed with high signal intensity. Myelolipomas may have a mild contrast enhancement depending on the amount of the soft-tissue component.
The differential diagnosis of fat-containing perirenal masses includes lipoma, liposarcoma, myelolipoma, myolipoma, and angiomyolipoma. Other disorders to be considered are extramedullary hemopoiesis, lymphoma, and amyloidosis. Differentiating extraadrenal myelolipoma from well-differentiated liposarcomas, retroperitoneal myolipomas, and extramedullary hemopoiesis can be difficult [1].
Because liposarcoma is the most common retroperitoneal fat-containing tumor, it is always a consideration when diagnosing retroperitoneal masses. The imaging characteristics of liposarcomas depend on their histologic subtype (i.e., well-differentiated, myxoid, round cell, and pleomorphic). Although well-differentiated liposarcomas often do not invade renal parenchyma, other types tend to be poorly marginated and infiltrative. In our patient, the bilateralism and well-encapsulated nature of the lesions pointed toward the benign nature of tumors. Also there was no communication of perirenal masses across the midline. The masses were strictly confined by Gerota's fascia. Histologically, liposarcomas have lipoblasts and zones of atypia, whereas myelolipomas have adipose tissue and hematopoietic tissue. The role of minimally invasive procedures such as fine-needle aspiration as a tool with which to diagnose liposarcoma has been equivocal, especially in well-differentiated types in which the zones of atypia are scant.
Extramedullary hemopoiesis requires special mention here because myelolipoma and extramedullary hemopoiesis can appear similar not only on imaging studies but also at histologic examination. It can even be difficult for pathologists to deal with hemopoietic precursors in a specimen mingled with fat. Fine-needle aspirates of both extramedullary hemopoiesis and myelolipoma have hemopoietic precursors and adipose tissue. A bone marrow biopsy helps to differentiate these two entities by excluding myelofibrosis and myeloid metaplasia. There have been case reports of extramedullary hemopoiesis involving the perirenal space in cases of agnogenic myeloid marrow metaplasia. A case described by Rapezzi et al. [5] had imaging features that were somewhat similar to those of our case but were diagnosed as extramedullary hemopoiesis. Angiomyolipomas may surround kidneys but have prominent vessels in the tumor. Because angiomyolipomas arise from renal parenchyma, a defect in the renal cortex should be visible. On the contrary, myelolipomas do not cause any defects but maintain a smooth interface between the tumor and the cortex. Myolipomas are very rare and may look like myelolipomas.
Scattered case reports of unilateral extraadrenal perirenal myelolipomas appear in the literature. For example, Wagner et al. [6] described a case of von Hippel-Lindau disease with bilateral renal cysts. One of the complex renal cysts was found to be a myelolipoma. Another case report by Snieders et al. [7] described a left retroperitoneal myelolipoma that was posterior to the kidney and displaced it anteriorly. None of these lesions presented a true global perirenal bilateral presence of the myelolipoma. We could find only one case report describing a unilateral mass that completely enveloped the right kidney and was diagnosed as a myelolipoma. However, this case report was published without radiologic images in a pathology journal [8]. We believe that ours is the only radiology case report describing large bilateral extraadrenal myelolipomas that completely occupied the perirenal spaces and were strictly confined by Gerota's fascia.
Making a precise diagnosis of retroperitoneal masses with imaging techniques is challenging. Although CT and MRI can characterize the tissues to a large extent, core biopsy is required to make a final diagnosis. Recently, fine-needle aspiration has been proposed as an aid to diagnosis of myelolipoma.
Conclusion
Although imaging characteristics of myelolipoma usually allow physicians to make a presumptive diagnosis of adrenal myelolipomas, percutaneous needle biopsy or core biopsy is often needed to confirm the diagnosis in cases of extraadrenal myelolipoma. Surgical excision is thought to be unnecessary unless the diagnosis is unclear or the lesion is symptomatic. Asymptomatic nonhemorrhagic myelolipomas are managed conservatively. Our case is a presentation of a rare condition—extraadrenal perirenal myelolipoma. The bilateral tumor entirely embedded both kidneys in enormous perirenal fat bags and was incidentally detected in a patient believed to have acute renal failure. The typical characteristics of adipose tissue mixed with hemopoietic tissue were displayed on various imaging techniques including conventional radiography, sonography, CT, and MRI. The diagnosis was made via CT-guided core biopsy; bone marrow biopsy was performed to rule out myelofibrosis.
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