Clinical note
Extraadrenal retroperitoneal myelolipoma
Paula Bartumeus Martínez, Tomás Ripollés González.
Servicio de Radiodiagnóstico. Hospital Universitario Doctor Peset. Valencia, Spain
ACTAS UROLÓGICAS ESPAÑOLAS 2009;33(4):439-442
Abstract
Extraadrenal myelolipomas are rare benign tumors and differentiating them from other soft
tissue tumors containing fat can be difficult. A retroperitoneal myelolipoma adjacent to right
renal hilum is presented in this case report. Initially diagnosed as liposarcoma, the final
diagnosis was obtained after surgery. Computed tomography (CT), ultrasound (US) and
magnetic resonance imaging (MRI) features may vary according to the major component of
the tumor. MRI with fat suppression and opposed-phase imaging are the best imaging tools to
demonstrate the adipose tissue. A histological confirmation may allow a better presurgical
planning in most cases.
Keywords: Myelolipoma. Retroperitoneum. Retroperitoneal tumor. Retroperitoneal neoplasm.
Myelolipoma, first described in 19051, is a rare benign tumor (<1% at autopsy)2, usually solitary and
nonfunctional, composed of a mixture of mature adipose tissue and hematopoietic cells (myeloid and
erythroid precursors) similar to those present in the bone marrow. It typically occurs in the fifth or sixth
decade of life3 and is usually located in the adrenal gland, although it has also been described in the
presacral retroperitoneum (the most common extraadrenal site)4, perirenal retroperitoneum, mesenterium,
mediastinum, liver, spleen, stomach and muscle fascia2,5-12, and may be confused with other soft tissue
tumors containing fat, mainly well differentiated liposarcomas, extramedullary hematopoietic tumors and
retroperitoneal myolipomas9,13. Imaging, pathologic and histologic features of extraadrenal myelolipomas
are identical those of adrenal myeolipomas9. The purpose of this article is to present the imaging findings
in a case of extraadrenal myelolipoma not located in the presacral region (the most common site),
confirmed surgically.
CASE REPORT
A 65-year-old male with a history of transurethral resection (TUR) for high grade superficial transitional
cell bladder tumor (pT1 G3), with subsequent low grade recurrence, who had the following medical
history of interest: smoker of more than 1 pack/day, arterial HT, severe COPD and dilated
cardiomyopathy. An non-intravenous (IV)-contrast enhanced abdominal computed tomography (CT)
performed as follow-up for his bladder tumor discovered as an incidental finding a heterogeneous nodular
lesion with fat content (hypodense) of 2 cm in diameter, with well defined edges and situated in a
retroperitoneal location posterior to the inferior vena cava (Figure 1). The imaging study was completed
with an abdominal ultrasound showing a retroperitoneal hyperechoic lesion in contact with the right lateral
branch of the inferior vena cava.
FIGURE 1: Non-IV-contrast-enhanced abdominal computed tomography at the level of the right
renal hilum showing a heterogeneous retroperitoneal mass of lower density than adjacent soft
tissues in contact with the inferior vena cava and right psoas muscle without evidence of invasion
of these structures.
Watchful waiting was decided and a follow-up CT was performed at 1 year, revealing growth of the mass
(4 cm maximum diameter), which displaced and compressed the vena cava inferior medially but did not
show signs of invasion. Given the suspicion of retroperitoneal liposarcoma, IV gadolinium-contrast
enhanced magnetic resonance imaging (MRI) was performed, which confirmed the fat content of the
lesion and slight heterogeneous uptake of contrast. T1 sequences showed a hyperintense lesion although
slightly less than retroperitoneal, mesenteric and subcutaneous fat (Figure 2A), with an area of lower
intensity in the upper pole of the lesion. MRI with fat suppression technique demonstrated a slight
hyperintensity compared with perilesional fat, mainly in the upper half. In opposed-phase images, the
lesion was hypointense (Figure 2B), showing a nodular area in the upper pole of higher signal intensity
(Figure 2C) that corresponded to the area of lower intensity in the T1 sequence and of highest intensity in
fat suppression. Following gadolinium administration, slight uptake of the contrast agent was noted
(Figure 2D). With radiological diagnosis of a fat-containing tumor, probably liposarcoma (given its
predominant frequency among retroperitoneal masses), surgical removal was decided. A reniform mass,
located posterior to the inferior vena cava and anchored in the retroperitoneum to the anterior vertebral
ligament, at 2 cm from the lower edge of the right renal vein, was surgically extirpated. The lesion did not
show a vascular pedicle, only small peripheral lymphatics. A pseudoencapsulated lobular 6 x 5 cm tumor
of fatty consistency and yellowish-grey color was sent for pathological study. Histological examination of
the surgical piece revealed a well-delineated fatty tumor, which showed large hematopoietic islands with
the presence of 3 series together with round lymphoid aggregates, diagnosed as lumbar prevertebral
myelolipoma although a bone marrow study was recommended to rule out primary disease
(extramedullary hematopoiesis in the context of a myelodysplastic-myeloproliferative syndrome). This
latter possibility was ruled out by analysis of the bone marrow, confirming the diagnosis of retroperitoneal
extraadrenal myelolipoma.
FIGURE 2. A) T1-weighted abdominal magnetic resonance image where the mass shows a slightly
lower intensity than adjacent retroperitoneal fat. B) Opposed-phase T1-weight magnetic
resonance image: the lesion appears hypointense because the voxels contain a mixture of fat and
soft tissue (myeloid tissue). C) Opposed-phase T1-weighted magnetic resonance image at a
slightly higher level than the previous figure: the lesion has an area of higher signal intensity
corresponding to an area with greater content in myeloid tissue than fat tissue. D) Gadoliniumenhanced fat-suppressed T1-weighted abdominal magnetic resonance image. Slightly
hyperintense lesion compared to adjacent fat.
DISCUSSION
The causes and natural history of extraadrenal myelolipoma are unknown, although it has been reported
that they can grow over time5. Although small lesions are usually asymptomatic, with growth they may
cause symptoms, mainly due to the mass effect and the associated bleeding. They are usually diagnosed
incidentally and no cases of malignant degeneration have been reported14.
The diagnosis of adrenal myelolipomas is less more difficult than extraadrenal ones, mainly because of
their greater frequency, the important relationship with their anatomical location and their predominantly
fat content9.
The appearance of extraadrenal lesions in imaging techniques may vary depending on their composition
(greater or less myeloid content) and associated bleeding and calcification. The typical ultrasound
appearance has been described as a hyperechoic mass with more hypoechoic regions corresponding to
the areas of predominantly myeloid components.
The CT image is characteristic. The lesion usually has negative Hounsfield unit (HU) attenuation values
due to the macroscopic fat, although occasionally this is higher in density than the adjacent
retroperitoneal fat. They may even show densities similar to those of soft tissues (but actually have
attenuation values of 20-30 HU), reflecting the admixture of fat and marrowlike elements. The regions of
greater density correspond to areas of hemorrhage or calcification15.
MRI with fat suppression is the best technique to demonstrate the presence of adipose tissue, although
the presence of marrowlike elements (as occurred in our case (Figure 2C) or hemorrhage may result in
persistent areas of hyperintensity. The signal intensity of the intralesional hemorrhage may vary
depending on how old the bleeding is. Opposed-phase imaging will show low signal intensity in voxels
containing both fat and water tissue, as has been described for adrenal adenomas14. The areas of
predominantly marrowlike elements will show a higher signal intensity.
Despite the large number of cases of adrenal myelolipomas published in the literature (more than 200),
extraadrenal presentation is much less common. In the review conducted by Kammen et al14 in 1998,
only 43 cases of extraadrenal myelolipomas had been published, and more than half were presacral in
location. In the review by Prahlow et al16, most of the cases (14 of 29) occurred in the presacral region,
followed by the retroperitoneum in 4 cases and the pelvis in 3. In the series described by Rao et al9 in
1997, 10 of the 67 myelolipomas studied were extraadrenal in location, 8 of them in the presacral or
perirenal retroperitoneum.
There are only 6 cases described in the perirenal tissue10,17-21, two of them in the renal hilum20,21. Among
the perirenal lesions are the one described by Brietta et al19 as a unilateral mass completely surrounding
the kidney, and the one described by Kumar et al10, similar to the previous one and bilateral in
presentation. In the latter case, both masses completely occupied the perirenal space, and were strictly
confined by Gerota’s fascia. Wagner et al17 report the case of a patient with von Hippel-Lindau disease
and bilateral renal cysts in whom one of the complex cysts was incidentally found to be a myelolipoma
adjacent to the renal hilum. The case published by Snieders et al18 described a left retroperitoneal
myelolipoma located posterior to the upper pole of the adjacent kidney and displacing it anteriorly.
Differential diagnosis of extraadrenal myelolipomas should be made9,13,14 with retroperitoneal tumors
containing fat, mainly well-differentiated liposarcoma (it usually has more poorly defined margins and
signs of infiltration) and myolipoma (a tumor composed of mature fat tissue and smooth muscle cells).
Although differential diagnosis of these tumors by imaging techniques may cause difficulties, pathological
differentiation does not usually represent a problem. Extramedullary hematopoietic tumors can be
pathologically confused with myelolipomas, and a bone marrow biopsy should be performed to rule out
the association with myeloproliferative disorders (myelofibrosis and myeloid metaplasia) and chronic
hemolytic anemias. There have been cases reports of extramedullary hematopoiesis involving the
perirenal space in cases of agnogenic myeloid marrow metaplasia. The case described by Rapezzi et al22
had imaging features similar to those of Kumar et al10, but the final diagnosis was extramedullary
hematopoiesis. Retroperitoneal teratomas and mesenchymal tumors may also contain mature adipose
tissue and hematopoietic components, but the presence of other characteristic tissues (mesenchymal
tissues, immature adipocytes or bone) allows them to be distinguished microscopically.
Therefore, despite the fact that imaging techniques may allow physicians to make an approximate
diagnosis, histological confirmation is often needed to confirm the diagnosis. Percutaneous biopsy or FNA
may be sufficient for this purpose, allowing better presurgical planning or indicating more appropriate
clinical management or even avoiding surgical excision, except in cases of symptomatic lesions or
unclear diagnoses11,23,24. Because our case was an incidental finding without associated clinical
symptoms, surgery could have been avoided with a previous percutaneous diagnosis.
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Correspondence author: Dra. Paula Bartumeus Martínez
Servicio de Radiodiagnóstico. Hospital Universitario Doctor Peset
Avda. Gaspar Aguilar, 90 - 46017 Valencia
Tel.: 963 862 500
Author e-mail: [email protected]
Paper information: Clinical note
Manuscript received: july 2007
Manuscript accepted: august 2007