British Journal of Radiology (2006) 79, e148-e151
© 2006 British Institute of Radiology
doi: 10.1259/bjr/28725217
Huge renal lipoma with prominent hypervascular non-adipose elements
I-C Chiang, MD1,3,
M-Y Jang, MD4,
K-B Tsai, MD4 and
T-J Hsieh, MD1,3
Departments of 1Radiology 4Pathology, , Kaohsiung Municipal Hsiao-Kang Hospital, Kaohsiung Medical University, 3 Division of Medical Imaging, 4 Department of Urology, Kaohsiung Medical University Chung-Ho Memorial Hospital, Kaohsiung Medical University, Kaohsiung, Taiwan
Correspondence: Tsyh-Jyi Hsieh, 13Fl, No. 392, Nan-Ping Road, Kaohsiung, Taiwan. E-mail: hh2001{at}ms69.hinet.net.
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Abstract
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A primary renal lipoma is a very rare neoplasm of the kidney and only 20 cases have been reported in the literature. We report a case of a huge retroperitoneal mass that presented as a hypervascular tumour with a prominent fat component, mimicking an angiomyolipoma or a liposarcoma in imaging studies. This finding presented a diagnostic challenge in terms of selecting medical versus surgical intervention. Surgical excision of the lesions was performed and the pathological evaluation revealed a renal lipoma with extrarenal growth.
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Introduction
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Retroperitoneal lipoma usually originates from the perirenal or pararenal adipose tissue. A primary renal lipoma is a very rare neoplasm of the kidney that is exclusively composed of adipose tissue. It occurs in less than 1% of autopsied kidneys and only 20 cases have been reported in published literature [1–3]. The origin of a renal lipoma is unknown, and one possible explanation is from lipomatous differentiation of primitive mesenchyma cells or from embryonal rests of adipose tissue in the kidney [4]. We report a case of a huge retroperitoneal mass that presented as a hypervascular tumour with prominent fat component, mimicking an angiomyolipoma or a liposarcoma in imaging studies. Surgical excision of the lesions was performed and the pathological evaluation revealed a renal lipoma with extrarenal growth.
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Case report
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A 42-year-old female presented with right flank soreness occurring over several months. She had a personal history of idiopathic thrombocytopenic purpura. Ultrasound revealed a huge hyperechoic mass in right retroperitoneum. Under the impression of a lipomatous tumour, she underwent CT examination. The lesion showed a huge mass (about 14 cmx10 cmx7 cm) with prominent fat components (Figure 1
). A small defect at the anterior portion of the middle pole of the right kidney was noted with fat density. The post-enhanced images showed some components with relatively intense enhancement. The findings suggested a peduncular angiomyolipoma of the kidney, or a retroperitoneal liposarcoma with renal invasion. In order to differentiate disease entities, MRI was performed. The MRI showed a huge mass (14.7 cmx10.7 cmx7.4 cm) in the anterior aspect of the right retroperitoneum (Figure 2). The lesion showed heterogeneous hyperintensity on in-phase T1 weighted images and significant signal-drop on out-of-phase and fat-suppression images. This finding supported the thesis that the mass had prominent fat component. The T2 weighted images with fat suppression revealed heterogeneously hyperintense components that had significant enhancement after intravenous gadolinium administration. It suggested that the mass had some soft tissue component with hypervascularity. These non-adipose components had ill-defined margins and infiltrates in the mass were also noted. There were some tubular enhancements in the mass and vessels were suspected. The renal defect that was noted in CT images was still noted in MRI and was connected with the huge retroperitoneal mass. It suggested that the mass had originated from the right kidney and had grown as a peduncular tumour. According to the findings of these imaging examinations, the diagnosis of a peduncular angiomyolipoma of the right kidney was made.
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Figure 1. The CT demonstrated a huge mass with a prominent fat component. A small defect at the anterior portion of the middle pole of the right kidney was noted with fat density. The post-enhanced images showed some components with relatively intense enhancement.
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Figure 2. The huge mass lesion in the anterior aspect of the right retroperitoneum showed heterogeneous hyperintensity on(a) in-phase T1 weighted images and heterogeneous hypointensity on (b) out-of-phase T1 weighted images. This supported the diagnosis that the mass had prominent fat component. (c) The T2 weighted images with fat suppression revealed heterogeneously hyperintense components that had significant enhancement in (d) axial and (e) coronal T1 weighted images after intravenous gadolinium administration. It suggested that the mass had some hypervascular components. There were some tubular enhancements in the mass and vessels were suspected. The renal defect was still noted in MRI and was connected with the huge retroperitoneal mass. It suggested that the mass originated from the right kidney and grew as a peduncular tumour.
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The patient received an angiogram (Figure 3), and transcatheter arterial embolisation with a metallic coil and gelfoam pledgets followed. The patient then underwent retroperitoneal exploration by way of a right flank incision. Dissection confirmed that involvement of the right kidney was only a peduncle. The mass was cut off from the peduncle because the frozen biopsy showed no malignant cells. Pathological examination revealed that the mass was composed mostly of mature fat cells (80%), focally mixed with fibrous tissue and congested blood vessels with the infiltration of inflammatory cells. The immunohistochemical stains showed no feature of angiomyolipoma.
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Figure 3. The angiogram demonstrated two feeding arteries supplying the huge mass at right retroperitoneum.(a) One of the feeding arteries came from one interlobar artery at the middle pole of right kidney and (b) another artery was from the right inferior artery that had a common trunk with right renal artery.
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Comment
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The fat component within a renal mass strongly suggests the diagnosis of angiomyolipoma [5, 6]. The diagnosis relies on the detection of intratumoural fat at CT or MRI. Surgery is therefore avoided in most cases. However, some renal neoplasms, including Wilms tumour, oncocytoma, liposarcoma, renal cell carcinoma and lipoma were once described with the detection of intratumoural fat.
A primary renal lipoma is a very rare neoplasm of the kidney with only 20 cases reported in published literature [1–3]. These lesions were predominant in middle-aged women. Small lesions were asymptomatic, but large neoplasms usually present with abdominal and/or flank pain and occasionally with haematuria. Histologically, they are composed of mature adipose tissue. The major differential diagnosis is with angiomyolipoma [4].
The imaging feature of a lipoma is a well-marginated mass with entirely fatty components that show low density in CT images and hyperintensity on in-phase T1 weighted images, hypointensity on out-of-phase T1 weighted images, fat-suppressed images and T2 weighted images in MRI. Chemical shift imaging success is attributed to the presence of abundant intracytoplasmic lipid, which causes a loss of signal on out-of-phase images. T1 weighted images, fat-suppressed images and chemical shift images make MRI more sensitive in the detection of fat components in small tumours than CT.
Usually, other soft tissue components are less noted in a lipoma, except for feeding vessels. These are used to differentiate from angiomyolipoma and liposarcoma.
In our case, the CT and MRI showed some abnormal soft tissue components in the huge mass with prominent fatty component. A small defect at the anterior portion of the middle pole of the right kidney was noted with fat component. The finding recommended the connection of the mass and the right kidney and the mass was considered as a peduncular lesion from the right kidney. Due to a distinct margin and small connection with the right kidney, a diagnosis of angiomyolipoma with extrarenal growth was made. However, well-differentiated liposarcoma was in the differential diagnosis. This finding presented a diagnostic challenge in terms of selecting medical versus surgical intervention.
However, the final pathological evaluation using immunohistochemical stains excluded the diagnosis of angiomyolipoma. These soft-tissue components with intense contrast-enhancement were composed of fibrous tissue, congested blood vessels and inflammatory cell infiltrate. It was considered as a result of fat necrosis that may be associated with the large tumour size.
Previous studies [7–9] showed that soft-tissue lipomas may have a more complex appearance with significant non-adipose elements than subcutaneous lipomas. On CT and MR images, these non-adipose elements were not only thick septa (>2 mm), but also nodular or globular regions (reported in 28–31% of cases) [8–11]. Such lipomas cannot be distinguished from well-differentiated liposarcoma with imaging alone. These papers suggest biopsy directed at these non-adipose regions, particularly if nodular or globular, both to diagnose well-differentiated liposarcoma and also to exclude the possibility of dedifferentiation [12]. These non-adipose regions in lipomas correspond to mesenchymal elements or areas of fat necrosis. In our case, the non-adipose elements showed neither septa-like nor nodular or globular appearances. The pathological findings of these elements showed infiltrates in the tumour mass. The appearance is not the usual findings in these reviewed papers [7–11] and it presented a diagnostic challenge in terms of selecting medical versus surgical intervention. The target of biopsy in our case is difficult to select because of infiltrative distribution of these non-adipose elements. Apart from surgical resection, close follow-up with or without biopsy should be recommended in cases with similar appearance to our case.
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Conclusions
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The final diagnosis of almost all fat-containing tumours of kidneys is angiomyolipoma and this is easily made by CT and MRI. However, other unusual fat-containing tumours make a diagnostic dilemma. Apart from tumour resection, close follow-up or CT- or MRI-guided biopsy is necessary to avoid a misdiagnosis.
Received for publication October 11, 2005.
Revision received December 8, 2005.
Accepted for publication January 3, 2006.
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References
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Abstract
Introduction
