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1 All authors: Department of Radiology, Division of Abdominal Imaging, HW 202, New York University Medical Center, 560 First Ave., New York, NY 10016.
Received January 8, 2002;
accepted after revision March 5, 2002.
Address correspondence to G. M. Israel.
Abstract
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MATERIALS AND METHODS. We retrospectively analyzed CT images of 15 large exophytic renal angiomyolipomas and 12 well-differentiated perirenal liposarcomas. Pathologic correlation was available for six of 15 angiomyolipomas and all of the liposarcomas. All examinations were evaluated for lesion size, renal parenchymal defect, enlarged vessels, kidney displacement, lesion encapsulation or margination, associated hemorrhage, and additional angiomyolipomas. The records of patients with tuberous sclerosis or the forme fruste of that condition were excluded from the study.
RESULTS. The average size of the angiomyolipomas was 14 x 10 cm. They showed a renal parenchymal defect (n = 15), enlarged vessels (n = 12), renal displacement (n = 14), good margination without a distinct capsule (n = 14), hemorrhage (n = 1), and additional (one or two) angiomyolipomas (n = 4). The average size of the liposarcomas was 18 x 11.6 cm. They showed enlarged vessels (n = 3), renal displacement (n = 11), and encapsulation (n = 4); none showed a renal parenchymal defect, hemorrhage, or associated angiomyolipomas.
CONCLUSION. Although large exophytic angiomyolipomas and well-differentiated retroperitoneal liposarcomas may have similar appearances on imaging, careful evaluation for a defect in the renal parenchyma combined with the presence of enlarged vessels in angiomyolipomas should enable accurate differentiation in almost all cases. Achieving an accurate diagnosis can have a significant impact on patient treatment.
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Six of 15 angiomyolipomas were surgically proven. Five patients underwent angiography and embolization and had follow-up examinations with CT from 4 months to 7 years 3 months later (mean follow-up time, 2 years 2 months). In one of the five patients who underwent embolization, symptoms persisted, and the tumor and kidney were removed. Lesions in the five remaining patients showed characteristic CT findings of angiomyolipoma (fat density mass, renal parenchymal defect, enlarged vessels) and were being followed clinically at the conclusion of this study. All 12 liposarcoma cases were surgically proven.
Because our study was retrospective, the examinations were performed on a variety of helical and conventional CT scanners using various slice collimations (range, 3-10 mm: 3 mm [n = 2]; 5 mm [n = 16]; 7 mm [n = 4]; 10 mm [n = 5]). All examinations were performed with IV contrast material, but the type and amount of contrast agent varied.
All cases were analyzed by two of the authors working in consensus. Each mass was evaluated for its size in the two greatest dimensions, presence or absence of a defect in the renal parenchyma associated with the lesion, presence and measurement of enlarged vessels in the mass (the two largest when present), extent of displacement of the kidney from the renal fossa, encapsulation or margination of the mass, and any associated hemorrhage.
A renal parenchymal defect (not an extrinsic pressure defect or deformity) was graded as definitely present, probably present, or not present. Displacement of the kidney from the renal fossa was graded as mild displacement (score of 1), moderate displacement (score of 2), or severe displacement (score of 3). In addition, both kidneys were evaluated for the presence of additional angiomyolipomas.
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In one of the six cases of pathologically proven angiomyolipoma, the kidney and tumor had been sent to pathology labeled as a liposarcoma. At pathology, the tumor was initially thought to represent a liposarcoma. However, when a defect in the renal parenchyma and the presence of enlarged vessels were revealed on CT, the findings at pathology were reviewed. The tumor was then confirmed to be an angiomyolipoma.
Liposarcomas
The average liposarcoma size was 18 x 11.6 cm. A sharp defect in the
renal parenchyma was not identified in any case. In a single case, in which
only 10-mm-thick images were available for review, renal parenchymal invasion
was questioned on imaging, but the renal capsule was found at pathology to be
intact and without parenchymal infiltration. Prominent enlarged vessels were
present in three cases (diameter range, 0.8-1.0 cm; mean diameter of the two
largest vessels, 0.9 and 0.9 cm). In 11 of the 12 cases, the kidney was
displaced from the renal fossa (mean, 2.5 on a 1-3 scale). In four cases, the
liposarcoma showed a distinct capsule; in the other eight cases, the margin of
the mass was not clearly defined. In none of the liposarcoma cases was there
associated hemorrhage or renal angiomyolipomas.
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Defect in the Renal Parenchyma
Because renal angiomyolipomas may grow exophytically, most of the neoplasm
may extend into the perirenal space. However, because these angiomyolipomas
arise from the kidney, a defect will be present in the renal parenchyma at
their origin (Figs.
1A,1B
and 2). This finding was
present in all of our cases of angiomyolipoma. In comparison, liposarcomas
arise in the retroperitoneal fat, including the perirenal fat within Gerota's
fascia. They are often closely associated with the renal capsule and are
sometimes called capsular liposarcomas. When these lesions grow, they
displace, compress, and distort the kidney but usually do not invade the
adjacent renal parenchyma
[11]. Therefore, liposarcomas
do not cause a defect in the renal parenchyma, and the interface of the lesion
with the kidney is smooth (Figs.
3A,3B,4A,4B,5).
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In our series, none of the perirenal liposarcomas invaded the renal parenchyma. In one case in which CT was performed with 10-mm-thick sections, parenchymal invasion was questioned on the CT scan. However, the renal capsule was found at pathology to be intact and without parenchymal infiltration. More aggressive liposarcomas could invade the kidney, but these are not the well-differentiated, noninvasive fatty liposarcomas being discussed in this article.
A liposarcoma originating within the renal sinus fat might be difficult to differentiate from a central angiomyolipoma. In this case, the renal parenchyma would be splayed around the neoplasm and any possible defect of the renal parenchyma might not be noted [9].
Vessels in the Lesion
Angiomyolipomas commonly contain enlarged vessels that can be seen on
contrastenhanced CT. In comparison, well-differentiated liposarcomas are
relatively avascular, and those vessels that are present are not usually
enlarged. In our series, 12 of the 15 angiomyolipomas contained enlarged
vessels as compared with only three of the 12 liposarcomas. Only one of the 12
liposarcomas had vessels as large as those present in the angiomyolipoma
group. Although the presence of these vessels is not as important in diagnosis
as the presence of a defect in the renal parenchyma, it is a significant
ancillary finding. If present, this finding should lead to a more thorough
search for a defect in the renal parenchyma if one was not originally
identified. The finding of enlarged vessels can be used as an additional sign
that the lesion is an angiomyolipoma.
For the two cases of angiomyolipoma in which a renal parenchymal defect was identified as not definitely but probably present, enlarged and apparently aneurysmal vessels were present within the tumor (Fig. 6A,6B). This additional finding was a strong indicator that the lesion was an angiomyolipoma and not a liposarcoma. At pathology, both lesions were proven to be angiomyolipomas.
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Presence of Additional Angiomyolipomas
Cases of tuberous sclerosis and the forme fruste of tuberous sclerosis
(bilateral multiple angiomyolipomas without the full spectrum of the disease)
were not included in this study. The occurrence of large angiomyolipomas in
these settings is common and will rarely create difficulty in making the
diagnosis. However, angiomyolipomas may be multiple, even without associated
tuberous sclerosis. This finding occurred in 27% (4/15) of patients with
angiomyolipomas in our study. The presence of other fatty lesions in the
ipsilateral or contralateral kidney, independent of the dominate lesion, is a
strong indicator that the tumor in question is an angiomyolipoma. This can be
a particularly important finding. As might be suspected, no associated
angiomyolipomas were seen in our liposarcoma cases.
Additional Considerations
The lesions in our study were large: the liposarcomas measured on average
18 x 11.6 cm, and the angiomyolipomas, 14 x 10 cm. Although the
liposarcomas were generally larger, overlap was found between the two groups,
and thus the size of the lesion was of no diagnostic help.
Liposarcomas vary in their aggressiveness and their histologic patterns and have been separated into five subtypes: well-differentiated lipomatous (the type under discussion), myxoid, round cell, pleomorphic, and mixed [2]. It is the well-differentiated form of liposarcoma that on occasion can be difficult to differentiate from the large exophytic angiomyolipoma both at gross inspection and microscopically. Many of the renal and perirenal liposarcomas previously reported in the literature were actually renal angiomyolipomas [10].
In many angiomyolipomas, the smooth muscle cells can show variation in nuclear size, which causes concern about the biologic potential of the tumor. Occasional mitosis in the smooth muscle portion compounds this misleading suggestion of malignancy [10]. This finding can be a problem in particular when a frozen section obtained at surgery is evaluated. A tissue sample containing fatty tissue and spindle cells could be called a liposarcoma or an angiomyolipoma. The pathologist must be alerted to the possibilities in the case so that enough tissue is assessed to make a correct diagnosis. This difficulty occurred in one case in this series in which the kidney and mass were removed and labeled as a liposarcoma. However, after review by the pathologist, it was realized that the lesion was a large exophytic angiomyolipoma.
It can even be difficult to determine pathologically whether the liposarcomatous mass represents the rather common retroperitoneal liposarcoma or the rare intrarenal tumor. The rarity of a primary renal liposarcoma is illustrated by a comprehensive review of the English-language literature in 1990, which found only eight cases of renal liposarcoma with unequivocal involvement of the renal parenchyma [12]. Angiomyolipomas can be found in perirenal lymph nodes [1], but usually they are small foci of this hamartoma—more often found in patients with tuberous sclerosis and rarely leading to the large and bulky lesions under discussion.
The prognosis for patients with angiomyolipoma is different than that for patients with liposarcoma. Well-differentiated liposarcomas grow slowly and do not tend to metastasize. However, when they are large, they are difficult to entirely remove, and recurrent tumor is a common result. Angiomyolipomas, on the other hand, are benign lesions that are considered to be hamartomas or choristomas, not neoplasms. Once removed, they will not recur. The major complication associated with angiomyolipomas is hemorrhage (Fig. 6A,6B). This problem may be controlled prophylactically or with embolization techniques at the time of such an event [13]. Alternatively, a "watchful waiting" approach may be used, in which the patient is informed of the potential complication and alerted to obtain treatment should a possible episode of hemorrhage occur [3].
Because angiomyolipomas and liposarcomas require different treatment, accurate diagnosis is important. Obviously, liposarcomas should be surgically removed, and almost always the adjacent kidney is removed as well. On the other hand, most angiomyolipomas can be treated with embolization techniques that will devascularize the lesion and possibly shrink it. This treatment will prophylactically address the major complication of angiomyolipoma, spontaneous hemorrhage [4].
In our series, five patients with angiomyolipomas underwent embolization. At shortterm follow-up, four patients showed minimal shrinkage of the tumor and were asymptomatic. The fifth patient had a 20-cm lesion. In this patient, initial shrinkage occurred after embolization, but the lesion reenlarged after a 7-year interval and was surgically removed because it caused discomfort. Six of the patients with angiomyolipomas underwent surgical removal of the tumor and the kidney. In these cases, either the surgeon or the patient decided on this course of treatment. Five patients had not undergone surgery or embolization at the conclusion of our study and were being followed up clinically. These cases were generally the smaller lesions in the series, and all of them were clearly seen to be angiomyolipomas according to imaging criteria. Three of these patients had one or two other small angiomyolipomas in the ipsilateral or contralateral kidney. In those few instances in which differentiation is ambiguous, surgery for removal of the lesion is indicated.
The CT protocol was not standard for all patients, and this represents a limitation in the study. Although most imaging was performed using 3- or 5-mm collimation, some patients were studied using 10-mm sections, and these were the cases that caused difficulty in determining the presence or absence of a parenchymal defect. In the two patients with angiomyolipomas in which a parenchymal defect was not definitely identified but considered probably present, having thinner sections might have increased the confidence level. In addition, for the single patient having liposarcoma in which renal parenchymal invasion was questioned, thinner sections might have helped clarify this finding. Having thinner sections is particularly helpful when a lesion arises from a pole of the kidney (Fig. 6A,6B). Furthermore, sagittal reconstructed images or the multiplanar capability of MR imaging may have been useful. Also, although pathologic proof is not available for all of the angiomyolipoma cases, these lesions clearly represent angiomyolipomas arising from the kidney when evaluated using established imaging criteria. Finally, the sample size is relatively small, and larger series of cases need to be studied to corroborate our findings.
Conclusion
Large exophytic angiomyolipomas and well-differentiated retroperitoneal
liposarcomas can have similar appearances on imaging studies. A careful
evaluation for a sharp defect in the renal parenchyma and the presence of
enlarged vessels and associated angiomyolipomas should enable accurate
differentiation of these lesions in almost all cases and should lead to proper
clinical management.
Acknowledgments
We thank Yale Shulman (Newark, NJ), Peter Nardi (Brooklyn, NY), and Daniel
Alterman (Bronx, NY) for their contributions of case material.
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