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Sunnybrook and Woman's College Health Science Centre University of Toronto Toronto, Ont. M4N 3M5, Canada
We read with interest the article in the AJR [1] on localized fat collections adjacent to the intrahepatic portion of the inferior vena cava.
In three similar cases, we found normal focal collections of fat that had the appearance of an intracaval mass on routine helically acquired axial CT images (Figs. 1A and 2A). Two of these patients had hepatic metastatic disease and the third had non-Hodgkin's lymphoma; all three patients had hepatic parenchyma fat infiltration. In our cases, however, unlike those in the article by Hines et al. [1] and other reports in the literature, the apparent intracaval masslike collection of fat or pseudolipoma disappeared on subsequent helically acquired axial CT scans obtained with identical imaging parameters (Figs. 1B and 2B). We conclude that the origin of these pseudomasses is extraluminal, a finding that was confirmed retrospectively by coronal reconstructions on both sets of images from each patient. To our knowledge, these cases are the first to be reported in which the follow-up helically acquired axial images did not show the lesion.
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A review of the literature [1,2,3] illustrates that these fat collections are at the level of, or above, the confluence of the hepatic veins and the intrahepatic inferior vena cava, and they are usually adjacent to and posteromedial to the inferior vena cava (the medial wall of the inferior vena cava can be identified interposed superiorly between apparently intraluminal fat and the extraluminal fat). The fat outside the inferior vena cava is continuous with the fat found medially around the subdiaphragmatic paraesophageal fat, which extends laterally and superiorly, indenting the cava.
In 1992, Miyake et al. [2] found that this phenomenon occurs in approximately 0.5% of the general population (11/2227 patients who had CT scans). The fat density ranges from 5 to 22 mm, is round or oval, and has attenuation values ranging from -138 to -30 H on unenhanced CT scans. Some reports [1, 2] suggest no change in size or appearance on follow-up scans (range, 2 months-7 years). Other reports [1, 4] have shown how coronal reformatted helical CT scans can confirm the extraluminal site of these pericaval fat collections, a finding not previously noted on routine axial scans.
Various theories as to the origin of these masses have been proposed [4]. Some authors believe they are caused by acute angulation of the inferior vena cava and narrowing of the intrahepatic inferior vena cava as a result of anatomic variation (which alters with respiration) or by chronic liver disease, or both [3]. Obesity, diabetes mellitus, and steroid medication are unproven associations.
The rare differential diagnoses of an apparent intraluminal inferior vena cava fat-attenuating mass include lipoma of the inferior vena cava or an angiomyolipoma that may extend into the inferior vena cava from the adjacent kidney.
Our cases concurs with the literature: that a masslike collection adjacent to the intrahepatic portion of the inferior vena cava is a normal incidental finding on CT scans that should not be mistaken for an abnormality and, therefore, is of no clinical significance [1, 3, 4]. However, we believe that this is a transient finding that may disappear, depending on the phase of respiration.
References
Winthrop University Hospital Mineola, NY 11501
We thank Drs. Ryan and Atri for their comments and for their interesting observations in these three patients. Since the publication of our article on this topic in the AJR in 1999 [1], we have seen intrahepatic pericaval fat on CT numerous times. We continue to believe that this fat collection is a normal incidental finding of no clinical significance, and the three patients reported here by Ryan and Atri further support this contention.
As they point out, there was no difference in the appearance of the pericaval fat in two of our patients who had previous CT [1]. Similarly, there was no change on follow-up CT scans (mean, 14 months) in the 11 patients reported by Miyake et al. [2] or in the one patient reported by Perry et al. [3]. To date, we have yet to see this finding disappear on follow-up CT, nor have we seen it appear when it was not present on initial CT. To our knowledge, the observations of Ryan and Atri have not been previously reported in the literature. However, although we cannot independently confirm their observations on the basis of our own clinical experience, we believe that the evidence they provide is irrefutable and definitively proves that the pericaval fat collection is in fact completely external to the inferior vena cava. We also agree with the hypothesis that the pericaval fat disappeared on follow-up CT as a result of differences in the respiratory phase. Other related explanations could also be postulated with respect to the blood volume within the inferior vena cava or the intracaval and extracaval pressures at the time of the CT acquisition.
References
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