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Case Report |
1 Department of Adult Radiology, Necker Hospital, 149 rue de Sèvres,
75015 Paris, France.
2 Department of Radiology, Fédération Mutualiste Parisienne, 75005
Paris, France.
3 Department of Radiology, Pellegrin Hospital, 33076 Bordeaux, France.
4 Department of Pathology, Cochin Hospital, 75014 Paris, France.
Received October 15, 2002;
accepted after revision December 9, 2002.
Address correspondence to O. Hélénon.
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This finding contradicts the generally accepted rule that the presence of fat within a renal tumor with no calcification and no evidence of macroscopic necrosis or perirenal fat invasion is consistent with an angiomyolipoma.
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A right nephrectomy and a left partial nephrectomy were performed. Pathology examination showed bilateral multifocal papillary renal cell carcinoma with low nuclear grade. No macroscopic necrosis was noted. The papillary cores were highly infiltrated by foam cells and cholesterol cleft deposits with microscopic cholesterol necrosis (Fig. 1C). Cholesterol clefts, histologically disseminated in the tumors, were generally not detectable on CT scans obtained with 5-mm-thick sections. Only the three foci of negative density, detected on a CT scan in this patient, were correlated histologically with the areas of large accumulation of amalgamated cholesterol clefts. Fat density was correlated with histology by rescanning the gross specimen. A gross specimen obtained from one tumor that did not exhibit intratumoral fat on in vivo CT was reevaluated on CT using thinner (1.3 mm) sections. Additional foci of fat attenuation (not detectable on the 5-mm-thick in vivo slices) were found and correlated closely with concentrations of cholesterol clefts at pathology.
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No family history of renal tumors was noted. Search for a c-met mutation known to be associated with hereditary forms in papillary renal cell carcinomas remained negative.
Our second patient was a 62-year-old man with multiple solid tumors of both kidneys. The largest tumor (13 cm) showed two small areas (1 cm in diameter) of fat with attenuation of -69 H (Fig. 2A) and -30 H. Enhanced CT showed moderate enhancement throughout the tumor (15-25 H) without evidence of macroscopic necrosis. No venous involvement or lymph node enlargement was noted. The patient underwent a right nephrectomy. Pathology examination showed multifocal tubulopapillary carcinomas. No macroscopic necrosis was noted. The largest tumor exhibited foam cell infiltration with accumulation of cholesterol clefts (Fig. 2B), especially in the center of the tumor. A subsequent left nephrectomy confirmed the diagnosis of bilateral multifocal tubulopapillary carcinomas.
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To our knowledge, 14 cases of intratumoral fat in renal cell carcinomas have been reported [1-9]. Seven cases exhibited calcifications [1, 3-8], and a fat-density tumor containing calcifications is likely to be a renal carcinoma and should be surgically removed. All reported cases, except those related to osseous metaplasia, were large heterogenous neoplasms with contrast-enhanced CT evidence of necrosis.
In the cases presented here, tumors were not associated with calcifications. The tumors exhibited smooth margins and a homogeneous pattern after contrast injection, findings that correlate with the absence of macroscopic necrosis within encapsulated neoplasms at histology. Previous CT studies for fat-containing renal cell carcinomas have not reported such a homogeneous, encapsulated tumor appearance and it does not fit with any of the previously mentioned mechanisms.
Papillary renal cell carcinomas represent 7-15% of renal carcinomas [10]. Prognosis for this form of carcinoma is better than for other renal cell carcinomas, although papillary renal cell carcinomas are often multifocal and bilateral. In both cases of papillary renal cell carcinoma we observed, fat-attenuation foci within the tumors showed extracellular amalgamated cholesterol clefts associated with foam cell infiltration. Foamy macrophages are observed in up to two thirds of papillary renal cell carcinomas and are considered one of the most sensitive and specific histologic criteria for their diagnosis [10]. When foamy macrophages are overloaded with cholesterol clefts, cellular necrosis may occur and lead to extracellular cholesterol deposits. This specific microscopic finding, cholesterol necrosis, has appeared to be very suggestive of papillary renal cell carcinoma [11]. Cholesterol necrosis should be differentiated from the more common ischemic necrosis which typically appears as nonenhancing areas on contrast-enhanced CT.
Our observations represent the first reported cases of large well-marginated, homogeneous, solid renal cell carcinomas with intratumoral fat. Such a CT finding does not meet any of the known criteria that indicate a fat-containing renal cell carcinoma. However, unlike angiomyolipomas with minimal fat components, papillary carcinomas are typically hypovascular and homogeneous [12] and do not contain enlarged vessels [13]. In a recent paper, Herts et al. [12] reported that a high (≥ 25%) tumor-to-aorta or tumor-to-parenchyma enhancement ratio on CT during the vascular or the nephrographic phase, respectively, generally excludes the possibility of a papillary renal cell carcinoma. Therefore, and in light of our observations of the two patients, we believe that the current recommendations relating to the diagnosis of fat-containing renal tumors should be modified: Malignancy, especially a tubulopapillary renal cell carcinoma containing fat-producing cholesterol necrosis, should be suspected in a large fat-containing renal tumor if the tumor tissue displays a homogeneous contrast-enhanced pattern (except for the fat-attenuating foci) and if a low (< 25%) tumor-to-aorta enhancement ratio is observed during vascular phase CT. Both signs should therefore be added to the widely accepted criteria that lead one to suspect a fat-containing renal cell carcinoma, including intratumoral calcifications, invasion of the perirenal or sinus fat, a large and heterogenous necrotic tumor, association with enlarged nonfatty lymph nodes, and venous tumor invasion [1].
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