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Case Report |
1 Department of Urology, University of Michigan, 2917 Taubman Center, 1500 E
Medical Center Dr., Ann Arbor, MI 48109-0330.
2 Department of Radiology, St. Joseph Mercy Hospital, 5301 E Huron River Dr.,
Ann Arbor, MI 48106.
3 Department of Pathology, St. Joseph Mercy Hospital, Ann Arbor, MI 48106.
4 Department of Urology, St. Joseph Mercy Hospital, Ann Arbor, MI 48106.
Received November 5, 2003;
accepted after revision December 1, 2003.
Address correspondence to T. G. Schuster.
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Because of the size of the mass and the associated risk of spontaneous bleeding, the patient underwent an uneventful subcapsular partial nephrectomy. Gross examination showed an encapsulated 9 x 7 x 4 cm yellow tumor with irregular zones of infarction. Near the capsule was a 1.7-cm bulging lobulated gray nodule. Microscopy revealed a low-grade epithelial tumor with grade 2 nuclei, occasional mitotic figures, and an architecture composed of trabeculae and papillae containing varying numbers of foamy histiocytes. On the basis of these findings, the tumor was classified as a low-grade papillary renal cell carcinoma (Fig. 1C). No smooth-muscle or angiomatous components were found in the tumor. The nodule near the capsule was a 1.7-cm well-circumscribed mass of mature adipose tissue surrounded by and containing rests of tumor (Fig. 1D), which corresponded to the CT finding. Additionally, multiple zones of coagulative ("cholesterol") necrosis were seen more centrally in the mass separate from the mature adipose tissue. Because no attempt was made to obtain a margin of normal renal parenchyma during surgery, the patient underwent a completion nephrectomy. No additional tumor was identified in this specimen, and the patient was discharged home after an uneventful postoperative course.
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More recently, two reports have been published describing three patients with renal cell carcinomas containing intratumoral fat without evidence of calcifications on CT [9, 10]. As in our patient, two of the reported patients had papillary renal cell carcinoma [10]; however, in those instances, the fat density seen on CT was due to tumor (cholesterol) necrosis. Also, the images presented in these reports show very small areas of fat relative to tumor size. The third reported patient had a granular cell carcinoma [9]. Although no calcifications were seen on CT, focal microscopic calcifications were seen at pathologic examination. Our case is unique in that it shows an encapsulated, noncalcified renal mass containing a distinct nodule of mature, well-circumscribed adipose tissue. Because no smooth-muscle or angiomatous components were found in the tumor and the rests of tumor were contained within the fat, it is unlikely that this was a collision tumor between an angiomyolipoma and a papillary renal cell carcinoma but rather a papillary renal cell carcinoma containing a discrete nodule of fat.
As shown by our case, renal cell carcinomas can, in rare instances, contain fat without associated calcifications. This fact raises concerns regarding the ability to correctly distinguish a benign from malignant renal epithelial tumor on CT only on the basis of identification of fat without associated calcifications. The risk of spontaneous hemorrhage for angiomyolipomas has been shown to be increased for lesions larger than 4 cm [12]. Therefore, many urologists recommend following up patients with asymptomatic angiomyolipomas 4 cm or smaller with serial imaging studies, whereas intervention is considered for patients with tumors larger than 4 cm. However, at surgery, the benign nature of the lesion makes it unnecessary to obtain a margin of normal renal parenchyma, in contrast to the need for a margin when performing a partial nephrectomy for malignancy.
Fat-containing renal cell carcinomas are extremely rare, and we believe that a lesion such as the one shown in our case, with a relatively large fatty nodule and without calcifications, is virtually unique in the reported medical literature. Obviously, the rarity of such a lesion must be weighed heavily against the expense and risk of routinely subjecting more patients with fat-containing renal masses to the surgical removal of a mass that in virtually all cases will be an angiomyolipoma. In the context of the current case report, we would recommend that for asymptomatic fat-containing renal masses smaller than 4 cm, serial imaging be performed and surgical resection be considered for masses with a rapid growth rate. For the rare patient with an underlying unsuspected renal cell carcinoma, the odds of metastasis for tumors of this size are still extremely low, and using 4 cm as the cutoff value would prevent unnecessary intervention in most patients with similar but smaller lesions. For those patients undergoing surgical resection of a suspected angiomyolipoma, if the urologist is not planning to obtain a margin of normal renal parenchyma, a frozen section should be acquired to verify the diagnosis and obviate a subsequent completion nephrectomy. Regardless of its size, if the decision is made to follow up a fat-containing renal mass, the patient should be counseled about the rare possibility of an underlying malignant neoplasm.
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