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Letters to the Editor |
,
Akihiro Tanimoto, MD,*,
Hiroshi Shinmoto, MD,* and
Sachio Kuribayashi, MD*
Department of Diagnostic Radiology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan*. e-mail: jinzaki@sc.itc.keio.ac.jp
Department of Radiology, Brigham and Women’s Hospital, Boston, Mass
Editor:
We read with interest the article by Dr Kim and colleagues in the March 2004 issue of Radiology, in which they used the enhancement pattern at computed tomography (CT) to differentiate angiomyolipoma (AML) with minimal fat from renal cell carcinoma (RCC) (1). The differentiation between RCC and AML with minimal fat is important because the treatment for these two diseases is different: Most AMLs can be ignored or followed, whereas RCCs are typically resected or ablated.
First, the article by Dr Kim and colleagues suggested that AML can be distinguished from RCC when a mass demonstrates homogeneous enhancement and prolonged enhancement pattern (1). We agree that AML can be distinguished from clear cell RCC, the most common subtype of primary renal cancer, because it typically exhibits heterogeneous enhancement and an early washout pattern (2). In our experience, however, 60% of small (<3.5-cm) papillary RCCs demonstrate homogeneous enhancement and prolonged enhancement pattern (2).
Indeed, Dr Kim and colleagues reported in a prior study (3) that 80% of small (<3-cm) papillary RCCs demonstrated homogeneous enhancement. Since enhancement patterns for papillary RCCs and AMLs with minimal fat can be similar, we would be interested to know the subtypes of RCC in their study and, specifically, the enhancement pattern of the papillary subtypes. Although AML may be distinguishable from clear cell RCC, we believe that it cannot be distinguished from papillary RCCs on the basis of homogeneous and prolonged enhancement alone.
Second, Dr Kim and colleagues use the term AML with minimal fat in their article to refer to renal masses that do not contain visible fat at CT. We use the same term to refer to a distinct entity and coined this term in the literature (4). We would prefer to use this term to refer only to the tumors that we described—tumors that are homogeneously hyperattenuating, enhancing at CT, hypointense at T2-weighted magnetic resonance (MR) imaging, and isoechoic at ultrasonography (US).
When we examined these tumors histologically, we found that they had little or no fat. Others have described similar findings (5). Dr Kim and colleagues used the same term in their article, but in our opinion, the tumors in their study included different types of AML. They were different in that some of the tumors were iso- or hypoattenuating and therefore probably contained more fat than did the tumors we described.
We believe that, in general, iso- and hypoattenuating AMLs (that contain no fat attenuation) contain more fat than do hyperattenuating AMLs. Alternatively, iso- and hypoattenuating AMLs might contain diffuse scattered distribution of fat cells, which, when volume averaged with neighboring smooth-muscle cells, result in iso- or hypoattenuating masses. Therefore, we would suggest that while all the masses in the article by Dr Kim and colleagues did not contain fat attenuation at unenhanced CT, we distinguish between those that are hyperattenuating from those that are iso- or hypoattenuating. We would suggest that the former group be called "AML with minimal fat" and the latter "AML with diffusely scattered fat." The precise explanation for why they look the way they do requires further radiologic-pathologic correlative study.
REFERENCES
Department of Radiology Asan Medical Center, University of Ulsan, 388–1 Poongnap-dong, Songpa-gu, Seoul 138–736, Korea. e-mail: rialto@amc.seoul.kr
We read with interest the comments from Dr Jinzaki and colleagues regarding (a) the limitation in differentiating AML with minimal fat from papillary RCC and (b) definition of AML with minimal fat (1).
First, they raised an argument that CT features of papillary RCC are similar to those of AML with minimal fat and requested that we present CT findings of papillary RCC in our study. Three of our 62 patients with RCC had papillary RCCs. We evaluated CT features of these patients and observed that there was no tumor with prolonged enhancement pattern, since two tumors had gradual enhancement pattern and one tumor showed early washout pattern.
Our results for papillary RCC seem to be different from those of Jinzaki et al (2), who noted prolonged enhancement pattern in three (60%) of five papillary RCCs. However, the criteria for classifying enhancement pattern over time are different between the two studies. According to our criteria for enhancement pattern over time, all papillary RCCs in the study of Dr Jinzaki and colleagues showed gradual enhancement pattern. Therefore, prolonged enhancement pattern according to our criteria and CT data acquisition technique seems to be a unique characteristic of AML with minimal fat. For the homogeneity of enhancement, the frequency (three of five [60%]) of homogeneous enhancement in the study of Dr Jinzaki and colleagues (2) is double that in ours (one of three [33%]). However, this difference in frequency may be unsubstantial because the number of cases is too small.
Despite our basis supporting the possibility of differentiating AML with minimal fat from papillary RCC, we agree that variable CT features of RCC according to its subtype can sometimes limit the differentiation between AML and RCC, as already discussed in our report (1).
Second, they considered the definition of AML with minimal fat in our study and proposed that this term should be used in specific cases described in their original report (3). As they pointed out, we did not perform histologic analysis for the amount of fat in the tumor. To our knowledge, however, there has been no published quantitative definition for "minimal fat." Moreover, it seems to be inappropriate to limit the definition of "minimal fat" by using the CT, MR imaging, and US findings described in their report, because those findings may be sensitive to but not specific for AML with minimal fat. Therefore, we suggest that the term AML with minimal fat would be better for clinical use to indicate the cases where the presence of fat is not demonstrated clearly with imaging studies.
We thank Dr Jinzaki and colleagues for their interest in our study and valuable comments.
REFERENCES
This article has been cited by other articles:
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  O. A. Catalano, A. E. Samir, D. V. Sahani, and P. F. Hahn Pixel Distribution Analysis: Can It be Used to Distinguish Clear Cell Carcinomas from Angiomyolipomas with Minimal Fat? Radiology, June 1, 2008; 247(3): 738 - 746. [Abstract] [Full Text] [PDF]
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