Small renal masses (SRMs) are defined as solid renal tumors that are enhanced on computed tomographic (CT) scan and magnetic resonance imaging and that are suspected as renal cell carcinomas (RCCs). With the introduction of sonographic and CT examinations, the number of SRMs detected has been greater than previously reported [1,2]. The number of renal masses <3 cm in diameter detected in a period of 5 years during the 1980s was five times greater than that in a similar period during the 1970s owing to the increased number of abdominal imaging studies carried out [1]. Tumors <3 cm in diameter may be more likely to be benign, whereas the aggressive potential of RCC increases dramatically beyond this size [3,4].

Over 70% of renal masses are diagnosed incidentally and have a small volume with the standard of treatment being partial or radical nephrectomy [5,6]. However, more recently, nephron-sparing surgery has become the gold standard for most SRMs. While refined detection technologies have led to the treatment of smaller and earlier-stage malignancies, the percentage of benign lesions discovered has also increased. Although most masses prove to be RCC on final histology, a high proportion of benign disease has also been identified (up to 46% for SRMs) [7]. Because of this, urologists need more preoperative predictors of pathology to aid in decision-making. Several studies have linked the likelihood of benign pathology with preoperative factors, such as age, sex, smoking history, and tumor size [7-11]. However, making a definitive diagnosis in SRM remains difficult. Angiomyolipoma (AML) is typically a solid benign lesion that exhibits fat density on CT scans [12]. However, some renal lesions may contain a very small amount of fat ("minimal fat AML") with microscopic fat and without demonstrable macroscopic fat; these lesions cannot be differentiated from RCCs.

Recently, Verma et al. [13] noted that some exophytic renal masses have a uniform rounded interface with the renal parenchyma, whereas others have a tapering, almost pyramidal, interface, with a definable apex within the parenchyma and an exophytic bulging of the mass beyond the renal capsule. Those authors reported a higher proportion of benign disease for renal masses shaped as an angular interface in the periphery of the kidney compared with renal cancer [13]. We term this renal mass shape with an angular interface in the periphery of the kidney an "ice-cream cone" shape. In the present study, we evaluated the ice-cream cone shape as a predictor of AML in patients who underwent partial nephrectomy of renal masses presumed to be RCC.

The morphologic contour method for assessing exophytic renal masses as benign versus malignant on the basis of the shape of the interface with the renal parenchyma is a recently developed concept [13]. Although the angular interface (which we referred to here as the ice-cream cone shape) could be speculated to be caused by differences in growth pattern between benign and malignant masses, which might influence the shape of their interfaces with the renal parenchyma, there is no convincing pathophysiologic explanation for the angular interface. We supposed that the ice-cream cone shape could be caused by characteristics of AML, such as a softer renal parenchyma. If this parameter could distinguish AML from RCC, it would be useful for reducing potentially unnecessary surgical interventions. The high incidence of the ice-cream cone shape caused by AML would be beneficial for distinguishing AML from RCC among SRMs, particularly in SRMs without a definitive fat component.

Clinical clues can help to distinguish AML from RCC. Sporadic AML, which accounts for about 80% of tumors, is typically small and asymptomatic and is an incidental finding in middle-aged women. This study also confirmed a high incidence of AML in females. The tumor is usually well-defined, and calcification or necrosis within the tumor is rare [14]. Although the fat content of an AML is often evident on thin-section unenhanced CT, about 5% of AMLs have insufficient fat to allow an unequivocal diagnosis [15,16].

When encountering an incidental SRM, AMLs should be excluded. Most AMLs can be diagnosed by identifying regions of fat within a noncalcified renal mass during unenhanced CT [17]. Chemical shift magnetic resonance imaging may also be used to diagnose an AML [18]. However, the clear cell subtype of RCC may lose signal intensity on out-of-phase images because the cells of this tumor, like fat cells, may also contain intracellular lipid [19].

Traditionally, tumor size has been highlighted as the only reliable preoperative factor for prediction of malignant histology. For tumors ≤4 cm in size, the percentage of benign masses has been reported to be 19.8% to 23% [20-22]. The smaller the mass, the greater the chance that it is benign. In a report on 2,770 surgically excised solid renal masses stratified according to size, 46% of masses <1 cm in diameter were benign, as were 22% of those that were 1 to 2.9 cm and 20% of those that were 3 to 3.9 cm [7]. Consistent with the latter study, our data highlighted tumor size as a reliable preoperative factor for prediction of malignant histology in univariate and multivariate logistic regression analysis.

Better preoperative diagnostic methods for SRMs are needed to reduce the number of major surgeries performed unnecessarily. One option is to use preoperative needle core biopsy for selected lesions, which has been shown to be an accurate method for pathological diagnosis [23,24]. It appears to be safe, and new techniques provide more accurate biopsy material [25,26]. Another option is initial active surveillance as part of the management of incidentally detected SRMs followed by treatment only for those that show progression [27]. However, the cost and burden of serial imaging is significant and others have expressed concern that local progression might require radical nephrectomy rather than nephron-sparing treatment. On the basis of the findings from the present study in which peripheral renal lesions with an ice-cream cone shape were associated with a significant probability of having AML pathology, we suggest that these data could be helpful in patient counseling concerning active surveillance or preoperative biopsy.

Several possible limitations of this study warrant discussion. The choice of 3 cm as a cutoff size may seem unusual. This was chosen because, from our experience, tumors smaller than 3 cm are difficult to classify by AML owing to the absence of a gross fat component and because tumors less than 3 cm in size may be more likely to be benign [3]. Thus, there is the question of whether a similar pattern would be found in larger masses. According to a previous report [13], the angular interface is also a useful predictor of benignity in exophytic renal masses larger than 3 cm in diameter. Another potential limitation of this study is that selection bias might result from the selection of all consecutive patients who underwent only partial nephrectomy for SRMs during the study period, but might be diminished by a study based on pathologic reports after partial nephrectomy.

Parameters that can distinguish AML from RCC would be useful for reducing unnecessary surgical interventions. This study confirmed a high incidence of AML in females. Also, tumor size and the ice-cream cone shape in CT imaging are beneficial for distinguishing AML from RCC among SRMs. Thus, a simple assessment of the ice-cream cone shape can be used as an additional parameter in patient counseling.