Differentiation of Renal Tumor Histotypes: Usefulness of Quantitative Analysis of Contrast- Enhanced Ultrasound

Genitourinary Imaging Original Research Lu et al. Contrast-Enhanced Ultrasound to Differentiate Renal Tumor Histotypes Genitourinary Imaging Original Research Qing Lu 1 Bei-jian Huang Li-yun Xue Pei-li Fan Wen-ping Wang Lu Q, Huang BJ, Xue LY, Fan PL, Wang WP Keywords: angiomyolipoma, contrast-enhanced ultrasound, histotype, quantitative analysis, renal cell carcinoma DOI:10.2214/AJR.14.14204 Received December 6, 2014; accepted after revision January 23, 2015. Q. Lu and B. J. Huang contributed equally to this work. 1 All authors: Shanghai Institute of Imaging, Department of Ultrasound, Zhongshan Hospital, Fudan University, 180 Fenglin Rd, Xuhui District, Shanghai 200032, China. Address correspondence to W. P. Wang (wang.wenping@zs-hospital.sh.cn). WEB This is a web exclusive article. AJR 2015; 205:W335 W342 0361 803X/15/2053 W335 American Roentgen Ray Society Differentiation of Renal Tumor Histotypes: Usefulness of Quantitative Analysis of Contrast- Enhanced Ultrasound OBJECTIVE. The purpose of this study is to evaluate quantitative analysis of contrastenhanced ultrasound (CEUS) in the differential diagnosis of renal tumor histotypes. MATERIALS AND METHODS. Between January 2010 and December 2013, 106 clear cell renal cell carcinomas (s) (mean [± SD] diameter, 3.7 ± 1.8 cm), 34 angiomyolipomas (mean diameter, 4.1 ± 1.4 cm), 25 papillary s (mean diameter, 3.5 ± 1.1 cm), and 28 chromophobe s (mean diameter, 2.9 ± 0.9 cm) underwent CEUS quantitative analysis. The dynamic vascular pattern was analyzed with the Fisher exact chi-square test, and rise time, time to peak (TTP), and tumor-to-cortex enhancement ratio were analyzed with the independent-sample t test. RESULTS. Dynamic vascular pattern types I and III (hyperenhancement) were more common among clear cell s, whereas type II (hypoenhancement) was more common among angiomyolipomas, papillary s, and chromophobe s. Irrespective of dynamic vascular pattern class, the rise time and TTP were the shortest in clear cell s and were equal in angiomyolipomas, papillary, and chromophobe s. The tumor-to-cortex enhancement ratio was the highest in clear cell s, was second highest in angiomyolipomas, and was lowest but equal in papillary and chromophobe s. Clear cell s and angiomyolipomas accounted for the majority of the hyperenhancing group. The tumor-to-cortex enhancement ratio of clear cell s was higher than that of angiomyolipomas. With tumorto-cortex enhancement ratio greater than 146.0% as the cutoff to differentiate clear cell from angiomyolipoma in the hyperenhanced group, the sensitivity and specificity were each 71.4%. In the hypoenhanced group, the tumor-to-cortex enhancement ratio was the same in clear cell s and angiomyolipomas but was higher in papillary and chromophobe s. With tumor-to-cortex enhancement ratio greater than 54.2% as the cutoff point to differentiate clear cell s from papillary and chromophobe s, the sensitivity and specificity were 95.5% and 94.8%, respectively, whereas with a tumor-to-cortex enhancement ratio greater than 57.4% as the cutoff point to differentiate angiomyolipomas from papillary and chromophobe s, the sensitivity and specificity were 90.0% and 96.4%, respectively. CONCLUSION. Quantitative analysis of CEUS can show quantification of enhancement features of different renal tumor histotypes and may be helpful in their differential diagnosis. R enal cell carcinoma () is the most common renal malignancy in adults [1], with three major histotypes clear cell, papillary, and chromophobe accounting for 75%, 10 15%, and 5% of cases, respectively [2]. Angiomyolipoma is the most common benign renal tumor [3]. Patients with chromophobe or papillary s, which are considered low-aggressive renal tumors, compared with clear cell s, may have a better prognosis [4]. Surgeons are demanding with increasing frequency histological proof of renal tumors before making therapeutic de- cisions; benign lesions or low-aggressive s are managed conservatively in selected cases, whereas clear cell s are a more aggressive histotype and require surgical intervention [4]. Though biopsy is helpful in differentiating renal lesions, the risk of procedural complications and the potential for sampling error have hindered its universal acceptance. Therefore, a method to accurately characterize renal tumor histotypes that is noninvasive and insensitive to sampling errors is urgent [5]. Although CT and MRI have been reported to be helpful in this field, with sensitiv- AJR:205, September 2015 W335

Lu et al. ity and specificity of 74 92% and 83 94%, respectively [6 8], their use is restrained under some circumstances because of radiation, nephrotoxicity, and metal implants, among other factors. Contrast-enhanced ultrasound (CEUS) has the potential to differentiate malignant from benign lesions by visualizing the tumor vasculature [9], and its sensitivity and specificity for predicting clear cell have been reported to be 48 97% and 45 82%, respectively [10, 11]. However, the qualitative description and interpretation of CEUS depend on operator experience, which is rather subjective, with a consequent low reproducibility and wide range in sensitivity and specificity. In recent years, software-based quantitative CEUS analysis has been reported to be valuable in obtaining a sonologist-independent assessment of tumor perfusion, which is more objective, reliable, and reproducible compared with qualitative CEUS analysis. Thus, in our study, quantitative analysis of CEUS, including quantitative parameters and consequent dynamic vascular pattern, is applied to evaluate the clinical value of quantitative analysis of CEUS in the differential diagnosis of renal tumor histotypes. Materials and Methods Patients The retrospective study was approved by the research ethics board of our institution, and written informed consent was obtained. Between January 2010 and December 2013, 219 consecutive patients with renal lesions underwent CEUS examination in our institution. In patients with more than one lesion, the largest and best visualized one was examined. Inclusion criteria were the presence of a solid or solid-cystic focal renal lesion that was well visualized on conventional ultrasound and pathologically proven by surgery and the absence of previous local treatments. Exclusion criteria for quantitative analysis were as follows: cystic lesions (n = 19); a contrastenhanced video that was too short (< 60 seconds from arrival of contrast agent to end of video) or too late recording (no black screen before contrast agent arrival) (n = 2); technical problems (e.g., wiggly recording, fragile breath-hold, imaging plane without adjacent renal cortex; n = 3); and artifacts or corrupted video quality (e.g., outof-plane movements or ultrasound absorption because of deep location of tumor; n = 2). Therefore, 106 clear cell s (diameter range, 1.2 6.8 cm; mean [± SD] diameter, 3.7 ± 1.8 cm), 25 papillary s (diameter range, 2.2 5.4 cm; mean diameter, 3.5 ± 1.1 cm), 28 chromophobe s (diameter range, 1.1 4.3 cm; mean diameter, 2.9 ± 0.9 cm), and 34 angiomyolipomas (diameter range, 2.5 7.3 cm; mean diameter, 4.1 ± 1.4 cm), identified by postradical or partial nephrectomy pathologic analysis, constituted our study cohort (71 women and 121 men; age range, 23 72 years; mean age, 42.6 ± 12.5 years). The serum creatinine level was normal in all patients. Video Acquisition A sonologist with 15 years of CEUS experience performed the CEUS using an ultrasound scanner (C1 5, 1.5 MHz; Logiq E9, GE Healthcare). The best plane to show the largest plane through the lesion should include the adjacent renal cortex. Each patient was IV injected with a bolus (2.0 ml) of aqueous suspension of phospholipidstabilized microbubbles filled with sulfur hexafluoride (Sonovue, Bracco) by the same operator to minimize variations in the injection rate, followed by a 5-mL flush of 0.9% NaCl. The dual mode of the scanner enabled simultaneous visualization of the conventional baseline image and the dark tissue suppressed contrast-enhanced image. To ensure good artifact-free video sequences, certain standard criteria were established: an initial image with no visible contrast agent (i.e., a black screen), a renal lesion centered recording, the presence of healthy adjacent renal cortex on the same depth, and a stable image with no undesired excursions or transducer movement. The patient was asked to slightly half-fill breath, and the probe was held steady to avoid strong motion of the lesions. The technical parameters were as follows: mechanical index less than 0.1, dynamic range of 65 70 db, temporal resolution of 10 13 frames per second, echo-signal gain below noise visibility, and one focus below the level of the lesion. Video clips of real-time CEUS were recorded on hard disc for offline analysis. The transfer materials were DICOM files. Software Analysis Another sonologist who had 8 years of experience with real-time CEUS description and interpretation performed the quantitative analysis with SonoLiver software (version 1.1, Bracco Research) and Image-Arena software (version 4.1, TomTec Imaging Systems), which included three consecutive steps [12]. First, out-of-plane images and images preceding contrast agent arrival in the interlobular renal artery were excluded from processing. Second, a representative image, which served as a reference position for motion compensation, equipped with SonoLiver to automatically minimize the influence of slight breath on the analysis, was selected where the lesion was well delineated and generally at peak enhancement. Last, two ROIs were manually drawn on the reference frame, avoiding artifacts, calcification, and renal capsule [12]. The analysis ROI, representing the area in which the quantitative analysis was computed, should encompass the major enhanced solid portion of the lesions, regardless of the enhancement degree. The reference ROI was drawn in adjacent renal cortex with homogeneous enhancement in the cortical phase and at the same depth as the lesion. The size and shape of the ROI, relative to the enhanced area of the lesion, were not stable in our study because they did not influence the quantitative analysis [13] (Figs. 1A and 1B). The video analysis began at time 0 second, on arrival of the contrast agent in the interlobular renal artery, not immediately upon injection of the bolus. Then the software generated the difference in signal between the original pixel signal in analysis ROI and the average reference ROI signal that is, the dynamic vascular pattern signal (Figs. 1C and 1D). Image Interpretation The dynamic vascular pattern signal of each lesion was categorized into one of four classes according to the temporal difference of contrast enhancement intensity between the tumor and cortex, as defined in Figure 2. When the pattern was predominately positive or negative, the assigned class was unipolar positive (type I, hyperenhanced) or unipolar negative (type II, hypoenhanced), respectively. When its amplitude changed from positive to negative, the assigned class was bipolar positive-negative (type III, hyperenhancement followed by hypoenhancement) or, conversely from negative to positive, it was bipolar negative-positive (type IV, hypoenhancement followed by hyperenhancement). To make the classes mutually exclusive, two decision rules were defined. The first one consisted in determining whether the dynamic vascular pattern signal was unipolar by thresholding the tumor-to-cortex enhancement ratio in positive and negative components at 10% of the cortex enhancement. Once the threshold was reached, the dynamic vascular pattern signal was considered to be bipolar and a second decision rule was applied, assigning the order of the polarity change. When the maximum positive amplitude occurred before the maximum negative amplitude, the class was set to bipolar positive-negative. Conversely, when the maximum negative amplitude occurred before the maximum positive amplitude, its class was set to bipolar negative-positive [14]. Moreover, SonoLiver also output data of quantitative parameters, including maximum intensity (in decibels), defined as the intensity on peak enhancement; rise time (in seconds), defined as the time that the agents move from 10% to 90% of W336 AJR:205, September 2015

Contrast-Enhanced Ultrasound to Differentiate Renal Tumor Histotypes maximum intensity; and time to peak (TTP; in seconds), defined as the time for the lesion reach maximum intensity. The two time-related parameters were associated with the wash-in speed of contrast agent. Because rise time and TTP showed good stability in different depths but maximum intensity varied according to the influence of depth [13], the measured maximum intensity of renal tumors was normalized by using the tumor-to-cortex enhancement ratio, defined as the ratio of intensity of tumor to the intensity of cortex, to ensure that it was independent of technical or individual variability. A C Fig. 1 46-year-old man with clear cell renal cell carcinoma (). SonoLiver (Bracco Research) screen shots of contrast-enhanced ultrasound images (Logiq E9, GE Healthcare) and dynamic perfusion images with motion compensation are shown. A and B, Ultrasound image (A) and color-coded display of dynamic perfusion model diagram (B) show. Area within blue line is motion compensation area, area within green line is ROI for analysis area, and area within yellow line is ROI for reference area. C, Contrast agent dynamics in reference area (yellow lines) and analysis area (green lines) are shown. Thin lines are original dynamic perfusion curve, and thick lines are perfusion curve after best-fitting analysis. D, Dynamic vascular pattern curve is difference between original signal in analysis ROI and reference signal averaged in reference ROI. Tumor-to-cortex enhancement ratio of 143.2% indicates type I dynamic vascular pattern. Reproducibility of Quantitative Analysis The imaging data of the first 50 renal tumors were used to analyze the reproducibility of quantitative analysis by two sonologists, both with 8 years of experience with CEUS description and interpretation. For the study of intraoperator reproducibility, one operator blinded to the pathologic findings repeated the measurements at an interval of at least 3 days. For the study of interoperator reproducibility, measurements were performed by two operators who were blinded to the results of measurements of the other operator and to the pathologic findings of the lesions. B Statistical Analysis Statistical analysis was performed using SPSS software (version 17.0, SPSS). Continuous data were expressed as mean ± SD. The interclass correlation coefficient (ICC) was used to assess the intra- and interoperator reproducibility of quantitative parameter analysis. The ICC values were interpreted as poor (ICC = 0 0.20), fair to good (ICC = 0.40 0.75), and excellent (ICC > 0.75) [15]; 95% CIs were calculated for the statistic tests. The Fisher exact chi-square test was used to compare the dynamic vascular pattern classes among different histotypes and between two operators. Independent-sample t test was applied to compare the difference of quantitative parameters in different histotypes. Concerning the parameters with statistically significant differences, the cutoff was calculated with ROC curve and the sensitivity and specificity were calculated. A two-tailed p < 0.05 was considered statistically significant. Results Intra- and Interoperator Reproducibility of Quantitative Analysis Among the first 50 renal lesions, there were 35 clear cell s, three papillary s, four chromophobe s, and eight angiomyolipomas. With regard to dynamic vascular pattern class, there was no statistically significant difference in the intra- and interoperator analysis (p = 0.368 and 0.287, respectively) and good reproducibility in both inter- and intraoperator analysis (κ = 0.935 and 0.963, respectively). The interoperator reproducibility for tumor-to-cortex enhancement ratio, rise time, and TTP were very good, with ICCs of 0.93 (95% CI, 0.90 0.95), 0.90 (95% CI, 0.85 0.93), and 0.88 (95% CI, 0.84 0.91), respectively. Similarly, the intraoperator reproducibility for tumor-tocortex enhancement ratio, rise time, and TTP were also very good, with ICCs of 0.97 (95% D AJR:205, September 2015 W337

Lu et al. Dynamic vascular pattern label Unipolar positive Difference signal + Vascular signature Hyperenhanced cular pattern type II (hypoenhancement) was more common among papillary s (88.0%) and chromophobe s (96.4%) than among angiomyolipomas (58.8%; p = 0.02 and 0.001, respectively) and clear cell s (20.8%; for both). Unipolar negative Bipolar positive Bipolar negative CI, 0.95 0.98), 0.93 (95% CI, 0.90 0.97), and 0.92 (95% CI, 0.88 0.94), respectively. +/ /+ Hypoenhanced Hyperenhancement followed by hypoenhancement Hypoenhancement followed by hyperenhancement Fig. 2 Dynamic vascular pattern classification according to differential signal with respect to adjacent renal cortex at same depth. No. of Lesions 70 60 50 40 30 20 10 0 58 9 2 1 22 20 22 27 I II III IV Dynamic Vascular Pattern Type 24 4 Clear Cell Angiomyolipoma Papillary Chromophobe 2 0 0 1 1 0 Fig. 3 Correlation between dynamic vascular pattern signal and renal tumor histotypes. = renal cell carcinoma. Dynamic Vascular Pattern Signal Features The dynamic vascular pattern signal class of each lesion is summarized in Figure 3. Though there are overlaps among different renal tumor histotypes, dynamic vascular pattern type I or III (hyperenhancement at peak) was more common among clear cell s (77.4%) than in angiomyolipomas (38.2%), papillary s (8.0%), and chromophobe s (3.6%) ( for all) (Figs. 1 and 4), whereas dynamic vas- Quantitative Parameter Analysis The time-related and enhancement degree related quantitative parameters for the different renal tumor histotypes are summarized in Table 1 and Figure 5, respectively. Irrespective of dynamic vascular pattern class, the rise time and TTP were the shortest in clear cell s and were equal in angiomyolipomas, papillary, and chromophobe s. The tumor-to-cortex enhancement ratio was the highest in clear cell s, was second highest in angiomyolipomas, and was lowest but equal in papillary and chromophobe s. For dynamic vascular pattern type I or III (hyperenhanced), clear cell s (83.7%; 82/98) and angiomyolipomas (13.3%; 13/98) accounted for most lesions. There was no statistically significant difference in rise time and TTP between them, whereas the tumor-to-cortex enhancement ratio was much higher in clear cell s than in angiomyolipomas (p = 0.005). With tumor-to-cortex enhancement ratio greater than 146.0% as the cutoff point to differentiate hyperenhanced clear cell s from hyperenhanced angiomyolipomas, the sensitivity and specificity were each 71.4%. For dynamic vascular pattern type II (hypoenhanced), clear cell s, papillary s, chromophobe s, and angiomyolipoma accounted for 24.2% (22/91), 24.2% (22/91), 29.6% (27/91), and 22.0% (20/91) of the lesions, respectively. There was no statistically significant difference among these histotypes concerning rise time and TTP. The tumor-to-cortex enhancement ratio was the same in clear cell s and angiomyolipomas but was higher in both papillary and chromophobe s (). With a tumor-to-cortex enhancement ratio of 57.4% as the cutoff point to differentiate angiomyolipomas from papillary and chromophobe s, the sensitivity and specificity were 90.0% and 96.4%, respectively; with a cutoff of 54.2% to differentiate clear cell s from papillary and chromophobe s, the sensitivity and specificity were 95.5% and 94.8%, respectively. In dynamic vascular pattern type IV group, statistical analysis could not be applied because of the small numbers of each tumor histotype. W338 AJR:205, September 2015

Contrast-Enhanced Ultrasound to Differentiate Renal Tumor Histotypes TABLE 1: Time-Related Quantitative Parameters of Different Renal Tumor Histotypes in Different Dynamic Vascular Pattern Types Dynamic Vascular Pattern Class, Time-Related Quantitative Parameter Clear Cell Angiomyolipoma Papillary Chromophobe Types I, II, III, and IV (n = 193) Rise time (s) 10.45 ± 3.36 a,b,c 12.47 ± 6.69 a 12.54 ± 5.17 b 12.35 ± 4.44 c Time to peak (s) 11.60 ± 3.61 d,e,f 14.39 ± 10.02 d 13.74 ± 5.34 e 13.31 ± 4.58 f Types I and III (n = 98) Rise time (s) 10.52 ± 2.81 10.13 ± 2.65 NA NA Time to peak (s) 11.62 ± 3.01 10.77 ± 2.58 NA NA Type II (n = 91) Rise time (s) 10.82 ± 4.67 12.87 ± 6.93 12.17 ± 5.03 11.96 ± 4.62 Time to peak (s) 12.24 ± 4.89 14.66 ± 10.28 12.98 ± 6.13 13.24 ± 4.72 Note Data are mean ± SD. = renal cell carcinoma, NA = not applicable. a p = 0.002, clear cell vs angiomyolipoma. b p = 0.021, clear cell vs papillary. c p = 0.017, clear cell vs chromophobe. d, clear cell vs angiomyolipoma. e p = 0.019, clear cell vs papillary. f p = 0.039, clear cell vs chromophobe. A C Discussion Renal cortical neoplasm is a complex family of tumors with varying histotypes, aggressiveness, and metastatic potential [16]. The differentiation of renal tumor histotypes plays an important role in the therapeutic planning. Compared with CT and MRI, the use of CEUS in this field has been less comprehensively studied, especially with quantitative analysis. Compared with visual analysis, a quantitative analysis technique may reduce the effect of the observer s experience in CEUS and may yield good reproducibility [17], which was proved in our study. We explored the value of dynamic vascular pattern class and quantitative parameters in the differentiation of renal tumor histotypes. Clear cell is the most common renal malignancy. Concerning the qualitative analysis of renal tumors, hyperenhancement has been reported to be a unique finding of clear cell s on both CT [8] and MRI [18]. However, quantitative CEUS analysis B D Fig. 4 37-year-old woman with angiomyolipoma. SonoLiver (Bracco Research) screen shots of ultrasound images (Logiq E9, GE Healthcare) are shown. A and B, Contrast-enhanced ultrasound image (A) and dynamic perfusion image (B) show angiomyolipoma. Area within blue line is motion compensation area, area within green line is ROI for analysis area, and area within yellow line is ROI for reference area. C, Contrast agent dynamics in reference area (yellow lines) and analysis area (green lines) are shown. Thin lines are original dynamic perfusion curve, and thick lines are perfusion curve after best-fitting analysis. D, Dynamic vascular pattern curve of difference signal between two ROIs, with tumor-to-cortex enhancement ratio 115.6% indicating type III dynamic vascular pattern. AJR:205, September 2015 W339

Lu et al. Tumor-to-Cortex Enhancement Ratio 400.00 300.00 200.00 100.00.00 87 76 60 52 Clear Cell Angiomyolipoma Papillary Histotypes p = 0.530 109 133 Chromophobe has been rarely applied in this field before. In our study, most clear cell s were categorized as dynamic vascular pattern type I or III, which means hyperenhancement at peak compared with the renal cortex. Moreover, with quantitative parameter analysis, the tumor-to-cortex enhancement ratio in clear cell s was also significantly higher than that in angiomyolipomas, papillary s, and chromophobe s. As for the time-related parameters, rise time and TTP in clear cell s were much shorter in our study. The rich vascular network and alveolar architecture seen on histologic analysis may allow the quick and strong enhancement of clear cell [19]. Much effort has been taken to evaluate quantitative analysis in the differentiation of renal tumor histotypes. For instance, Zhang et al. [20] reported clear cell s to be the most hyperenhanced tumor compared with papillary s, chromophobe s, and angiomyolipoma on contrast-enhanced CT; using an MRI technique, Roy et al. [18] reported that TTP was shorter in clear cell s than in papillary and chromophobe s; and Gerst et al. [10] also reported that maximum intensity and TTP in clear cell s were significantly higher and shorter, respectively, than those in low-grade malignancy, according to CEUS. Those studies examined only malignant lesions or, in some cases, subgroups of malignant lesions or relatively small cohorts of benign lesions examined with different modalities. To our knowledge, our study is the largest series to examine both benign lesions and 120 116 A Tumor-to-Cortex Enhancement Ratio 350.00 300.00 250.00 200.00 150.00 100.00 68 60 Clear Cell Angiomyolipoma Papillary Histotypes Chromophobe Tumor-to-Cortex Enhancement Ratio 100.00 80.00 60.00 40.00 20.00.00 Clear Cell p = 0.115 Angiomyolipoma Papillary Histotypes p = 0.900 Chromophobe Fig. 5 Box plots of tumor-to-cortex enhancement ratios of different renal tumor histotypes in different dynamic vascular pattern classes. Lines in boxes denote medians, whiskers denote 95% CIs, circles denote outliers, stars denote statistical significance, and numbers are numbers of lesions. A, Irrespective of dynamic vascular pattern class, sequence of tumor-to-cortex enhancement ratio is clear cell renal cell carcinoma () first, followed by angiomyolipoma, and then papillary and chromophobe s. B, In dynamic vascular pattern type I and III (hyperenhancement) group, clear cell s and angiomyolipomas account for most lesions, and tumor-to-cortex enhancement ratio of clear cell s is much higher than that of angiomyolipomas. C, In dynamic vascular pattern II (hypoenhancement) group, sequence of tumor-to-cortex enhancement ratio is clear cell followed by angiomyolipoma, and then papillary and chromophobe s. B 90 different histotypes of s with the utility of dynamic vascular pattern class and quantitative parameter analysis using CEUS. However, as a previous study reported [21], oncocytoma is indistinguishable from clear cell on the basis of imaging findings alone because of its hyperenhancement, which is similar to that of clear cell. Though central stellate scar [22] and segmental enhancement inversion [23] on CT have been suggested as characteristics of oncocytoma, they are not considered diagnostic characteristics of oncocytoma because of its considerable overlap with clear cell. Quantitative CEUS analysis can provide information about the enhancement degree in different lesions; however, it has not been applied in the differential diagnosis between clear cell and oncocytoma in our study cohort and should be explored in future studies. Compared with clear cell s, papillary and chromophobe s are low-aggressive malignant histotypes of renal tumors and are typically hypovascular on histologic analysis [24, 25]. In our study, during quantitative CEUS analysis, both the time-related and the enhancement degree related parameters showed no statistically significant differences between these two histotypes. Under certain circumstances (e.g., for fragile and elderly patients), the treatment strategy is the same for papillary and chromophobe s but different from that for clear cell s, so their quantitative features should be analyzed to distinguish them from clear cell s. Fan et al. [26] reported that papillary and chromophobe s showed hypoenhancement with CEUS. On contrast-enhanced CT and MRI, slight enhancement is also their main imaging feature [27, 28]. In our series, the majority of papillary s (88.0%) and chromophobe s (96.4%) were classified as being of the type II dynamic vascular pattern class, which means that they were hypoenhanced compared with the renal cortex throughout the CEUS process. This hypoenhancement may be attributed to the vascularized stalks of papillary s, which are characterized by the presence of only small vessels without any enlarged vessels or arteriovenous shunts [10], and to the compact growth pattern of tumor cells in chromophobe s [29]. However, it is noteworthy that 20.8% (22/106) of clear cell s in our study were also classified as type II dynamic vascular pattern, similar to most papillary s and chromophobe s. In the previously published literature [18], the proportion of hypoenhancing clear cell s ranged from 10% to 38% and presented a diagnostic dilemma. The differentiation of hypoenhancing clear cell s and papillary and chromophobe s has been rarely reported, especially with CEUS quantitative analysis. In our series, the tumor-to-cortex enhancement ratio in hypoenhanced clear cell s was higher than that in papillary and chromophobe s. On this basis, a tumor-to-cortex enhancement ratio greater than 54.2% was considered as the cutoff point for the differentiation between them, with sensitivity and specificity of 95.5% and 94.8%, respectively. Areas of hyalinization have been report- 31 C W340 AJR:205, September 2015

Contrast-Enhanced Ultrasound to Differentiate Renal Tumor Histotypes ed to account for the hypoenhancement of clear cell, to some extent [18]. However, as a high-aggressive malignant tumor, the existence of enlarged vessels or arteriovenous shunts may explain the relatively high tumor-to-cortex enhancement ratio in type II dynamic vascular pattern clear cell s, compared with that in papillary and chromophobe s. This may be explored in the future combined with pathologic analyses. Angiomyolipoma is the most common benign kidney neoplasm. Most angiomyolipomas in our study showed a hypoenhancement pattern (type II dynamic vascular pattern) on CEUS, which was inconsistent with the findings of a previous study [26]. In the type II dynamic vascular pattern, it is essential to differentiate angiomyolipomas from papillary and chromophobe s as well as from hypoenhanced clear cell s. The rise time and TTP were not statistically significantly different in these histotypes. However, tumor-to-cortex enhancement ratio in angiomyolipomas is much higher than that in papillary and chromophobe s. With a tumor-to-cortex enhancement ratio of 57.4% as the cutoff point for the differentiation between angiomyolipomas and papillary and chromophobe s, the sensitivity and specificity were 90.0% and 96.4%, respectively. Unfortunately, there is no statistically significant difference between hypoenhanced clear cell and angiomyolipoma. However, some qualitative features, such as homogeneous enhancement at peak and slow centripetal enhancement pattern, which was reported in our previous study [30], may help with their differentiation. It has also been reported that angiomyolipomas with typical patterns on ultrasound (hyperechoic angiomyolipomas) have a low degree of contrast enhancement, whereas angiomyolipomas with atypical patterns (hypoechoic due to minimal fat within the lesion) show more intense contrast enhancement [31]. The comparison between them was not conducted in our study because of the small number of hyperechoic angiomyolipomas, and this may be studied in the future. Furthermore, in the hyperenhanced group (dynamic vascular pattern type I or III), the tumor-to-cortex enhancement ratio of angiomyolipomas is much lower than that of clear cell s. Zhang et al. [20] also found that clear cell s showed a higher degree of enhancement than did hypervascular angiomyolipomas, which was in accordance with our results, despite the different enhanced imaging modalities in these studies. The differences of enhancement degree between them need to be further studied in combination with the pathologic analysis, such as microvessel density. Our study still has some limitations. First, pathologic findings proven by surgical resection as an inclusive criterion excludes all already characterized nonsurgical masses, such as small typical angiomyolipomas. Furthermore, successful and qualified CEUS is essential for quantitative analysis. These factors may have caused selection bias in our study. Second, quantitative analysis based on CEUS has good proven reproducibility, but the reproducibility of CEUS itself was not analyzed, and this would require further study. Third, an ROI of relatively large size covering the enhanced solid portion of the lesion just reflects the averaged perfusion information of the tumor tissue. The inhomogeneity of tumor blood perfusion in the ROI was ignored because blood perfusion within tumors is spatially and temporally heterogeneous [20]. Last, our study involved only clear cell s, papillary s, chromophobe s, and angiomyolipomas, without any other benign (e.g., oncocytoma or metanephric adenoma) or malignant (e.g., metastases or lymphoma) histotypes. Further studies should be performed for the differentiation of additional renal tumor histotypes. 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