We generated a ROC curve to compare the performance of DW-MRI alone, nonenhanced MRI alone and a combination of the two imaging methods for diagnosing ccRCC [Figure 4]. Using a cut-off ADC of 1.36 × 10−3 mm2/s, DW-MRI resulted in an area under the curve (AUC), sensitivity, and specificity equal to 0.839, 75.8%, and 87.5%, respectively. Nonenhanced MRI led to an AUC, sensitivity, and specificity equal to 0.919, 93.9%, and 81.2%, respectively. Finally, the combination of imaging methods produced an AUC, sensitivity, and specificity equal to 0.998, 97%, and 100%, respectively.
The Logistic regression showed that the location of center of the tumor (inside the contour of the kidney) (P < 0.001) and appearance of stiff blood vessels (P = 0.01) were significantly helpful for diagnosing ccRCCs. We found that the signal intensity of lesions, necrosis of tumors, the lifted cortex, and the angular interface had no significant effect on diagnosing ccRCCs in our study.
This study supports the potential utility of DW-MRI for the characterization of ccRCCs in human small solid renal tumors (≤4 cm). Currently, CE-CT and CE-MRI are the two conventional imaging techniques used to evaluate RCC characteristics, while dynamic CE-CT and MRI are serving as the “gold standard” for solid renal mass differentiation. Mileto et al. reported that CE dual-energy MDCT with iodine-related attenuation and iodine quantification allows accurate evaluation of iodine uptake in renal lesions on a single-phase nephrographic image, which could reach a sensitivity and specificity of 100% and 97.7%, respectively. Kim et al. included 466 nonfatty solid renal masses to evaluate the clinico-radio-pathologic features of a solitary solid renal mass at MDCT examination and concluded that MDCT accuracy for detection of RCC was 94%. In our study, the AUC, sensitivity, and specificity of nonenhanced MRI for diagnosing ccRCCs were 0.919, 93.9%, and 81.2%, respectively, while the AUC, sensitivity, and specificity of DW-MRI combined with nonenhanced MRI for diagnosing ccRCCs were 0.998, 97%, and 100%, respectively. This indicated that DW-MRI provided additional information to that acquired with nonenhanced MRI, and the combination yielded a diagnostic accuracy similar to that of CE-CT. In addition, use of DW-MRI and nonenhanced MRI could also decrease the risk of contrast-induced nephropathy and nephrogenic systemic fibrosis in CE-CT or CE-MRI, supporting its potential.
Previous studies reported the diagnostic utility of DW-MRI in the identification of the renal diseases like hydronephrosis and pyonephrosis, and determination of renal tumors when DW-MRI was combined with CE-MRI. Taouli et al. reported that the AUC, sensitivity, and specificity of DW-MRI for diagnosing RCCs (excluding angiomyolipomas) were 0.856, 86%, and 80%, respectively. The AUC and sensitivity were higher than our results, which may be due to our small sample size and the different categories of benign lesions in our study. In addition, Taouli's study demonstrated the mean ADC of RCCs was significantly lower than that of benign lesions (P < 0.0001), which contradicts our result. There are several potential reasons for this difference: (i) In Taouli's study, 80% of benign tumors were cystic lesions while in our study, 87.5% of the benign tumors were MFAMLs, and MFAMLs have the lowest ADCs among focal renal lesions; and (ii) the benign lesions in Taouli's study included solid tumors, simple cysts, and renal abscesses etc., but the benign lesions in our study only included solid tumors.
Our results are consistent with other studies in the literature. For example, Cova et al. reported that the mean ADC of solid renal tumors was significantly lower than that of normal renal parenchyma. Zhang et al. showed that the value of ADC in normal renal parenchyma was higher than RCCs and angiomyolipomas by comparing the exponential ADCs and ADCs of 101 renal tumors to that of 20 healthy volunteers. Tanaka et al. studied 41 solid renal tumors without visible macroscopic fat on unenhanced CT and DW-MRI, and observed that the mean and maximum ADC values of MFAMLs were significantly lower than those of ccRCCs (P = 0.0030 and 0.0009, respectively). A study by Rosenkrantz et al. indicated that ADCs of high-grade ccRCC were significantly lower than that of low-grade ccRCC with a b value of 400 s/mm2 and a b value of 800 s/mm2 in 57 patients with pathologically proven ccRCC. Our study results also found the ADCs of high-grade ccRCCs were significantly lower than that of low-grade ccRCCs.
Our study had several advantages. First, by using consecutive patients in our prospective study, we reduced the selection bias. Second, our study focused on small solid renal tumors ≤4 cm in size, which correlates with T1a staging of renal malignant tumors. Early diagnosis of renal tumors leads to early intervention and subsequent improved prognosis in clinical practice. Third, our study determined the most suitable b-value in DW-MRI to distinguish malignant from benign lesions using b values of 0, 50, 400, 600 s/mm2 at 3.0T. We based our choice of b values on the theories that low b values (b ≤ 200 s/mm2) would lead to intravoxel incoherent motion effects contributing to the ADC values, high b values (b ≥ 800 s/mm2) would lead a decreased signal-to-noise ratio, and multiple b values would improve the accuracy of ADCs. Jie et al. showed that the use of high field strength resulted in greater sensitivity and specificity for the detection of prostate cancer with DWI.
There were some limitations in our study as well. First, since the sample size was relatively small and all RCCs included in our study were ccRCCs, our study did not compare the ADC of RCCs subtypes. Second, our study concentrated on solid renal tumors without evaluating the cystic renal lesions. Third, the cut-off ADC proposed in our study cannot be easily reproduced because it is dependent on the MR scanner and sequence, which are inherent limitations of DW-MRI. Thus, in the future, it will be necessary to establish standardized DW-MRI acquisition parameters and processing procedures.
In conclusion, our study has shown the potential of DW-MRI in differentiating ccRCCs from benign small solid renal tumors (≤4 cm), which when combined with nonenhanced MRI, could lead to an increase in accuracy for diagnosing ccRCCs.
We shall extend our thanks to Dr. Wu Ying-Hua and Dr. Liu Rong-Bo for all the kindness and help. We would also like to thank Moira R. Hitchens (Department of Radiology, University of Pittsburgh) for revising our manuscript.
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Edited by: Xiu-Yuan Hao
Source of Support: This work was supported by Sichuan Provincial Science and Technology plan grants (No. 2011SZ0160).
Conflict of Interest: None declared.