Comparisons among different tumor subtypes
We compared the imaging and basic features of the four subtypes of renal tumors, ccRCC, AML, pRCC, and other types of RCC. There were significant differences among the four groups of tumors in terms of age, AWI, ADC, size, T2-weighted signal intensity, intravoxel fat, bulk fat, and enhancement pattern (p = 0.033, p < 0.001, p < 0.001, p = 0.012, p < 0.001, p < 0.001, p < 0.001, and p < 0.001, respectively). There were no significant differences among these four subtypes regarding sex (p = 0.056) and grade (p = 0.426). Finally, the pairwise comparisons that showed the significant differences in the above parameters among the groups of tumors are listed in Table 2 and Table 3.
In the comparison of ccRCC and AML, ccRCC exhibited a higher T2-weighted signal intensity (high: 81.6% vs 16.7%, and low: 5.3% vs 66.7%) (p < 0.001); less intravoxel fat (present: 10.5% vs 75.0%, and absent: 89.5% vs 25.0%) (p < 0.001); less bulk fat (present: 0.0% vs 50.0%, and absent: 100.0% vs 50.0%) (p < 0.001); more type 2 and 3 enhancement patterns (type 2 and type 3: 100.0% vs 83.3%) (p = 0.023); a higher AWI (260.9 ± 116.7 vs 165.4 ± 56.5) (p = 0.015); and a higher ADC value (1.907 ± 0.336 vs 1.191 ± 0.180) (p < 0.001).
In the comparison of ccRCC and pRCC, ccRCC exhibited a higher T2-weighted signal intensity (high: 81.6% vs 22.2%, and low: 5.3% vs 77.8%) (p < 0.001); more type 3 enhancement patterns (type 3: 75.0% vs 0.0%, and type 1: 0.0% vs 60.0%) (p < 0.001); a higher AWI (260.9 ± 116.7 vs 30.5 ± 13.7) (p < 0.001); and a higher ADC value (1.907 ± 0.336 vs 0.986 ± 0.202) (p < 0.001).
In the comparison of ccRCC and other types of RCC, ccRCC exhibited a higher T2-weighted signal intensity (high: 81.6% vs 37.5%, and low: 5.3% vs 37.5%) (p = 0.026) and larger size (2.8 ± 1.0 vs 1.5 ± 0.6) (p = 0.012).
In the comparison of AML and pRCC, AML exhibited more intravoxel fat (present: 75.0% vs 0.0%, and absent: 25.0% vs 100.0%) (p = 0.003) and more type 3 enhancement patterns (type 3: 83.3% vs 0.0%, and type 1: 16.7% vs 60.0%) (p = 0.010).
In the comparison of AML and other types of RCC, AML exhibited more intravoxel fat (present: 75.0% vs 0.0%, and absent: 25.0% vs 100.0%) (p = 0.003).
In the comparison of pRCC and other types of RCC, pRCC exhibited more type 1 enhancement patterns (type 3: 0.0% vs 83.3%, and type 1: 60.0% vs 0.0%) (p = 0.023) and a lower AWI (30.5 ± 13.7 vs 225.2 ± 49.8) (p = 0.014).
Box-and-whisker plots of the AWI and ADC values of ccRCC, pRCC, other types of RCC, and AML are shown in Fig. 1.
Fig. 2, Fig. 3 and Fig. 4 demonstrate the typical image features of the major types of SRMs. Fig. 2 illustrates the MRI images of a Fuhrman grade II clear cell RCC. The images show that the T2-weighted signal intensity of the tumor is higher than that of the normal renal cortex. There is no obvious signal dropout on the fat-suppressed images compared with nonfat-suppressed T2-weighted images, which indicates that there is no detectable bulk fat. Additionally, there is no obvious signal dropout on the opposed-phase images compared with in-phase images, which indicates that there is no detectable intravoxel fat; the AWI is 244.16, with the presence of a plateau pattern and an ADC value =1.911 × 10−3 mm2/s.
Fig. 3 illustrates the MRI images of a benign angiomyolipoma. The images show that the T2-weighted signal intensity of the tumor is lower than that of the normal renal cortex. There is a detectable signal dropout on the opposed-phase images compared with that on the in-phase images, which indicates the presence of intravoxel fat. The AWI was 150.00, with the presence of a washout enhancement pattern and an ADC value =1.201 × 10−3 mm2/s.
Fig. 4 illustrates the MRI images of a Fuhrman grade III papillary RCC. The images show that the T2-weighted signal intensity of the tumor is lower than that of the normal renal cortex. There was no bulk or intravoxel fat component detected in fat-suppressed images or opposed-phase sequences. The AWI was 28.10, with the presence of a progressive enhancement pattern and an ADC value =0.896 × 10−3 mm2/s.
Tumor grade comparison
The study also analyzed differences in imaging and basic features between low-grade and high-grade RCCs, and these results are shown in Table 4. Compared with patients with low-grade ccRCCs, those with high-grade ccRCCs were significantly older (p = 0.024), and the ADC value was lower for high-grade ccRCCs (p < 0.001). Compared with patients with any subtype of RCC (ccRCC, pRCC, chRCC, and other type RCCs) considered low grade, those with high-grade RCCs were also older (p = 0.015), and the ADC value was lower for high-grade RCCs (p = 0.001). There were no significant differences between low-grade and high-grade ccRCCs or among all subtypes of RCCs in the parameters of sex, AWI, size, T2-weighted signal intensity, intravoxel fat or enhancement pattern.
The box-and-whisker plot of ADC values between low-grade and high-grade ccRCCs and the corresponding data for all subtypes of RCC are shown in Fig. 5.