Renal masses are frequently encountered on cross-sectional imaging done either for kidney-related symptoms or as an incidental finding. The increased incidence especially in elderly people and those with multiple co-morbidities warrants the need for a non-invasive tool to characterize those lesions.
Renal cell carcinoma is the most common renal neoplasm and of which clear cell subtype is the most common subtype. Each subtype has a different aggressiveness and thus a different prognosis.
Multiple studies aiming at investigating different parameters in a trial to differentiate those subtypes based on imaging were done. Finding a method to distinguish the lesion along with its extensions is important for the management, and as contrast-enhanced studies are not always feasible (either due to patient-related factors or technical difficulties as well as cost-related issues), our study aimed at studying the role of DWI/ADC as a tool in diagnosis and characterization, as well as comparing ADC value and ADC ratio and their roles in the characterization of the renal masses.
ccRCC was the most common subtype with a higher mean ADC than other benign and malignant lesions showing a mean ADC value of 1.4 ± 0.19 × 10−3 mm2/s, and the results are consistent with those by Zhang et al., de Silva et al., Sevcenco et al., Serter et al., and el Serougy et al. reporting mean ADC value of 1.53 ± 0.31 × 10−3 mm2/s [15], 1.50 × 10−3 mm2/s [16], 1.38 0.56 × 10−3 mm2/s [4], 1.474 ± 0.575 × 10−3 mm2/s [17], and 1.56 ± 0.27 × 10−3 mm2/s [18], respectively. Mirka et al. used the median ADC to report their results, reporting a median of 1.365 × 10−3 mm2/s for ccRCC [19], which is close to our results having a median of 1.2 × 10−3 mm2/s for ccRCC.
Razek et al. had a higher mean ADC for ccRCC of 1.74 ± 0.12 × 10−3 mm2/s [20], which could be attributed to the fact that their study was conducted on a 1.5T machine whereas ours was performed on a 3T machine, in addition to the different sample sizes (27 cases of ccRCC in our study compared to 19 in the study by Razek et al.).
In our study, we had a mean ADC value for pRCC of 0.80 ± 0.18 × 10−3 mm2/s; results are close to those by de Silva et al., Sevcenco et al., and el-Serougy et al. reporting a mean ADC value of 0.76 × 10−3 mm2/s [16], 1.016 ± 0.377 × 10−3 mm2/s [4], and 0.96 ± 0.25 × 10−3mm2/s [18], respectively. Mirka et al. used the median ADC to report their results, reporting a median of 1.0 × 10−3 mm2/s for pRCC [19], which is close to our results having a median of 0.8 × 10−3 mm2/s for pRCC.
However, Razek et al. had different results reporting a mean ADC value for pRCC of 1.65 ± 0.26 × 10−3 mm2/s [20], the difference again is likely because their study was conducted on a 1.5T machine while our study was performed on a 3T machine, in addition to the different sample sizes (15 cases of pRCC in our study compared to six in the study by Razek et al.).
In our study, we had nine cases of AML showing restriction with a mean ADC of 0.89 ± 0.16 × 10−3mm2/s, which aligns with other studies by Emad-Eldin et al. and Sevcenco et al. who reported a mean ADC of 0.95 ± 0.3 × 10−3 mm2/s [21], and 0.828 ± 0.227 × 10−3 mm2/s [4], respectively. While Ghoneim et al. and de Silva et al. had different results reporting a mean of 0.75 × 10−3 mm2/s [22] and 0.69 × 10−3 mm2/s [16], respectively, this can be attributed to the difference in magnet strength (both studies performed on 1.5-tesla machine while ours was on 3-tesla machine).
In our study, we had four cases of chrRCC with a mean ADC value of 0.85 ± 0.13 × 10−3 mm2/s, similar to findings by el-Serougy et al. who reported a mean ADC value of 0.89 ± 0.13 × 10−3mm2/s [18]. This is incompatible with other studies by de Silva et al., Razek et al., and Sevcenco et al. who reported a mean ADC of 1.11 × 10−3 mm2/s [16], 1.44 ± 0.12 × 10−3 mm2/s [20], and 1.239 ± 0.334 × 10−3mm2/s [4], respectively. This can be attributed to the different magnet strength in the former and the larger sample size in the latter two compared to our study. Mirka et al. used the median ADC to report their results, reporting a median of 1.06 × 10−3 mm2/s for chrRCC [19], this was not in concordance with our calculated median of 0.85 × 10−3 mm2/s for chrRCC; this can be attributed to the small sample size.
In the current study, we had three out of four cases of oncocytoma showing restricted diffusion with a mean ADC value of 1.10 ± 0.26 × 10−3 mm2/s, which is not in concordance with other studies by Razek et al. and Sevcenco et al. who reported a mean ADC of 2.10 ± 0.10 × 10−3mm2/s [20], and 1.603 ± 0.636 × 10−3 mm2/s [4], respectively. This can be attributed to the different magnet power in the former study, and the larger sample size in the latter study. Mirka et al. used the median ADC to report their results, reporting a median of 1.65 × 10−3 mm2/s for oncocytoma [19], this was not in concordance with our study; the median in ours was 1 × 10−3 mm2/s, which can be attributed to the different sample size (three cases in our study and 11 in the study by Mirka et al.).
In our study, we only had one case of TCC which had an ADC value of 1.1 × 10−3 mm2/s, and this aligns with the mean calculated in the studies by Ghoneim et al., Emad-Eldin et al., and Sevcenco et al. of 1.1 × 10−3 mm2/s [22], 1.15 ± 0.3 × 10−3 mm2/s [21], and 1.2 ± 0.81 × 10−3 mm2/s [4], respectively; each of the studies included 3 cases, as well as the median calculated by Mirka et al. of 1.028 × 10−3 mm2/s [19].
In our study, we had only three cases of renal infection, with a mean ADC value of 0.87 ± 0.06 × 10−3 mm2/s, and two masses of T.B nephritis with a mean ADC value of 0.70 × 10−3 mm2/s.
This is not in concordance with the study by Goyal et al. who reported a mean ADC value for the inflammatory lesions of 1.12 ± 0.21 × 10−3 mm2/s. The difference can be attributed to the different sample sizes (20 cases in their study compared to three cases in our study) [23].
In our study, we only had one case of collecting duct carcinoma; it had an ADC value of 0.7 × 10−3 mm2/s.
While Razek et al. had two cases with a higher mean ADC value of 1.1 ± 0.14 × 10−3 mm2/s [20]. The difference in the ADC value can be attributed to the reason that their study was conducted on a 1.5T machine, whereas our study was performed on a 3T machine.
In our study, we reached a cutoff value to differentiate between neoplastic and non-neoplastic lesions, where a cutoff ADC value of > 0.85 × 10−3 mm2/s can be used with a sensitivity of 69.35% and specificity of 60.0%, a negative predictive value of 13.6%, and a positive predictive value of 95.6% (p = 0.045), and a cutoff ADC ratio of > 0.41 × 10−3 mm2/s can be used with a sensitivity of 85.48% and specificity of 80.00%, a negative predictive value of 30.8%, and a positive predictive value of 98.1% (p = 0.003).
None of the previous studies used the ADC ratio in comparing between neoplastic and non-neoplastic lesions.
Also, Goyal et al. showed a statistical difference between inflammatory masses and RCC, however, at a much higher value of 1.41 × 10−3 mm2/s, with a sensitivity of 100% and specificity of 78.1%. The difference in the cutoff value could be attributed to the different sample sizes and the fact that our study was conducted on a 3T machine compared to a 1.5T machine in the study by Goyal et al. [23].
Although a cutoff value to differentiate benign and malignant lesions cannot be reached using ADC value, the use ADC ratio calculated a cutoff value of > 0.52 × 10−3 mm2/s to differentiate between malignant and benign lesions, this can be used with a sensitivity of 65.31% and specificity of 92.1%, a negative predictive value of 41.1%, and a positive predictive value of 97% (p = 0.038); this was not in concordance with the study by Ludwig et al. who reported an ADC ratio of < 0.89 × 10−3 mm2/s to discriminate malignant from benign lesions, with a sensitivity of 74% and specificity of 78% [24]. This can be attributed to the fact that their study was restricted to T1 hyperintense small lesions while our study had more cases diversity.
Sevcenco et al. in their study also reported no cutoff value between benign and malignant masses using ADC value [4], while Razek et al. reported an ADC value of 1.84 × 10−3 mm2/s as a threshold for differentiating malignant from benign renal tumors, with a sensitivity of 89%, a specificity of 89%, a positive predictive value of 89%, and a negative predictive value of 89% [20], and Ghoneim et al. reported an ADC value of 1.11 × 10−3 mm2/s, with a sensitivity of 60%, a specificity of 75%, a positive predictive value of 70.5%, and a negative predictive value of 65.2% [22]. This can be attributed to the difference in magnet strength (1.5 T in both studies compared to 3 T in ours.).
Our findings also contributed to suggesting a cutoff value to help differentiate ccRCC from other neoplastic lesions; a cutoff value of > 1.1 × 10−3 mm2/s had a sensitivity of 72.41% and a specificity of 95.74%.
The cutoff value using ADC ratio to differentiate between ccRCC and other masses was > 0.56 × 10−3 mm2/s, with a sensitivity of 88.89% and a specificity of 90%; thus, the ADC ratio is more sensitive with slightly less specificity.
El Serougy et al. calculated a cutoff ADC value in the differentiation between clear cell RCC and non-clear cell RCC of 1.35 × 10−3 mm2/s with a sensitivity of 91.7% and specificity of 76.7%. However, the ADC ratio demonstrates a cutoff value of 0.64 × 10−3mm2/s with a sensitivity of 91.7% and a specificity of 81.4% [18]. The difference can be attributed to the diversity of cases in our study including both benign and malignant neoplastic as well as non-neoplastic lesions compared to only RCC subtypes in their study.
De Silva et al. also calculated a cutoff value of differentiate between the different subtypes of RCC, where lesions having ADC value < 1.3705 × 10−3 mm2/s being more likely a non-clear cell RCC of low malignant potential (chrRCC or pRCC) rather than ccRCC [16]. Again, the difference can be attributed to the diversity of cases in our study including both neoplastic and non-neoplastic lesions.
Razek et al. suggested a cutoff value to differentiate RCC from other renal malignancies, not to differentiate between different subtypes, they used an ADC value of 1.15 × 10−3 mm2/s which revealed an accuracy of 72%, a sensitivity of 95%, a specificity of 50%, a positive predictive value of 66%, and a negative predictive value of 57% [20].
A cutoff value to differentiate pRCC from other neoplastic lesions was also reached in our study by ROC analysis with a value of ≤ 1 × 10−3 mm2/s with a sensitivity of 88.24% and specificity of 49.15%.
This was close to the cutoff value suggested by Sevcenco et al. (≤ 0.954 × 10−3 mm2/s), with a sensitivity of 64.3% and a specificity of 77.1% for the diagnosis of papillary RCC [4].
Using ADC ratio, a cutoff value of ≤ 0.56 × 10−3 mm2/s can be used to differentiate pRCC from other neoplastic lesions with a sensitivity of 88.67% and specificity of 50%, a negative predictive value of 92.9%, and a positive predictive value of 33.3%. This point was not addressed by other studies.
Although in the current study, a pathological correlation was performed for all neoplastic lesions as well as granulomatous lesions, the small sample size along with the paucity of some varieties, such as granulomatous lesions, TCC, and sarcoma, was a limiting factor.