Purpose:
To evaluate the diffusion properties of clear cell renal cell carcinoma (ccRCC) on magnetic resonance imaging (MRI) concerning their Fuhrman nuclear grades and sizes, and to compare the diagnostic performance of two ROI placement techniques for apparent diffusion coefficient (ADC) measurement (entire mass vs. only the darkest region of the mass).

Material and methods:
Fifty-one ccRCC were enrolled in the study and grouped into low-grade ccRCC (Fuhrman grade 1 and 2, n = 37) and high-grade ccRCC (Fuhrman grade 3 and 4, n = 14). Selective ADC (Sel-ADC) measurement was performed by placing a circular ROI that included the darkest region of the tumour on ADC map images. Extensive ADC (Ext-ADC) measurement was performed by drawing an ROI that covered the entire tumour.

Results:
The Sel-ADC value was lower in high-grade ccRCC (p = 0.019), whereas the Ext-ADC value did not show a statistically significant difference (p = 0.42). Sel-ADC value of a ≤ 1.405 mm2/s has a sensitivity, specificity, positive predictive value, negative predictive value, and accuracy value of 78.6, 72.2, 73.87, 77.13, and 75.4, respectively, to differentiate high-grade from low-grade ccRCC. The size and Fuhrman grade of the ccRCC were inversely correlated with the Sel-ADC value; however, the correlations were weak (r = –0.322, p = 0.021 and r = –0.376, p = 0.006, respectively). There was no difference between ADC values of small (≤ 4 cm) and large (> 4 cm) ccRCCs.

Conclusions:
The ADC value of the darkest region in solid part of the ccRCC may play a role in predicting the nuclear grade of ccRCC.