Abstract: Objective  To evaluate the diagnostic value of apparent diffusion coefficient（ADC） histogram for the classification of clear cell renal cell carcinoma（CCRCC）. Methods A retrospective analysis was performed on 34 patients with CCRCC by conventional MRI and diffusion-weighted imaging（DWI）. According to pathological findings，the patients with CCRCC were divided into low grade group（19 cases） and high grade group（15 cases）. The ADC histogram parameters of different grades of CCRCC were measured and recorded，including maximum，minimum，mean，median，skewness and kurtosis. Independent sample t test or Mann-Whitney U test were used to compare the difference of ADC parameters. Results The minimum，mean and median of ADC histogram of low grade CCRCC were higher than that of high grade CCRCC ［（1.09±0.23） ×10^-3  mm²/s vs （0.81±0.09）×10^-3 mm²/s 、（1.86±0.27）×10^-3 mm²/s  vs （1.58±0.14）×10^-3 mm²/s and （1.85±0.23）×10^-3 mm²/s  vs （1.59±0.12）×10^-3 mm²/s ，P＜0.05］. Moreover，the low grade CCRCC was dominated by negative skewness. There were no significant difference in the maximum and peak values between the two groups （P>0.05）. In the whole tumor ADC histogram，the diagnostic efficiency of the ADC minimum was the highest，the area under the curve （AUC） was 0.895，and the corresponding optimal diagnostic threshold index，Youden index，sensitivity and specificity were 0.898×10^-3 mm²/s，0.761，86.67% and 89.47%，respectively. Conclusions The ADC histogram helps to identify different grade of CCRCC，and the ADC minimum has the best diagnostic efficiency.

Key words: Clear cell renal cell carcinoma, Magnetic resonance imaging, ADC histogram, Pathological grading, ROC curve