Introduction:
Renal cell carcinoma (RCC) is the most common malignant epithelial tumour of the kidney, accounting for 85–90% of all solid renal tumours in adults and comprising 1–3% of all malignant visceral neoplasms. Recently, computed tomography (CT) has been considered the ‘gold standard’ in the diagnostic imaging of RCC; however, the use of CT is always associated with radiation exposure and consequently carries a significant increase in the risk of malignancy for patients with neoplastic processes. In recent years, magnetic resonance imaging (MRI) is increasingly attracting the attention of clinicians as the method of choice for the diagnosis and staging of RCC, due to several advantages over CT.

Material and Methods:
The study involved 62 adult patients with a pathologically verified clear cell subtype of the renal cell carcinoma (ccRCC) and 15 healthy volunteers. All patients underwent renal MRI which included diffusion-weighted imaging (DWI) with subsequent apparent diffusion coefficient (ADC) measurement.

Results:
A significant difference was observed in the mean ADC value of the normal renal parenchyma and ccRCC – 1.82 ± 0.16 × 10– 3 mm2/s vs 2.15 ± 0.12 × 10– 3 mm2/s, respectively (р < 0,05). Additionally, statistically reliable differences in ADC values in patients with high and low ccRCC grades were obtained: in patients with the I grade, the mean ADC value was 1.92 ± 0.12 × 10– 3 mm2/s, in patients with the II grade, this value was 1.84 ± 0.14 × 10– 3 mm2/s, in patients with the III grade, the mean ADC value was 1.79 ± 0.12 × 10– 3 mm2/s, and in patients with the IV grade of nuclear polymorphism the mean ADC value was 1.72 ± 0.11 × 10– 3 mm2/s (p <0.05).

Conclusions:
The data obtained in the survey show a significant restriction in the diffusion of hydrogen molecules in tissues of ccRCC, compared to the healthy renal parenchyma due to the tumour’s greater density. A statistically significant difference was observed in the mean ADC values of ccRCC tumours with different Fuhrman nuclear grades: tumours with a low grade of differentiation demonstrated higher mean ADC values compared to highly differentiated tumours. Application of DWI modality of MR imaging with ADC calculation allows to obtain valuable information that is vital for the diagnosis of ccRCC and differentiation of its degree of malignancy.