Estimating the Sodium Content: A Case Series of Benign and Malignant Renal Tumours Using ²³Na-MRI at 3 T

Accurate and non-invasive subtyping of localised renal tumours is an important unmet clinical challenge in uro-oncology and has significant implications for patient mortality and quality of life. Developing novel imaging methods to characterise and stratify indeterminate kidney tumours at an early stage has the potential to address this clinical challenge. Here we applied sodium MRI (²³Na-MRI) to estimate kidney tumour sodium content in a prospectively recruited case series of 10 patients (mean age ± SD 64 ± 8 years; 7:3 male:female ratio). The patients had localised renal tumours which included six renal oncocytomas (ROs), two chromophobe renal cell carcinomas (chRCCs), three clear cell RCCs (ccRCC) and one papillary RCC (pRCC). The patients underwent ²³Na-MRI at 3 T (3D sodium cones and double-angle B₁ mapping) and ¹H-MRI which included R₂* mapping and intravoxel incoherent motion (IVIM) diffusion weighted imaging (DWI). The following imaging parameters were quantified within the renal tumours and in the normal-appearing kidney parenchyma: apparent total sodium concentration (TSC); apparent ²³Na and ¹H relaxation rates (R₂*); perfusion fraction (f_p); and diffusion coefficient (D_t). ²³Na-MRI findings were correlated with conventional ¹H-MRI measures of perfusion, hypoxia and cellularity. The mean apparent TSC in ccRCC and in pRCC were 135 ± 59 mM and 81 mM, respectively. The apparent TSC was significantly higher in ROs compared to chRCCs: 162 ± 58 mM vs. 71 ± 2 mM (p < 0.05). The apparent TSC inversely correlated with ¹H-R₂* (Spearman r = -0.39, p < 0.05). In conclusion, this study showed the feasibility and potential of using ²³Na-MRI in renal tumours to probe sodium concentrations. These preliminary findings suggest a differential sodium content between benign ROs and malignant chRCCs. The inverse correlation between sodium concentration and ¹H-R₂* as a surrogate of hypoxia may indicate a biophysical relationship between the two which requires further validation in larger patient cohorts.