Quantitative sodium-MRI detects differential sodium content in benign vs. malignant oncocytic renal tumours

Background: Accurate non-invasive subtyping of localised kidney tumours is an unmet clinical question in uro-oncology. Differentiation of benign renal oncocytomas (RO) from malignant chromophobe renal cell carcinomas (chRCC) is not possible using conventional imaging. Despite the importance of renal function for sodium regulation, little is known about sodium handling in kidney tumours. Purpose: Here we used non-invasive sodium MRI (23Na-MRI) to quantify sodium concentration and relaxation dynamics across a range of different kidney tumour subtypes and have correlated these findings with imaging surrogates for perfusion, hypoxia, and cellularity. Materials and Methods: Between January and April 2023, patients with localised renal masses were prospectively recruited and underwent 23Na and proton (1H) MRI at 3T to acquire 3D maps of B1, total sodium concentration (TSC), proton and sodium relaxation rates (R2*), and diffusion weighted imaging (DWI). Statistical analysis included comparison and correlation of quantified imaging parameters across kidney tumour subtypes. Results: Ten patients were included in the final analysis (mean age ± S.D. = 64 ± 8 years; 7:3 male:female ratio) encompassing seven ROs, two chRCCs, two clear cell RCCs (ccRCC), and one papillary RCC (pRCC). The TSC was significantly higher in the ROs compared to the chRCCs: 162 ± 58 mM vs. 71 ± 2 mM (P < 0.05). The mean TSC in ccRCC was 135 ± 59 mM, and 81 mM in pRCC. The 23Na-derived and 1H-derived R2* values showed a weak correlation (Spearman r = 0.17; P = 0.50). There was a significant inverse correlation between TSC and 1H-R2* (Spearman r = -0.39, P < 0.05), but TSC was independent of the DWI-derived imaging parameters. Conclusion: 23Na-MRI detected markedly different sodium concentrations within benign ROs and malignant chRCCs. In addition, the sodium signal inversely correlated with 1H-R2* as a surrogate for hypoxia. Therefore we have shown the feasibility and potential of 23Na-MRI for future research in renal tumours.