Unenhanced CT as an Alternative to Contrast-Enhanced CT in Evaluating Renal Cryoablation Zones

Background Advances in imaging technology and the increased use of abdominal imaging have led to a rise in renal cell carcinoma (RCC) detection. While surgery remains the primary treatment for small RCCs, minimally invasive procedures like cryoablation are gaining popularity, particularly for patients with comorbidities or renal dysfunction. CT-guided cryoablation offers advantages, including high spatial resolution and real-time visualization during the procedure. Post-procedure imaging is essential for assessing treatment success, with contrast-enhanced CT (CE-CT) typically considered vital. However, many patients, especially older individuals, have renal dysfunction that limits the use of contrast agents. In such cases, unenhanced CT (UE-CT) presents a viable alternative for post-procedural evaluation. This study explored the effectiveness of UE-CT in assessing cryoablation zones as a substitute for CE-CT. Materials and Methods This retrospective study included 54 patients (58 tumors) who underwent cryoablation at a single institution between 2014 and 2024. Only patients with available early follow-up CT (within three days post-cryoablation) and subsequent follow-up were included. Tumors marked with lipiodol prior to cryoablation and cases requiring transcatheter arterial embolization due to extravasation immediately after cryoablation were excluded. Percutaneous renal cryoablation was performed under CT fluoroscopy, and the ablation zone was assessed using a 64-channel multi-slice CT scanner. UE-CT was conducted before the procedure, followed by both UE-CT and CE-CT within three days after cryoablation. CT attenuation values were measured for pre-procedure UE-CT (kidneys and tumor), post-procedure UE-CT (kidneys, cryoablation zone, and tumor), and post-procedure CE-CT (kidneys, cryoablation zone, and tumor). Tumor volumes in the post-procedure regions were evaluated on both UE-CT and CE-CT. Statistical analyses were performed using Wilcoxon's signed-rank test and Spearman's rank correlation coefficient, with interobserver agreement determined by the intraclass correlation coefficient. Results The median tumor diameter was 1.56 cm (IQR: 1.33-2.00 cm). On UE-CT, the cryoablation zone exhibited high attenuation, while it showed low attenuation on CE-CT. The median attenuation values of the kidneys on UE-CT before and after cryoablation were not significantly different (33.6 Hounsfield unit (HU) vs. 34.3 HU, P = 0.17). However, on CE-CT, the median attenuation values of normal kidneys and the cryoablation zone significantly differed (171.7 HU vs. 55.7 HU, P < 0.0001). Similarly, on UE-CT, there was a significant difference in the median attenuation values between normal kidneys and the cryoablation zone (34.3 HU vs. 47.4 HU, P < 0.0001). The median renal volumes of the unenhanced regions on CE-CT and those with attenuation changes on UE-CT were not significantly different (26.52 cm³ vs. 28.83 cm³, P = 0.86). These values showed a strong correlation (r = 0.95; 95% CI: 0.91-0.97). Conclusions This study showed that UE-CT can reliably estimate the ablation zone in RCC patients post-cryoablation. While the contrast between the ablation zone and normal renal parenchyma was lower on UE-CT compared to CE-CT, the ablation zone was still detectable and highly correlated with CE-CT results. Further research with larger sample sizes is needed to validate the clinical utility of UE-CT and assess the reproducibility of these findings.