Percutaneous Microwave versus Radiofrequency Ablation of Colorectal Liver Metastases: Ablation with Clear Margins (AO) Provides the Best Local Tumor Control

Purpose: To identify and compare predictors of local tumor progression (LTP)-free survival (LTPFS) after radiofrequency (RF) ablation and microwave (MW) ablation of colorectal liver metastases (CLMs). Materials and Methods: This is a retrospective review of CLMs ablated from November 2009 to April 2015 (110 patients). Margins were measured on contrast-enhanced computed tomography (CT) 6 weeks after ablation. Clinical and technical predictors of LTPFS were assessed using a competing risk model adjusted for clustering. Results: Technique effectiveness (complete ablation) was 93% (79/85) for RF ablation and 97% (58/60) for MW ablation (P =.47). The median follow-up period was significantly longer for RF ablation than for MW ablation (56 months vs. 29 months) (P <.001). There was no difference in the local tumor progression (LTP) rates between RF ablation and MW ablation (P = 0.84). Significant predictors of shorter LTPFS for RF ablation on univariate analysis were ablation margins 5 mm or smaller (P <.001) (hazard ratio [HR]: 14.6; 95% confidence interval [CI]: 5.2-40.9) and perivascular tumors (P =.021) (HR: 2.2; 95% CI: 1.1-4.3); both retained significance on multivariate analysis. Significant predictors of shorter LTPFS on univariate analysis for MW ablation were ablation margins 5 mm or smaller (P <.001) (subhazard ratio: 11.6; 95% CI: 3.1-42.7) and no history of prior liver resection (P <.013) (HR: 3.2; 95%: 1,3-7.8); both retained significance on multivariate analysis. There was no LTP for tumors ablated with margins over 10 mm (median LTPFS: not reached). Perivascular tumors were not predictive for MW ablation (P =.43). Conclusions: Regardless of the thermal ablation modality used, margins larger than 5 mm are critical for local tumor control, with no LTP noted for margins over 10 mm. Unlike RF ablation, the efficiency of MW ablation was not affected for perivascular tumors.