Background/Aims: In July 2017, the Emprint™ next-generation microwave ablation system using thermosphere technology (Covidien, Boulder, CO, USA) was approved for use in Japan. This system can produce a predictable spherical ablation zone at higher temperatures than radiofrequency ablation (RFA). The aim of the present study was to elucidate whether this new microwave thermosphere ablation (MTA) could safely improve outcome compared to RFA, which is the standard of care for small hepatocellular carcinoma (HCC). Methods: This retrospective study analyzed 513 patients with 630 HCCs (≤3 cm) who were performed by percutaneous RFA (174 patients, 214 HCCs) or MTA (339 patients, 416 HCCs) between January 2016 and March 2020. Results: Median ablation time was significantly shorter for MTA (240 seconds) than for RFA (721 seconds; P<0.001). A significant difference in 3-year local tumor progression rate was evident between the RFA group (22%) and MTA group (8%; P<0.001). Multivariable analysis revealed ablation procedure and tumor diameter as independent factors contributing to local tumor progression (MTA; P<0.001; hazard ratio, 0.565; 95% confidence interval, 0.437–0.731). In patients with primary HCC, a significant difference in overall survival was evident (RFA vs. MTA, 3-year, 77% vs. 95%, P=0.029). Ablation procedure and Child-Pugh score were independent factors contributing to survival. The total complication rate was significantly lower for MTA (8%) than for RFA (14%, P<0.05), particularly for bile duct injury (3% vs. 9%, respectively; P<0.05). Conclusions Next-generation MTA for small HCC could provide safer, more curative treatment in a shorter ablation time than RFA.

Background/Aims: In July 2017, the Emprint™ next-generation microwave ablation system using thermosphere technology (Covidien, Boulder, CO, USA) was approved for use in Japan. This system can produce a predictable spherical ablation zone at higher temperatures than radiofrequency ablation (RFA). The aim of the present study was to elucidate whether this new microwave thermosphere ablation (MTA) could safely improve outcome compared to RFA, which is the standard of care for small hepatocellular carcinoma (HCC). Methods: This retrospective study analyzed 513 patients with 630 HCCs (≤3 cm) who were performed by percutaneous RFA (174 patients, 214 HCCs) or MTA (339 patients, 416 HCCs) between January 2016 and March 2020. Results: Median ablation time was significantly shorter for MTA (240 seconds) than for RFA (721 seconds; P<0.001). A significant difference in 3-year local tumor progression rate was evident between the RFA group (22%) and MTA group (8%; P<0.001). Multivariable analysis revealed ablation procedure and tumor diameter as independent factors contributing to local tumor progression (MTA; P<0.001; hazard ratio, 0.565; 95% confidence interval, 0.437–0.731). In patients with primary HCC, a significant difference in overall survival was evident (RFA vs. MTA, 3-year, 77% vs. 95%, P=0.029). Ablation procedure and Child-Pugh score were independent factors contributing to survival. The total complication rate was significantly lower for MTA (8%) than for RFA (14%, P<0.05), particularly for bile duct injury (3% vs. 9%, respectively; P<0.05). Conclusions Next-generation MTA for small HCC could provide safer, more curative treatment in a shorter ablation time than RFA.