Tissue shrinkage in microwave thermal ablation: comparison of three commercial devices

Purpose: To evaluate, characterise and compare the extent of tissue shrinkage induced from three different commercial microwave ablation devices, and to elucidate the mechanism behind the distinctive performances obtained. Materials and methods: Microwave ablation (N = 152) was conducted with three different commercial devices on cubes of ex vivo liver (10–40 ± 2 mm/side) embedded in agar phantoms. 50–60 W was applied for 1–10 min duration. Pre- and post-ablation dimensions of the samples, as well as the extent of carbonisation and coagulation were measured and correlated. ANOVA was performed to evaluate statistical significance. Results: For all devices, logarithmic correlations with time were observed for both tissue shrinkage (R2= 0.84–1.00) and induced carbonisation (R2= 0.73–0.99) radially to the antenna axis. Along the longitudinal axis of the antenna, for two of the devices shrinkage did not appreciably change with time (p > 0.05), yet carbonisation increased linearly (R2= 0.57–0.94). For the third fully internally-cooled device, both carbonisation and shrinkage showed logarithmic trends (R2= 0.85–0.98 and R2= 0.78–0.94, respectively) based upon delayed carbonisation appearing only 5 min into ablation and onward. For all devices, non-uniform shrinkage was noted within the coagulated area increasing from the boundary of the ablated area (14%) to the limit of carbonisation (39%) in a linear fashion (R2= 0.88) Conclusions: Microwave ablation device construction can alter the extent of post-ablation coagulation and tissue shrinkage. Given that tissue shrinkage in the coagulated area shows non-uniform behaviour, observed differences can be attributed in part to the applicator cooling system that alters the ablation temperature profile.