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

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.

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.

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.