SIR 2024
Interventional Oncology
Scott M. Thompson, MD., Ph.D (he/him/his)
Associate Professor of Radiology
Mayo Clinic Rochester
Financial relationships: Full list of relationships is listed on the CME information page.
Thomas Atwell, MD (he/him/his)
Professor of Radiology
Mayo Clinic Rochester
Disclosure information not submitted.
Daniel Adamo, M.D.
Assistant Professor of Radiology
Mayo Clinic
Disclosure information not submitted.
Christopher Favazza, Ph.D
Assistant Professor of Medical Physics
Mayo Clinic
Financial relationships: Full list of relationships is listed on the CME information page.
Cryoablation systems offer flexibility to adjust both freezing and thawing functions; however, vendors provide performance data across a single (freeze only) operation mode. Here, we developed a robust, 3D-printed phantom set-up that enables repeatable cryoprobe performance measurements across experimental variables and used it to investigate the effect of different modes of cryoprobe operation on iceball size, thermal gradients and thaw rates.
Materials and methods: Two IceFORCE cryoprobes were positioned 2 cm apart and inserted into a tissue mimicking gel phantom1 at room temperature (~21°C). Ten-minute freeze cycles were performed at 25, 50, and 100% freeze settings. The experimental setup was brought back to the original conditions between experiments. Using 8 fiber optic sensors, temperatures were sampled throughout the volume every 1s. Two sensors were positioned 1 and 4 mm off the cryoprobe tip and 6 sensors were positioned at systematic locations between the two cryoprobes. CT images were acquired at 20s intervals to track iceball size and confirm sensor position. Thermal gradients and maximum potentially lethal zones (defined as T< -20°C) were measured. In a set of thaw experiments, a similar phantom set-up was utilized to evaluate thaw rates achievable with the available 3 options, one using He gas and two using proprietary resistive heating—iThaw and FastThaw.
Results:
The iceball and gradients after a 10-minute freeze cycle varied significantly between the 3 freeze settings; maximum radial distances from the probe shaft for iceball size/lethality zone were: 15/8, 22/16 and 27/19 mm for 25, 50 and 100% freeze settings, respectively. Notably, midplane between the two probes didn’t reach -25°C when freezing at 25%, despite the full coalescence of ice from both probes. Temperatures 1 mm past the probe tip were -23, -32 and -37°C for the 3 freeze settings, respectively, and increased sharply farther from the tip. During the thaw experiments, warming rates of the ice to 0°C were similar across modes of operation. iThaw and He thaw yielded very similar melting rates; whereas, the FastThaw mechanism melted the internal volume of the iceball noticeably faster ( >2min).
Conclusion: Cryoablation vendors provide options to tailor ablation parameters to better meet treatment requirements and sculpt the ice to avoid sensitive organs. Our investigation demonstrated that using a low freezing rate setting does not result in lethal temperatures in the center of the treatment zone created by 2 probes positioned 2 cm apart, despite generating a sizeable iceball (38 x 20 mm, midplane).