The study, published as a short communication in CVIR Oncology, analyzed isotherm charts from three FDA-certified cryoablation systems: two argon-based Joule–Thomson platforms, Boston Scientific’s System A and Endocare/Varian’s System B, and a liquid nitrogen system, IceCure Medical’s System C. All measurements had been performed in ultrasonic gels under standardized conditions, with 216 data points covering single needles and clustered needle configurations. The authors defined the LIR as the width of the −20 °C or −40 °C isotherm divided by the width of the visible ice ball, expressed as a percentage. A higher ratio means a larger share of the frozen zone is cold enough to guarantee cell death.

The physics behind the metric is rooted in how cells die under extreme cold. Below −40 °C, intracellular ice crystals form and physically shred cellular structures, triggering immediate necrosis. At temperatures as high as −20 °C, secondary mechanisms come into play, including osmotic stress and ischemic injury that starve tissue of blood supply. Because these lethal zones cannot be seen during a procedure, operators currently rely on the visible ice ball diameter and probe spacing as proxies, a practice the authors argue leaves a dangerous margin of uncertainty that can lead to inadequate tumor coverage and local recurrence.

The results were striking. Across all systems, isotherm widths correlated strongly and linearly with ice ball size, with correlation coefficients ranging from 0.880 to 0.999. But the proportion of lethal territory within the ice ball differed dramatically by technology. The liquid nitrogen System C achieved the highest ratios, with the −20 °C isotherm spanning 80.6 percent of the ice ball on average and the −40 °C isotherm covering 60.3 percent. The argon-based Systems A and B trailed significantly, both reaching about 69.6 percent at −20 °C, while their −40 °C ratios fell to 44 percent and 51.4 percent respectively, differences the authors report as statistically significant.

The explanation lies in the cooling technologies themselves. Joule–Thomson systems drop their temperature by expanding high-pressure argon gas, at 3200 PSI, through a throttling orifice, a process that supports multiple cryoprobes working in parallel. Liquid nitrogen systems operate at far lower pressures, around 100 PSI, but reach colder temperatures faster. The trade-off, the study notes, is that liquid nitrogen systems may struggle with very large ice balls, where peripheral warming erodes the lethal core, while clustered cryoprobes enhance energy removal but demand more complex insertion techniques.

That trade-off produced the study’s most clinically actionable finding: a threshold effect at roughly 45 millimeters. Linear regression analysis showed that for ice balls smaller than about 45 mm in width, a single liquid nitrogen needle delivers larger lethal isotherms than any single argon needle. Beyond that threshold, the regression lines intersect and the picture reverses: clusters of Joule–Thomson needles produce bigger lethal zones, with an almost perfect correlation of 0.998 to 0.999. Notably, when System A’s needles were clustered, their LIRs rose to 79.3 percent at −20 °C and 60.9 percent at −40 °C, statistically indistinguishable from the liquid nitrogen system’s single-needle performance.

For interventional radiologists, the implications could reshape device selection and preoperative planning. If a tumor is expected to generate an ice ball under 45 mm, the data suggest a single liquid nitrogen probe may be the more efficient choice. For larger targets, clustered argon probes offer superior coverage. Because the LIR quantifies the relationship between what is visible on screen and the effective ablation zone, it could allow operators to estimate the required number of probes and their optimal spacing before the first incision, potentially shortening procedures and reducing complications. The finding that the −20 °C isotherm can encompass up to 80 percent of the ice ball even hints that real-time thermometry might become unnecessary if LIR values reliably predict ablation zones.

The metric also fills a gap in how the field compares competing technologies. Until now, manufacturers’ claims rested on raw ice ball dimensions and probe spacing, figures that say nothing about the lethal fraction of the frozen volume. By normalizing lethal isotherm widths to the visible freezing front, the LIR offers a common currency for benchmarking cryoprobes, needle gauges, and chamber lengths across vendors. The authors suggest it could become a standardized yardstick for regulators, buyers, and clinicians alike, in the same way that metrics like specific absorption rate structure comparisons in other energy-based therapies.

The study is not without caveats, and the authors are candid about them. All isotherm data came from ultrasonic gels, not living tissue. Biological tissue brings perfusion, heterogeneity, and variable thermal conductivity, all of which alter heat transfer in vivo and could shift LIR values in real patients. Testing conditions also varied somewhat between manufacturers, with ambient temperatures of 20 to 23 °C, probe depths of 2 to 3 cm, and freeze durations of 10 to 15 minutes, and although FDA certification imposed standardized protocols, cross-vendor methodological differences could introduce bias. The corresponding author has consulted for all three companies whose systems were analyzed, a potential conflict the authors state was mitigated through standardized data collection and analysis.

Future work, the team argues, should validate LIR against histologically confirmed ablation zones in animal and human tissue, run prospective trials comparing procedures planned with and without the metric, and build computational models that fold tissue-specific properties into the calculation. If those steps succeed, the Lethal Isotherm Ratio could transform a field that has long judged its instruments by the size of the ice they make rather than the death they deliver. For now, the message is clear: not all ice balls are created equal, and the difference between 44 percent and 80 percent lethal coverage may be the difference between a cured tumor and one that comes back.

Subject of Research: A standardized metric, the lethal isotherm ratio, for comparing cryoablation needles and systems based on lethal temperature zones within visible ice balls.

Article Title: Lethal isotherm ratio to standardize the comparison of cryoablation needles and systems

Article References: Cornelis, A. A., Bodard, S., & Cornelis, F. H. (2025). Lethal isotherm ratio to standardize the comparison of cryoablation needles and systems. CVIR Oncology, 1(1), Article 3. https://doi.org/10.1007/s44343-025-00004-4

Image Credits: AI Generated

DOI: 10.1007/s44343-025-00004-4

Keywords: cryoablation, lethal isotherm ratio, cryoprobes, liquid nitrogen, Joule-Thomson, interventional radiology, tumor ablation, isotherms, ice ball, device comparison, oncology, Lethal