Assessment of Cryosurgical Device Performance Using a 3D Tissue-Engineered Cancer Model

Abstract

As the clinical use of cryoablation for the treatment of cancer has increased, so too has the need for knowledge on the dynamic environment within the frozen mass created by a cryoprobe. While a number of factors exist, an understanding of the iceball size, critical isotherm distribution/penetration, and the resultant lethal zone created by a cryoprobe are critical for clinical application. To this end, cryoprobe performance is typically characterized based on the iceball size and temperature penetration in phantom gel models. Although informative, these models do not provide information as to the impact of heat input from surrounding tissue nor give any information on the ablative zone created. As such, we evaluated the use of a tissue-engineered tumor model (TEM) to assess cryoprobe performance including iceball size, real-time thermal profile distribution, and resultant ablative zone. Studies were conducted using an Endocare V-probe cryoprobe, with a 10/5/10 double freeze–thaw protocol using prostate and renal cancer TEMs. The data demonstrate the generation of a 33- to 38-cm3 frozen mass with the V-Probe cryoprobe following the double freeze of which ∼12.7 and 6.5 cm3 was at or below −20°C and −40°C, respectively. Analysis of ablation zone using fluorescence microscopy 24 hours postthaw demonstrated that the internal ∼40% of the frozen mass was completely ablated, whereas in the periphery of the iceball (outer 1 cm region), a gradient of partial to minimal destruction was observed. These findings correlated well with clinical reports on renal and prostate cancer cryoablation. Overall, this study demonstrates that TEMs provide an effective model for a more complete characterization of cryoablation device performance. The data demonstrate that while the overall iceball size generated in the TEM was consistent with published reports from phantom models, the integration of an external heat load, circulation, and cellular components more closely reflect an in vivo setting and the impact of penetration of the critical (−20°C and −40°C) isotherms into the tissue. This is important as it is well appreciated in clinical practice that the heat load of a tissue, cryoprobe proximity to vasculature, and so on, can impact outcome. The TEM model provides a means of characterizing the impact on ablative dose delivery allowing for a better understanding of probe performance and potential impact on ablative outcome.

Evaluation of a new epicardial cryoablation system for the treatment of Cardiac Tachyarrhythmias

“…Isothermal profiling of the frozen mass created by the epicardial cryoprobes revealed penetration of the -20°C, -30°C and -40°C isotherms to a diameter of 2.2 cm, 2.1 cm and 1.9 cm in a 2.3 cm iceball following a 3 minute freeze resulting in ~90% of the frozen volume below -20°C, 80% below -30°C and >65% below -40°C for the various SCN epicardial cryoprobe configurations (Table 4). This significantly outperformed previous reports on an argon cryodevice which was reported to yield ~35% and ~20% of the iceball being below -20°C and -40°C, respectively [53,61]. Other studies focusing on analysis of the -30°C isotherm have reported that an argon based cryoprobe produces ~25% of the frozen mass under -30°C under similar conditions [54,55].…”

“…It is important to understand the model utilized and recognize the benefits and limitations in order to frame the results. Several studies have shown that the distribution of the -20°C and -40°C critical isotherms is often restricted to the center of the frozen mass (closest to the cryoprobe), thereby comprising less than 35% and 20% of the frozen mass, respectively (53)(54)(55)58,61,67) The typical freeze times reported in these and other studies range from 5 to 10 minutes which are applied in multiple (repeat) freeze/ thaw scenarios. While limited, these data provide a reference point for comparison of modern technologies.…”

“…As such, the use of ex vivo models to assess the performance characteristics of thermal ablation devices, such as cryoablation devices and probes, is important. Serving as a comparative standard, studies have also shown that the depth of penetration of the critical -20°C and -40°C isotherms is an important indicator of ablative success both ex vivo and in vitro (24,(53)(54)(55)59,62,65,66) While serving as a good comparative standard, the model in which the isotherm penetration is assessed can significantly impact results. For example, at a given temperature, agarose and ultrasound gel models provide for more favorable isotherm penetration than water baths.…”

“…The probe height was adjusted such that half of the ablation segment probe (1 mm for HRS-1 and 1.7 mm for HRS-2) was submerged, and a 2 min freeze was performed. Following the freeze procedure, ice ball diameter was measured at the midpoint of the freeze zone [53] The thermocouples and mandrill were situated such that temperature recordings were taken at 1 mm, 6 mm, 11 mm 13.5 mm and 16 mm extending radially from the cryoprobe surface.…”

“…Knowledge of the expected performance of a cryosurgical probe is critical to obtain optimal outcomes [53][54][55]. With good contact with the tissue to insure efficient heat transfer, a probe temperature of about -80°C, cooled by pressurized argon or nitrous oxide, takes ~5 minutes to freeze tissues to a depth of about 0.5 cm.…”