A model for the time-dependent thermal distribution within an iceball surrounding a cryoprobe

A three-dimensional lattice-free agent-based model of intracellular ice formation and propagation and intercellular mechanics in liver tissues

A successful cryopreservation of tissues and organs is crucial for medical procedures and drug development acceleration. However, there are only a few instances of successful tissue cryopreservation. One of the main obstacles to successful cryopreservation is intracellular ice damage. Understanding how ice spreads can accelerate protocol development and enable model-based decision-making. Previous models of intracellular ice formation in individual cells have been extended to one-cell-wide arrays to establish the theory of intercellular ice propagation in tissues. The current lattice-based ice propagation models do not account for intercellular forces resulting from cell solidification, which could lead to mechanical disruption of tissue structures during freezing. Moreover, these models have not been expanded to include more realistic tissue architectures. In this article, we discuss the development and validation of a stochastic model for the formation and propagation of ice in small tissues using lattice-free agent-based model. We have improved the existing model by incorporating the mechanical effects of water crystallization within cells. Using information from previous research, we have also created a new model that accounts for ice growth in tissue slabs, spheroids and hepatocyte discs. Our model demonstrates that individual cell freezing can have mechanical consequences and is consistent with earlier findings.

Infrared thermal imaging controls freezing and warming in skin cryoablation

The purpose of this study was to assess the possibilities of intraoperative control of the current parameters of frozen biological tissues in the cryoablation area, including the instant location of primary necrosis isotherm, based on the dynamics of thermal fields on skin surface. Cryoablation of skin was performed in 30 rats with exposure durations of 0.5, 1 and 2 min. The contact cryoprobe actively cooled with liquid nitrogen was used. The dynamics of animal's skin thermal field during freeze/thaw cycle was quantitatively controlled by the original infrared camera with an extended range of measurable temperatures. The obtained by us ratio of the maximal diameters of primary necrosis and ice spots was 0.64 ± 0.03 for cryoexposure durations of 0.5 and 1 min. During thawing, a quasi-stable stage was observed both in the dynamics of ice spot diameters and their temperature distribution. The effect is presumably associated with structural rearrangements of ice in the frozen tissue volume. The results indicate that thermal imaging can be effectively used for quantitative control of freezing and warming of biological tissues in vivo, including current control of the position of necrotic and cryoscopic isotherms, distortion of their thermal symmetry, thermal response of other skin areas, etc.

Experimental Investigation on Thermal Conductivity and Thermal Diffusivity of Ex-Vivo Bovine Liver from Room Temperature down to -60 °C

Ex vivo animal tissues (e.g., bovine liver) as well as water-agar gel are commonly used to simulate both experimentally and numerically the response of human tissues to cryoablation treatments. Data on the low temperature thermal properties of bovine liver are difficult to find in the literature and very often are not provided for the whole temperature range of interest. This article presents the thermal conductivity and thermal diffusivity measurements performed on ex-vivo bovine liver samples using the transient plane source method. Regression coefficients are provided to determine these properties in different temperature ranges except for the phase transition during which no results were obtained, which suggests an ad hoc calorimetric analysis. A quick procedure is also suggested to determine the water mass fraction in the tissue. Moreover, an attempt to estimate the liver density in the frozen state using measurements performed solely at room temperature is also presented. The measured thermal conductivity and thermal diffusivity values are compared with data reported in literature highlighting a spread up to 40%. Moreover, it emerges that water-agar gel usually made with 2% by weight of agar does not show the same thermal properties as the bovine liver.

Optimization of prostatic cryosurgery with multi-cryoprobe based on refrigerant flow

Cryosurgery is a promising novel minimally invasive surgical technique to eradicate carcinoma and non-carcinoma tissues by freezing. In this research, we applied a transient 3D two-phase refrigerant flow model inside the LN₂ boiling chamber as well as a bioheat transfer model inside the tissues to evaluate the optimized ablation outcome during prostatic cryosurgery. For the evaluation of the scenarios, a defect function was used that considers non-ablated target tissue (prostate/cancer tissue) as well as ablated healthy tissue, in which the ablated tissue was evaluated using a temperature threshold. Three different configurations using three LN₂ cryoprobes were analyzed during the modeling study, and the best configuration with the three LN₂ cryoprobes positioned isoscelesly was found. For this configuration, temperature distributions and temperature profiles at specific points within the tissue were investigated numerically. Owing to its low computational cost, the 3D coupled model has an advantage in accurate modeling cryosurgery for curing numerous diseases.

Cryosurgery for Breast Cancer

As the majority of breast cancers present as small non-palpable lesions, alternatives for surgical lumpectomy come into consideration. Breast tumor ablation without surgical excision may be a less morbid procedure without sacrificing cancer control. Cryosurgery is one of several ablative options for the treatment of small unifocal breast cancer. The potential advantages include avoidance of a surgical procedure, less overall discomfort, improved cosmesis, quicker recovery and the prospect of overall cost benefits. Clinical experience in 29 patients is reviewed demonstrating effectiveness in properly chosen patients. A comparison of ablation methods is discussed. Finally, future research of the role of cryosurgery in the management of breast cancer is described.

Liver Tumor Cryoablation: A Commentary on the Need of Improved Procedural Monitoring

Cryoablation is a method used for in situ destruction of liver tumors not eligible for surgical resection. Local recurrences following such treatment have been reported at rates of 5–44%. Insufficient procedural monitoring of the ablation is one plausible explanation for these recurrences. The cryoablative procedure is usually monitored by ultrasonography, but acoustic shadowing and loss of signals, compromise visualisation of the cryolesion circumference. Other monitoring modalities such as computer tomography and invasive methods like the use of thermocouples and impedance measurements have also been studied, but are not in common clinical use as single monitoring modalities. Thermodynamic conditions assumed adequate for tumor eradication are likely to occur only in parts of the cryolesion. This tumoricidal part of the cryolesion is not adequately depicted using any of these modalities.

Magnetic resonance imaging (MRI) provides a clear delineation of the cryolesion circumference. Noninvasive temperature measurements assisted by MRI indicate which parts of the cryolesion that may be subject to complete necrosis. In this article MRI monitored cryoablation of liver tumors is discussed. Improved peroperative monitoring as offered by MRI may reduce the rates of local recurrences after treatment, but further technological improvements are required.

Methods for characterizing convective cryoprobe heat transfer in ultrasound gel phantoms

While cryosurgery has proven capable in treating of a variety of conditions, it has met with some resistance among physicians, in part due to shortcomings in the ability to predict treatment outcomes. Here we attempt to address several key issues related to predictive modeling by demonstrating methods for accurately characterizing heat transfer from cryoprobes, report temperature dependent thermal properties for ultrasound gel (a convenient tissue phantom) down to cryogenic temperatures, and demonstrate the ability of convective exchange heat transfer boundary conditions to accurately describe freezing in the case of single and multiple interacting cryoprobe(s). Temperature dependent changes in the specific heat and thermal conductivity for ultrasound gel are reported down to -150 °C for the first time here and these data were used to accurately describe freezing in ultrasound gel in subsequent modeling. Freezing around a single and two interacting cryoprobe(s) was characterized in the ultrasound gel phantom by mapping the temperature in and around the "iceball" with carefully placed thermocouple arrays. These experimental data were fit with finite-element modeling in COMSOL Multiphysics, which was used to investigate the sensitivity and effectiveness of convective boundary conditions in describing heat transfer from the cryoprobes. Heat transfer at the probe tip was described in terms of a convective coefficient and the cryogen temperature. While model accuracy depended strongly on spatial (i.e., along the exchange surface) variation in the convective coefficient, it was much less sensitive to spatial and transient variations in the cryogen temperature parameter. The optimized fit, convective exchange conditions for the single-probe case also provided close agreement with the experimental data for the case of two interacting cryoprobes, suggesting that this basic characterization and modeling approach can be extended to accurately describe more complicated, multiprobe freezing geometries. Accurately characterizing cryoprobe behavior in phantoms requires detailed knowledge of the freezing medium's properties throughout the range of expected temperatures and an appropriate description of the heat transfer across the probe's exchange surfaces. Here we demonstrate that convective exchange boundary conditions provide an accurate and versatile description of heat transfer from cryoprobes, offering potential advantages over the traditional constant surface heat flux and constant surface temperature descriptions. In addition, although this study was conducted on Joule-Thomson type cryoprobes, the general methodologies should extend to any probe that is based on convective exchange with a cryogenic fluid.

HEATING EFFECT OF THERMALLY SIGNIFICANT BLOOD VESSELS IN PERFUSED TUMOR TISSUE DURING CRYOSURGERY

The purpose of this work is to evaluate the heating effect of thermally significant blood vessels (TSBVs) during cryosurgery process. A theoretical model is developed to describe the heat transfer of perfused tissue containing a 6 mm-radius tumor and a countercurrent artery–vein pair in the vicinity of the tumor. The novelty of the model is that both the Pennes bio-heat transfer equation and the energy transport equation are used to govern the heat transfer of perfused tumor tissue and the heat convection and diffusion of the blood flow. The explicit finite different method is used to solve the transient equations with the second boundary condition set for the boundary of both the blood vessels and the tissue. The results indicate that the heating effect of the blood on the tissue surrounding the countercurrent artery–vein pair may cause parts of the tumor to be insufficiently frozen for a pre-designed cryosurgery protocols, and which may lead to regeneration of the tumor cells. A quite important issue has been raised by this study, i.e., the cryosurgery protocols should be carefully designed with full attention being paid to the heating effect of TSBVs in case of a countercurrent artery–vein pair in the vicinity of the tumor.

Nanoparticle preconditioning for enhanced thermal therapies in cancer

Nanoparticles show tremendous promise in the safe and effective delivery of molecular adjuvants to enhance local cancer therapy. One important form of local cancer treatment that suffers from local recurrence and distant metastases is thermal therapy. In this article, we review a new concept involving the use of nanoparticle-delivered adjuvants to 'precondition' or alter the vascular and immunological biology of the tumor to enhance its susceptibility to thermal therapy. To this end, a number of opportunities to combine nanoparticles with vascular and immunologically active agents are reviewed. One specific example of preconditioning involves a gold nanoparticle tagged with a vascular targeting agent (i.e., TNF-α). This nanoparticle embodiment demonstrates preconditioning through a dramatic reduction in tumor blood flow and induction of vascular damage, which recruits a strong and sustained inflammatory infiltrate in the tumor. The ability of this nanoparticle preconditioning to enhance subsequent heat or cold thermal therapy in a variety of tumor models is reviewed. Finally, the potential for future clinical imaging to judge the extent of preconditioning and thus the optimal timing and extent of combinatorial thermal therapy is discussed.

Cell accelerated cryoablation simulation

Tumor cryoablation is a clinical procedure where supercooled probes are used to destroy cancerous lesions. Cryoablation is a safe and effective palliative treatment for skeletal metastases, providing immediate and long term pain relief, increasing mobility and improving quality of life. Ideally, lesions are encompassed by an ice ball and frozen to a sufficiently low temperature to ensure cell death. "Lethal ice" is the term used to describe regions within the ice ball where cell death occurs. Failure to achieve lethal ice in all portions of a lesion may explain the high recurrence rate currently observed. Tracking growth of lethal ice is critical to success of percutaneous ablations, however, no practical methods currently exist for non-invasive temperature monitoring. Physicians lack planning tools which provide accurate estimation of the ice formation. Simulation of ice formation, while possible, is computationally demanding and too time consuming to be of clinical utility. We developed the computational framework for the simulation, acceleration strategies for multicore Intel x86 and IBM Cell architectures, and performed preliminary validation of the simulation. Our results demonstrate that the streaming SIMD implementation has better performance and scalability. Both accelerated and non-accelerated algorithms demonstrate good agreement between simulation and manually identified ice ball boundaries in phantom and patient images. Our results show promise for the development of novel cryoablation planning tools with real-time monitoring capability for clinical use.

The apparent critical isotherm for cryoinsult-induced osteonecrotic lesions in emu femoral heads

Cryoinsult-induced osteonecrosis (ON) in the emu femoral head provides a unique opportunity to systematically explore the pathogenesis of ON in an animal model that progresses to human-like femoral head collapse. Among the various characteristics of cryoinsult, the maximally cold temperature attained is one plausible determinant of tissue necrosis. To identify the critical isotherm required to induce development of ON in the cancellous bone of the emu femoral head, a thermal finite element (FE) model of intraoperative cryoinsults was developed. Thermal material property values of emu cancellous bone were estimated from FE simulations of cryoinsult to emu cadaver femora, by varying model properties until the FE-generated temperatures matched corresponding thermocouple measurements. The resulting FE model, with emu bone-specific thermal properties augmented to include blood flow effects, was then used to study intraoperatively performed in vivo cryoinsults. Comparisons of minimum temperatures attained at FE nodes corresponding to the three-dimensional histologically apparent boundary of the region of ON were made for six experimental cryoinsults. Series-wide, a critical isotherm of 3.5 degrees C best corresponded to the boundary of the osteonecrotic lesions.

Finite-element analysis of ex vivo and in vivo hepatic cryoablation

Cryoablation is a widely used method for the treatment of nonresectable primary and metastatic liver tumors. A model that can accurately predict the size of a cryolesion may allow more effective treatment of tumor, while sparing normal liver tissue. We generated a computer model of tissue cryoablation using the finite-element method (FEM). In our model, we considered the heat transfer mechanism inside the cryoprobe and also cryoprobe surfaces so our model could incorporate the effect of heat transfer along the cryoprobe from the environment at room temperature. The modeling of the phase shift from liquid to solid was a key factor in the accurate development of this model. The model was verified initially in an ex vivo liver model. Temperature history at three locations around one cryoprobe and between two cryoprobes was measured. The comparison between the ex vivo result and the FEM modeling result at each location showed a good match, where the maximum difference was within the error range acquired in the experiment (< 5 degrees C). The FEM model prediction of the lesion size was within 0.7 mm of experimental results. We then validated our FEM in an in vivo experimental porcine model. We considered blood perfusion in conjunction with blood viscosity depending on temperature. The in vivo iceball size was smaller than the ex vivo iceball size due to blood perfusion as predicted in our model. The FEM results predicted this size within 0.1-mm error. The FEM model we report can accurately predict the extent of cryoablation in the liver.

Isothermal volume contours generated in a freezing gel by embedded cryo-needles with applications to cryo-surgery

Three-dimensional numerical simulations of multi-cryo-needle surgery were performed with cryo-needle temperature variations taken from matched experimental data. The transient temperatures and frozen volumes generated by simultaneously operating up to three 1.47 mm OD cryo-needles embedded in a phase-changing gel simulating the properties of biological tissues, were studied. In all cases studied, the volumes enclosed by the "lethal", -40 degrees C isotherm, achieved most of their final size in the first few minutes of operation, thus obviating the need for long application times. After 30 min of application of the one-, two- or three-cryo-needles, the ablation ratio attained 3%, 3-6% and 3-8%, respectively, depending on cryo-needle placement configurations. Synergistic effects of using multi-cryo-needles were reflected in the increased expansion of both the radial and axial locations of the isothermal contours. Within each number of cryo-needles used, however, the differences in these locations were rather small, and, as a general rule, tended to somewhat decrease with increasing the placement "density" of the cryo-needles. For each two- and three-cryo-needle application, there is a certain combination of placement configuration and application time that would produce the largest, temperature-specific, volume. As a general guideline, multiple cryo-needles should not be placed too close to each other in order to enhance their synergistic effect. Results of this study should be useful in the design of cryo-needle placement and operation protocols and in understanding the limitations of the freezing-ablation process.

Laser-assisted cryosurgery of prostate: numerical study

A new methodology for preventing freezing damage beyond pre-specified boundaries during prostate cryosurgery is proposed herein. It consists of emitting controlled laser irradiation from the urethra, across the wall and into the prostate while conventional cryoprobes freeze the unwanted prostate tissue. The purpose of this methodology is to protect the urethral wall better and confine the desired cryoinjured region more accurately than the current cryosurgery approach. We also explore the potential use of light-absorbing dyes to further enhance the laser light absorption and corresponding heat generation to increase the thickness of the protected region. A finite difference heat diffusion model in polar coordinates with temperature-dependent thermophysical properties simulates the prostate freezing while laser irradiation across the urethral wall is emitted. This approach maintains the temperature of the urethral wall and the adjacent tissue above a pre-specified threshold temperature of -45 degrees C, independent of application time. Temperature contours resulting from prostate cryoablation with (a) conventional constant temperature heating; (b) laser irradiation heating; and (c) laser irradiation heating with pre-injected light-absorbing dye layers indicate that the thickness of the protected region increases in this order, and that the latter two methodologies may be more effective in limiting cryoinjury to a predefined region compared to constant temperature heating. An analysis of laser power requirements and sensibility of laser-assisted cryosurgery (LAC) of prostate is also presented. It is shown that tissue temperature may vary as much as +/-20 degrees C with variations of +/-10% in laser power relative to the nominal power required to maintain the tissue at 37 degrees C. This demonstrates the sensitivity to laser power and the need of an accurate laser power control algorithm.

Office-Based Cryoablation of Breast Fibroadenomas with Long-Term Follow-up

Approximately 10% of women will experience a breast fibroadenoma in their lifetime. Cryoablation is a new treatment that combines the better attributes of the current standards: surveillance and surgery. It is a minimally invasive office-based procedure that is administered without the use of general anesthesia, involving minimal patient discomfort and little to no scarring. This work aimed to establish the long-term (2-3 years) efficacy, safety, and satisfaction of the procedure, as well as the impact of cryoablation on mammogram and ultrasound images. Thirty-seven treated fibroadenomas were available for assessment with an average follow-up period of 2.6 years. Of the original 84% that were palpable prior to treatment, only 16% remained palpable to the patient as of this writing. Of those fibroadenomas that were initially < or = 2.0 cm in size, only 6% remained palpable. A median volume reduction of 99% was observed with ultrasound. Ninety-seven percent of patients and 100% of physicians were satisfied with the long-term treatment results. Mammograms and ultrasounds showed cryoablation produced no artifact that would adversely affect interpretation. Cryoablation for breast fibroadenomas has previously been reported as safe and effective both acutely and at the 1-year follow-up mark, and thus has been implemented as a treatment option. At long-term follow-up, cryoablation as a primary therapy for breast fibroadenomas demonstrates progressive resolution of the treated area, durable safety, and excellent patient and physician satisfaction. The treatment is performed in an office setting rather than an operating room, resulting in a cost-effective and patient-friendly procedure. Cryoablation should be considered a preferred option for those patients desiring definitive therapy for their fibroadenomas without surgical intervention.

Numerical Simulation for Heat Transfer in Prostate Cancer Cryosurgery

A comprehensive computational framework to simulate heat transfer during the freezing process in prostate cancer cryosurgery is presented. Tissues are treated as nonideal materials wherein phase transition occurs over a temperature range, thermophysical properties are temperature dependent and heating due to blood flow and metabolism are included. Boundary conditions were determined at the surfaces of the commercially available cryoprobes and urethral warmer by experimental study of temperature combined with a mathematical optimization process. For simulations, a suitable computational geometry was designed based on MRI imaging data of a real prostate. An enthalpy formulation-based numerical solution was performed for a prescribed surgical protocol to mimic a clinical freezing process. This computational framework allows for the individual planning of cryosurgical procedures and objective assessment of the effectiveness of prostate cryosurgery.

Focal cryogen insults for inducing segmental osteonecrosis: computational and experimental assessments of thermal fields

Orthopaedic management of femoral head osteonecrosis is a common clinical problem for which there presently is no good solution. Current animal models are inappropriate to study potential new solutions, since it has been difficult to replicate the natural history of structural collapse seen in the human disorder. Recently, progression to collapse was obtained for cryogenically induced osteonecrosis in emus, although the lesions involved were imprecisely controlled in terms of size or location. A new cryo-insult probe is here reported for the purpose of delivering well-prescribed local thermal insults in this new animal model, while minimizing damage to non-targeted regions. Finite element analysis was used to elucidate the influence of operator-controlled parameters upon the temporal/spatial variation of the thermal field. The numerical formulation includes convective heat transfer attributable to tissue bed perfusion. The computational results agreed closely with the results of thermocouple recordings in a companion bench-top experiment. The cryo-insult probe successfully produced segmental lesions in the emu model of sizes comparable to the computed freeze front diameters.

A finite element model for ice ball evolution in a multi-probe cryosurgery

The ice formation in a water body is examined for the computation of temperature field, phase change and a moving ice-water interface whose location is not known á priori. This is classically referred to as the Stefan problem [Rubinstein, L.I. (1971) The Stefan Problem (American Mathematical Society, Providence, Rhode Island 02904]. Based on the Duvaut [Duvaut, G. (1973) "Résolution d'un probléme Stefan" C.R. Acad Sci. Paris 276, 1461-1463] transformation, the governing equations for heat conduction are formulated within a variational principle that is readily amenable to a standard finite element solution without remeshing. Numerical simulation results pertaining to the freezing of tumour tissue in a multi-cryoprobe cryosurgery are presented. These results lend both quantitative and graphical support to the current empirical standards of "effective therapy" in view of refining clinical applications.

The use of a new miniature cryoprobe for ablation of bone tissue: in vivo assessment of the probe and application of the method to bone in a sheep model

BACKGROUND

So far, modern miniature cryoprobes were used for local destruction of soft tissue tumours without damaging the adjacent healthy tissue. In this study, cryoablation methodology was applied to bone and the cooling capacity of the probe was examined in vitro and in vivo.

METHOD

Freezing was performed by cooling one or two probes (diameter 3.2 mm) to -180 degrees C with liquid nitrogen. The cooling capacity of the probes was determined optically and thermally against a homogeneous reference gel, followed by in vivo measurements on femoral and tibial sheep bone followed by histological examination.

RESULTS

Thanks to the synergistic effect, the simultaneous use of 2 probes produced an almost spherical expansion of cold in the homogenous gelatin. During the in vivo freezes, the temperature curves showed a more moderate trend. Nevertheless, due to the synergistic effect, temperatures below -50 degrees C could be reached at a distance of 1 cm from the probe. No local or systemic intraoperative complications were observed. Histological examination revealed cell necrosis up into the -10 degrees C isotherm.

CONCLUSIONS

Adequate tissue cooling of the bone matrix can be achieved with in vivo freezes by means of one or more miniature cryoprobes. Therefore, this probe could provide an alternative to or supplement surgical resection of pathological bone processes.

The evolution and state of modern technology for prostate cryosurgery

Cryosurgery is the in situ ablation of a target tissue by application of extreme cold temperature. The ability of cryosurgery to ablate tissue is unquestioned. It is the controlled application of a cryoinjury in a manner to minimize morbidity that is problematic. Prostate cryosurgery is complicated by the proximity of the prostate to adjacent structures that are sensitive to a freeze injury, namely the urethra, rectal wall, and neurovascular bundles. Several recent technological advances have led to the development of an effective treatment protocol with acceptable morbidity. These include the advent of real-time transrectal ultrasound, cryomachines with almost instant freeze-thaw control through the use of the Joule-Thompson effect, and warming catheters to effectively preserve the integrity of the urethra and external sphincter. Further, temperature monitoring at the posterior margin of the prostate sometimes combined with an injection of saline solution into Denonvilliers fascia has reduced the occurrence of urethrorectal fistula formation to 0% to 0.5% in modern series. We review the key innovations of prostate cryosurgery that differentiate this state-of-the-art procedure from that used by early investigators to even that of the early 1990s. Potential future innovations, specifically related to image guidance of the procedure, are also addressed.

Cryosurgery as primary treatment for localized prostate cancer: a community hospital experience

The technique and recent experience incorporating cryosurgery into our community practice for primary treatment of localized prostate cancer is described. Between December 2000 and December 2001, a total of 93 patients underwent targeted cryoablation for localized prostate cancer. Of the 93 patients, 18 had failed radiotherapy, and cryotherapy was used as salvage therapy. The remaining 75 patients underwent targeted cryoablation of the prostate as primary therapy. A single urologist using an argon-based cryoablation system performed the procedure. Cryoprobes and thermosensors were placed under transrectal ultrasound guidance via a transperineal route. A double freeze-thaw cycle was used with anterior-to- posterior probe operation. Strategically placed thermosensors were used to monitor and control the freezing, and a warming catheter was used to protect the urethra. We achieved a nadir prostate-specific antigen level of < or =0.4 ng/mL in 84% of the entire population we studied (63 of 75 patients). Postsurgery complications were minimal. Incontinence developed in 4 patients, as did postsuprapubic catheter removal urinary retention. Erectile dysfunction developed in 28 of 34 patients who were potent preoperatively, with 6 of the 34 patients regaining potency after surgery. No rectourethral fistula formation occurred. Urethral sloughing was observed in 5 patients, 1 of whom developed a scrotal abscess during treatment of the sloughing. The use of cryoablation of the prostate for the treatment of localized adenocarcinoma of the prostate is feasible and can easily be transferred from the pioneering centers to the community hospitals without sacrificing safety or efficacy.

Cryotherapy in the treatment of liver metastases from colorectal cancer

Although hepatic resection remains the treatment of choice for metastatic disease to the liver from colorectal cancer, the procedure is only possible for approximately 10% of patients. Ninety percent of patients with liver metastases ultimately die of liver failure. Thus, attention has turned to other, locoregional techniques that may be used alone or in conjunction with resection to increase the eligibility for some type of surgical, potentially curative treatment. One such option is cryotherapy. This review provides technical, physiologic, and outcome information regarding this technique. Semin Oncol 29:183-191.

Cryosurgery of normal and tumor tissue in the dorsal skin flap chamber: Part I--thermal response

Current research in cryosurgery is concerned with finding a thermal history that will definitively destroy tissue. In this study, we measured and predicted the thermal history obtained during freezing and thawing in a cryosurgical model. This thermal history was then compared to the injury observed in the tissue of the same cryosurgical model (reported in companion paper (Hoffmann and Bischof, 2001)). The dorsal skin flap chamber, implanted in the Copenhagen rat, was chosen as the cryosurgical model. Cryosurgery was performed in the chamber on either normal skin or tumor tissue propagatedfrom an AT-1 Dunning rat prostate tumor. The freezing was performed by placing a approximately 1 mm diameter liquid-nitrogen-cooled cryoprobe in the center of the chamber and activating it for approximately 1 minute, followed by a passive thaw. This created a 4.2 mm radius iceball. Thermocouples were placed in the tissue around the probe at three locations (r = 2, 3, and 3.8 mm from the center of the window) in order to monitor the thermal history produced in the tissue. The conduction error introduced by the presence of the thermocouples was investigated using an in vitro simulation of the in vivo case and found to be <10 degrees C for all cases. The corrected temperature measurements were used to investigate the validity of two models of freezing behavior within the iceball. The first model used to approximate the freezing and thawing behavior within the DSFC was a two-dimensional transient axisymmetric numerical solution using an enthalpy method and incorporating heating due to blood flow. The second model was a one-dimensional radial steady state analytical solution without blood flow. The models used constant thermal properties for the unfrozen region, and temperature-dependent thermal properties for the frozen region. The two-dimensional transient model presented here is one of the first attempts to model both the freezing and thawing of cryosurgery. The ability of the model to calculate freezing appeared to be superior to the ability to calculate thawing. After demonstrating that the two-dimensional model sufficiently captured the freezing and thawing parameters recorded by the thermocouples, it was used to estimate the thermal history throughout the iceball. This model was used as a basis to compare thermal history to injury assessment (reported in companion paper (Hoffmann and Bischof, 2001)).

Two freezing cycles ensure interface sterilization by cryosurgery during bone tumor resection

Cryosurgery utilizing an argon-based system allows bone-tumor interface sterilization, while avoiding the risks of conventional cryosurgery. This study was conducted in order to evaluate the number of freezing cycles required for interface sterilization in cases of aggressive human bone tumors. Sixteen tumors were included (six chondrosarcomas, eight metastatic carcinomas, and two giant cell tumors). All occurred within long bones. In all cases a standardized marginal resection was performed. Following thorough curettage, we sampled five different locations within the tumor interface by a cylindrical hollow trephine. The interface viability was assessed using the XTT method. Quantitative histological evaluation was based on the percentage of live cells divided by total lacunae number in five random medium-power fields. One freezing cycle (5 min, -40 degrees C) reduced tumor viability to approximately 5% of prefreezing. However, there were still live specimens. Two or three freezing cycles led to complete interface sterilization. The difference between a single freezing cycle and two freezing cycles was significant (ANOVA, F = 130, P < 0.01). The difference between two freezing cycles and three freezing cycles was not significant (ANOVA, F = 0.14, P < 0.6). The results of the XTT method for the assessment of interface viability correlated well with histological evaluation of the percentage of viable cells (r = 0.89), as well as with cell culture results of frozen vs. prefreezing tumor samples. In conclusion, two freezing cycles are adequate to achieve tumor-bone interface sterilization in aggressive human bone tumors.

Validation of estimated 3D temperature maps during hepatic cryo surgery

A simple model for estimating temperature distribution within the frozen region during cryo ablation was tested for accuracy. Freezing experiments were conducted in both ex vivo and in vivo porcine livers. Temperature was measured during freezing using a fiber-optic temperature sensor. Three-dimensional MR images were obtained at the end of each freezing cycle. From the MR image volumes, three-dimensional temperature maps were calculated numerically using a simplified bio-heat model. Estimated temperatures were compared to measured temperatures. The median difference between measured and estimated temperature was 3.03 degrees C. The median distance from a sensor element to the closest point on a isotherm surface with the corresponding estimated temperature was 0.70 mm. The accuracy of this model is acceptable. Temperature maps as outlined here may be used for monitoring of cryotherapy in order to increase clinical effectiveness.

A model for the time dependent three-dimensional thermal distribution within iceballs surrounding multiple cryoprobes

A time dependent three-dimensional finite difference model of iceball formation about multiple cryoprobes has been developed and compared to experimental data. Realistic three-dimensional probe geometry is specified and the number of cryoprobes, the cryoprobe cooling rates, and the locations of the probes are arbitrary inputs by the user. The simulation accounts for observed longitudinal thermal gradients along the cryoprobe tips. Thermal histories for several points around commercially available cryoprobes have been predicted within experimental error for one, three, and five probe configurations. The simulation can be used to generate isotherms within the iceball at arbitrary times. Volumes enclosed by the iceball and any isotherms may also be computed to give the ablative ratio, a measure of the iceball's killing efficiency. This ratio was calculated as the volume enclosed by a critical isotherm divided by the total volume of the iceball for assumed critical temperatures of -20 and -40 degrees C. The ablative ratio for a single probe is a continuously decreasing function of time but when multiple probe configurations are used the ablative ratio increases to a maximum and then essentially plateaus. Maximum values of 0.44 and 0.55 were observed for three and five probe configurations, respectively, with an assumed critical temperature of -20 degrees C. Assuming a critical temperature of -40 degrees C, maximum ablative ratios of 0.21 and 0.3 for three and five probe configurations, respectively, were observed.

Comparison of iceball diameter and temperature distribution achieved with 3-mm accuprobe cryoprobes in porcine and human liver tissue and human colorectal liver metastases in vitro

We aimed to assess the thermal profile and size of iceballs produced by Accuprobe cryoprobes in fresh porcine and human liver and human colorectal cancer liver metastases in vitro to allow better planning of cryosurgical treatment of liver metastases. Iceballs were produced by a 20-min single freeze cycle using 8-mm cryoprobes in pig liver in a waterbath at 37 degrees C (n = 8) and 3-mm cryoprobes in pig liver (n = 8), human liver (n = 3), and human colorectal cancer liver metastases (n = 8). The iceball diameters and the temperatures at different distances from the cryoprobe were measured. Mean iceball diameters produced by 8-mm cryoprobes in pig liver were 56.3 mm and varied from 38.7 to 39.6 mm for 3-mm cryoprobes in the different tissues used. There was no significant difference in iceball size in the different tissues. The diameter of the zone of -40 degrees C or less was approximately 44 mm using 8-mm cryoprobes in porcine liver and between 27 and 31 mm using 3-mm cryoprobes in the different tissues examined. The results may allow better preoperative planning of the cryosurgical treatment of liver metastases with Accuprobe cryoprobes.

Considerations during clinical operation of two commercially available cryomachines

BACKGROUND AND OBJECTIVES

Advances in the technology of cryomachines in the last 10 years have led to the development of both liquid nitrogen and argon-based Joule-Thompson cryomachines. Theoretical and practical evaluation of the CMS Accuprobe and the ENDOcare CRYOcare was performed as respective examples of these technologies.

METHODS

Thermal gradients were calculated about both probes for the best case scenario of probe surface temperature equaling that of the cryogen used. Also, experimental evaluation in gelatin phantoms was performed with five probe arrays.

RESULTS

Theoretically, a liquid nitrogen-cooled probe provides only a slight advantage over one cooled with liquid argon. However, the experimental performance evaluation demonstrated that the CRYOcare system creates an iceball faster with steeper internal temperature gradients than the Accuprobe. Further, temperature outputs from the Accuprobe were shown to be in error, likely due to the position of the thermocouple within the probe.

CONCLUSIONS

Cryomachine performance is determined more by technological innovations than by cryogen temperature. Thermocouple monitoring is urged for users of the Accuprobe.