Radiofrequency ablation: variability in heat sensitivity in tumors and tissues

Radiofrequency ablation: mechanisms and clinical applications

Radiofrequency ablation (RFA), a form of thermal ablation, employs localized heat to induce protein denaturation in tissue cells, resulting in cell death. It has emerged as a viable treatment option for patients who are ineligible for surgery in various diseases, particularly liver cancer and other tumor-related conditions. In addition to directly eliminating tumor cells, RFA also induces alterations in the infiltrating cells within the tumor microenvironment (TME), which can significantly impact treatment outcomes. Moreover, incomplete RFA (iRFA) may lead to tumor recurrence and metastasis. The current challenge is to enhance the efficacy of RFA by elucidating its underlying mechanisms. This review discusses the clinical applications of RFA in treating various diseases and the mechanisms that contribute to the survival and invasion of tumor cells following iRFA, including the roles of heat shock proteins, hypoxia, and autophagy. Additionally, we analyze‌ the changes occurring in infiltrating cells within the TME after iRFA. Finally, we provide a comprehensive summary of clinical trials involving RFA in conjunction with other treatment modalities in the field of cancer therapy, aiming to offer novel insights and references for improving the effectiveness of RFA.

Retrospective evaluation of radiofrequency volumetric tissue reduction for hypertrophic turbinates in dogs with brachycephalic obstructive airway syndrome

OBJECTIVE

The objective of this study was to retrospectively assess the effect of Radiofrequency Volumetric Tissue Reduction (RFVTR) on hypertrophic turbinates and clinical outcome in brachycephalic dogs when included in multi-level surgery (MLS).

STUDY DESIGN

Clinical retrospective multicenter study.

ANIMALS

132 client-owned brachycephalic dogs.

METHODS

132 brachycephalic dogs with high-grade Brachycephalic Obstructive Airway Ayndrome (BOAS) and hypertrophic turbinates were treated with RFVTR as part of MLS of the upper airways. Intranasal obstruction was evaluated by computer tomography (CT) and antero-/retrograde rhinoscopy before and 6 months after RFVTR. The clinical records, the CT images and the rhinoscopy videos were reviewed and clinical evolution was evaluated using a standardized questionnaire. The data was scored semi-quantitatively.

RESULTS

In this study, 132 patients were included for a follow-up period of 120 weeks. RFVTR resulted in minor complications, including serous nasal discharge within the first postoperative week in all dogs, and intermittent nasal congestion between 3-8 weeks after treatment in 24.3% of the patients. Rhinoscopy and CT follow-ups were available for 33 patients. Six months after treatment intranasal airspace was increased (p = 0.002) and the presence and overall amount of mucosal contact points was reduced (p = 0.039).

CONCLUSION

MLS with RFVTR led to a significant reduction in turbinate volume at the 6-month follow-up examination and significant clinical improvement over a long-term period of 120 weeks. This suggests the viability of RFVTR as a turbinate-preserving treatment for intranasal obstruction in dogs with BOAS.

CLINICAL SIGNIFICANCE

RFVTR is a minimally invasive turbinoplasty technique for intranasal obstruction in dogs with BOAS and can be included in MLS without increasing complication rates.

Ablation Techniques in Cancer Pain

Painful bone metastases are a frequently encountered problem in oncology practice. The skeletal system is the third most common site of metastatic disease and up to 85% of patients with breast, prostate, and lung cancer may develop bone metastases during the course of their disease.

Twelve-Month Volume Reduction Ratio Predicts Regrowth and Time to Regrowth in Thyroid Nodules Submitted to Laser Ablation: A 5-Year Follow-Up Retrospective Study

Objective

Laser ablation is a therapeutic modality used to reduce the volume of large benign thyroid nodules. Unsatisfactory reduction and regrowth are observed in some treated nodules. The aim of the study was to evaluate the long-term outcomes of laser treatment for solid nodules during a 5-year follow-up period, the regrowth rate, and the predictive risk factors of nodule regrowth.

Materials and Methods

We retrospectively evaluated patients with benign, solid, cold thyroid nodules who underwent laser ablation and were followed-up for 5 years. According to the selection criteria, 104 patients were included (median baseline nodule volume, 12.5 mL [25.0–75.0%, 8–18 mL]; median energy delivered, 481.5 J/mL [25.0–75.0%, 370–620 J/mL]). Nodule volume, thyroid function test results, and ultrasound were evaluated at baseline and then annually after the procedure.

Results

Of 104 patients, 31 patients (29.8%) had a 12-month volume reduction ratio (VRR) < 50.0% and 39 (37.5%) experienced nodule regrowth. Of these 39 patients, 17 (43.6%) underwent surgery and 14 (35.9%) underwent a second laser treatment. The rate of nodule regrowth was inversely related to the 12-month VRR, i.e., the lower the 12-month VRR, the higher the risk of regrowth (p < 0.001). The mean time for nodule regrowth was 33.5 ± 16.6 months. The 12-month VRR was directly related to time to regrowth, i.e., the lower the 12-month VRR, the shorter the time to regrowth (p < 0.001; R² = 0.3516). Non-spongiform composition increased the risk of regrowth with an odds ratio of 4.3 (95% confidence interval [CI] 1.8–10.2; p < 0.001); 12-month VRR < 50.0% increased the risk of regrowth with an odds ratio of 11.7 (95% CI 4.2–32.2; p < 0.001).

Conclusion

The VRR of thyroid nodules subjected to similar amounts of laser energy varies widely and depends on the nodule composition; non-spongiform nodules are reduced to a lesser extent and regrow more frequently than spongiform nodules. A 12-month VRR < 50.0% is a predictive risk factor for regrowth and correlates with the time to regrowth.

Radiofrequency ablation of atypical cartilaginous tumors in long bones: a retrospective study

Purpose: To determine the size of the ablation zone after radiofrequency ablation (RFA) of atypical cartilaginous bone tumors (ACT) using temperature-controlled 20 and 30 mm RFA straight non-cooled electrodes.Materials and methods: Sixteen patients with ACT in their long bones, who had undergone a single-session single-application CT-guided temperature-controlled RFA, were included retrospectively in the study. Tumors with a diameter of 10-25 mm were treated with 20 mm electrodes (n = 10), and tumors of 25-35 mm, with 30 mm electrodes (n = 6). The ablated zone was measured after three months on MRI images.Results: All the tumors were within the ablated zone on the 3-month follow-up MRI scan. The mean ablation time with the electrode, at a target temperature of 90 °C, was 7.6 minutes (range 6-10). The median of the largest ablation diameters, on applying the 20 and 30 mm electrodes, were 42 mm (IQR 8.5, range 30-51 mm) and 44.5 mm (IQR 4.5, range 42-63 mm), respectively.Conclusions: All the retrospectively viewed tumors in the long bones of ACT patients treated with RFA were completely ablated. The ablation zone diameters in the bones were larger than expected, when compared to other tissues, such as the liver.

Unfavorable Outcomes in Solid and Spongiform Thyroid Nodules Treated with Laser Ablation. A 5-Year Follow-up Retrospective Study

OBJECTIVE

Laser Ablation (LA) is a therapeutic modality for reducing the volume of large benign thyroid nodules. This retrospective study was aimed at assessing the outcome of LA in patients with benign nonfunctioning thyroid nodules in a 5-years follow-up.

METHODS

Sixty-two patients (47 females; mean age 54.7±12 yr) with benign cold thyroid nodules underwent LA from July 2009 to March 2012. Nodule volume, thyroid function test, and ultrasound were monitored at baseline, and at 3, 6 and 12 months after the procedure, then annually. After dividing nodules in solid and spongiform, we evaluated unfavourable outcomes: 1) nodule's volume reduction <50%; 2) need for surgery; 3) need for additive LA session (due to nodule re-growth with persistence of cosmetic concern or compressive symptoms).

RESULTS

Baseline volume did not differ between solid and spongiform nodules as well as energy delivered and the number of needles used. Unfavourable outcomes occurred in 24 patients (38.7%). Nineteen/ 24 (79.2%) patients who experienced unfavourable outcomes belonged to the solid nodules group (P<0.01). When considering only those who benefited from LA, the 5-years reduction was 59.7% for solid and 78.6% for spongiform nodules (P<0.05). One/6 patients who underwent surgery (solid nodules group) had a final diagnosis of Follicular Variant of Papillary Thyroid Cancer (FVPTC).

CONCLUSION

Large solid nodules, unlike spongiform, submitted to LA are characterized by a long-term unfavourable outcome and entail a potential risk of false negative cytologic results.

Defeating Cancers' Adaptive Defensive Strategies Using Thermal Therapies: Examining Cancer's Therapeutic Resistance, Ablative, and Computational Modeling Strategies as a means for Improving Therapeutic Outcome

BACKGROUND

Diverse thermal ablative therapies are currently in use for the treatment of cancer. Commonly applied with the intent to cure, these ablative therapies are providing promising success rates similar to and often exceeding "gold standard" approaches. Cancer-curing prospects may be enhanced by deeper understanding of thermal effects on cancer cells and the hosting tissue, including the molecular mechanisms of cancer cell mutations, which enable resistance to therapy. Furthermore, thermal ablative therapies may benefit from recent developments in computer hardware and computation tools for planning, monitoring, visualization, and education.

METHODS

Recent discoveries in cancer cell resistance to destruction by apoptosis, autophagy, and necrosis are now providing an understanding of the strategies used by cancer cells to avoid destruction by immunologic surveillance. Further, these discoveries are now providing insight into the success of the diverse types of ablative therapies utilized in the clinical arena today and into how they directly and indirectly overcome many of the cancers' defensive strategies. Additionally, the manner in which minimally invasive thermal therapy is enabled by imaging, which facilitates anatomical features reconstruction, insertion guidance of thermal probes, and strategic placement of thermal sensors, plays a critical role in the delivery of effective ablative treatment.

RESULTS

The thermal techniques discussed include radiofrequency, microwave, high-intensity focused ultrasound, laser, and cryosurgery. Also discussed is the development of thermal adjunctive therapies-the combination of drug and thermal treatments-which provide new and more effective combinatorial physical and molecular-based approaches for treating various cancers. Finally, advanced computational and planning tools are also discussed.

CONCLUSION

This review lays out the various molecular adaptive mechanisms-the hallmarks of cancer-responsible for therapeutic resistance, on one hand, and how various ablative therapies, including both heating- and freezing-based strategies, overcome many of cancer's defenses, on the other hand, thereby enhancing the potential for curative approaches for various cancers.

Motion Correction in Proton Resonance Frequency-based Thermometry in the Liver

The unique ability of magnetic resonance imaging to measure temperature noninvasively, in vivo, makes it an attractive tool for monitoring interventional procedures, such as radiofrequency or microwave ablation in real-time. The most frequently used approach for magnetic resonance-based temperature measurement is proton resonance frequency (PRF) thermometry. Although it has many advantages, including tissue-independence and real-time capability, the main drawback is its motion sensitivity. This is likely the reason PRF thermometry in moving organs, such as the liver, is not commonly used in the clinical arena. In recent years, however, several developments suggest that motion-corrected thermometry in the liver is achievable. The present article summarizes the diverse attempts to correct thermometry in the liver. Therefore, the physical principle of PRF is introduced, with additional references for necrosis zone estimation and how to deal with fat phase modulation, and main magnetic field drifts. The primary categories of motion correction are presented, including general methods for motion compensation and library-based approaches, and referenceless thermometry and hybrid methods. Practical validation of the described methods in larger patient groups will be necessary to establish accurate motion-corrected thermometry in the clinical arena, with the goal of complete liver tumor ablation.

Temperature sensitive liposomes combined with thermal ablation: Effects of duration and timing of heating in mathematical models and in vivo

BACKGROUND

Temperature sensitive liposomes (TSL) are nanoparticles that rapidly release the contained drug at hyperthermic temperatures, typically above ~40°C. TSL have been combined with various heating modalities, but there is no consensus on required hyperthermia duration or ideal timing of heating relative to TSL administration. The goal of this study was to determine changes in drug uptake when heating duration and timing are varied when combining TSL with radiofrequency ablation (RF) heating.

METHODS

We used computer models to simulate both RF tissue heating and TSL drug delivery, to calculate spatial drug concentration maps. We simulated heating for 5, 12 and 30 min for a single RF electrode, as well as three sequential 12 min ablations for 3 electrodes placed in a triangular array. To support simulation results, we performed porcine in vivo studies in normal liver, where TSL filled with doxorubicin (TSL-Dox) at a dose of 30 mg was infused over 30 min. Following infusion, RF heating was performed in separate liver locations for either 5 min (n = 2) or 12 min (n = 2). After ablation, the animal was euthanized, and liver extracted and frozen. Liver samples were cut orthogonal to the electrode axis, and fluorescence imaging was used to visualize tissue doxorubicin distribution.

RESULTS

Both in vivo studies and computer models demonstrate a ring-shaped drug deposition within ~1 cm of the visibly coagulated tissue. Drug uptake directly correlated with heating duration. In computer simulations, drug concentration increased by a factor of 2.2x and 4.3x when heating duration was extended from 5 to either 12, or 30 minutes, respectively. In vivo, drug concentration was by a factor of 2.4x higher at 12 vs 5 min heating duration (7.1 μg/g to 3.0 μg/g). The computer models suggest that heating should be timed to maximize area under the curve of systemic plasma concentration of encapsulated drug.

CONCLUSIONS

Both computer models and in vivo study demonstrate that tissue drug uptake directly correlates with heating duration for TSL based delivery. Computational models were able to predict the spatial drug delivery profile, and may serve as a valuable tool in understanding and optimizing drug delivery systems.

Immunological effect of local ablation combined with immunotherapy on solid malignancies

Recent comprehensive investigations clarified that immune microenvironment surrounding tumor cells are deeply involved in tumor progression, metastasis, and response to treatment. Furthermore, several immunotherapeutic trials have achieved successful results, and the immunotherapeutic agents are available in clinical practice. To enhance their demonstrated efficacy, combination of immunotherapy and ablation has begun to emerge. Local ablations have considerable advantages as an alternative therapeutic option, especially its minimal invasiveness. In addition, local ablations have shown immune-regulatory effect in preclinical and clinical studies. Although the corresponding mechanisms are still unclear, the local ablations combined with immunotherapy have been suggested in the treatment of several solid malignancies. This article aims to review the published data on the immune-regulatory effects of local ablations including stereotactic body radiotherapy, cryoablation, radiofrequency ablation, and high-intensity-focused ultrasound. We also discuss the value of local ablations combined with immunotherapy. Local ablations have the potential to improve future patient outcomes; however, the effectiveness and safety of local ablations combined with immunotherapy should be further investigated.

In vivo effects of radiofrequency ablation on long bones and the repair process in swine models

PURPOSE

To investigate in vivo effect of radiofrequency ablation (RFA) on swine long bones and the repair process.

MATERIALS AND METHODS

RFA was performed in six swine at the end and middle part of the tibia or femur. After RFA, radiological examinations were performed, and the swine were killed immediately and at different time points post-RFA for histopathological examination.

RESULTS

All swine had successful RFA. The RFA-induced elliptical necrotic area ranged from 3.81-5.24 cm² (mean 4.08 ± 0.73 cm²) at the bone end but 5.60-8.98 cm² (mean 7.58 ± 1.41) at the middle part immediately after RFA until 10 days, with the necrosis area significantly smaller (P = 0.000) at the end than at the middle. RFA only damaged the cortical bone slightly (0.01 cm thick) with no damage to the soft tissues outside the compact bone at both the end and middle. Surrounding the elliptic pale zone of coagulative necrosis was a narrow brown band of hemorrhage and inflammatory exudate. From day 10 until week 12, tissue proliferation and repair became increasingly apparent, with proliferated granulation, fibrous tissue, and fresh and mature bone trabecula.

CONCLUSION

RFA can quickly and effectively destroy the cancellous bone tissue without affecting the cortical bone and activate bone remodeling.

Heat Stress-Induced PI3K/mTORC2-Dependent AKT Signaling Is a Central Mediator of Hepatocellular Carcinoma Survival to Thermal Ablation Induced Heat Stress

Thermal ablative therapies are important treatment options in the multidisciplinary care of patients with hepatocellular carcinoma (HCC), but lesions larger than 2–3 cm are plagued with high local recurrence rates and overall survival of these patients remains poor. Currently no adjuvant therapies exist to prevent local HCC recurrence in patients undergoing thermal ablation. The molecular mechanisms mediating HCC resistance to thermal ablation induced heat stress and local recurrence remain unclear. Here we demonstrate that the HCC cells with a poor prognostic hepatic stem cell subtype (Subtype HS) are more resistant to heat stress than HCC cells with a better prognostic hepatocyte subtype (Subtype HC). Moreover, sublethal heat stress rapidly induces phosphoinositide 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) dependent-protein kinase B (AKT) survival signaling in HCC cells in vitro and at the tumor ablation margin in vivo. Conversely, inhibition of PI3K/mTOR complex 2 (mTORC2)-dependent AKT phosphorylation or direct inhibition of AKT function both enhance HCC cell killing and decrease HCC cell survival to sublethal heat stress in both poor and better prognostic HCC subtypes while mTOR complex 1 (mTORC1)-inhibition has no impact. Finally, we showed that AKT isoforms 1, 2 and 3 are differentially upregulated in primary human HCCs and that overexpression of AKT correlates with worse tumor biology and pathologic features (AKT3) and prognosis (AKT1). Together these findings define a novel molecular mechanism whereby heat stress induces PI3K/mTORC2-dependent AKT survival signaling in HCC cells and provide a mechanistic rationale for adjuvant AKT inhibition in combination with thermal ablation as a strategy to enhance HCC cell killing and prevent local recurrence, particularly at the ablation margin.

Predictive factors for complete renal tumor ablation using RFA

BACKGROUND

Radiofrequency ablation (RFA) can be used to treat renal masses in patients where surgery is preferably avoided. As tumor size and location can affect ablation results, procedural planning needs to identify these factors to limit treatment to a single session and increase ablation success.

PURPOSE

To identify factors that may affect the primary efficacy of complete renal tumor ablation with radiofrequency after a single session.

MATERIAL AND METHODS

Percutaneous RFA (using an impedance based system) was performed using computed tomography (CT) guidance. Fifty-two renal tumors (in 44 patients) were retrospectively studied (median follow-up, 7 months). Data collection included patient demographics, tumor data (modified Renal Nephrometry Score, histopathological diagnosis), RFA treatment data (electrode placement), and follow-up results (tumor relapse). Data were analyzed through generalized estimating equations.

RESULTS

Primary efficacy rate was 83%. Predictors for complete ablation were optimal electrode placement (P = 0.002, OR = 16.67) and increasing distance to the collecting system (P = 0.02, OR = 1.18). Tumor size was not a predictor for complete ablation (median size, 24 mm; P = 0.069, OR = 0.47), but all tumors ≤2 cm were completely ablated. All papillary tumors and oncocytomas were completely ablated in a single session; the most common incompletely ablated tumor type was clear cell carcinoma (6 of 9).

CONCLUSION

Optimal electrode placement and a long distance from the collecting system are associated with an increased primary efficacy of renal tumor RFA. These variables need to be considered to increase primary ablation success. Further studies are needed to evaluate the effect of RFA on histopathologically different renal tumors.

Radiofrequency ablation-combined multimodel therapies for hepatocellular carcinoma: Current status

Radiofrequency ablation (RFA) is widely accepted as a first-line interventional oncology approach for hepatocellular carcinoma (HCC) and has the advantages of high treatment efficacy and low complication risk. Local control rates equivalent to hepatic resection can be reached by RFA alone when treating small HCCs (<2 cm) in favorable locations. However, local tumor progression and recurrence rates with RFA monotherapy increase sharply when treating larger lesions (>3 cm). To address this clinical problem, recent efforts have focused on multimodel management of HCC by combining RFA with different techniques, including percutaneous ethanol injection, transarterial chemo-embolization, targeted molecular therapy, nanoparticle-mediated therapy, and immunotherapy. The combination strategy indeed leads to better outcomes in comparison to RFA alone. In this article, we review the current status of RFA-combined multimodal therapies in the management of HCC.

The advent of ultrasound-guided ablation techniques in nodular thyroid disease: towards a patient-tailored approach

Surgery is the long-established therapeutic option for benign thyroid nodules, which steadily grow and become symptomatic. The cost of thyroid surgery, the risk of temporary or permanent complications, and the effect on quality of life, however, remain relevant concerns. Therefore, various minimally invasive treatments, directed towards office-based management of symptomatic nodules, without requiring general anaesthesia, and with negligible damage to the skin and cervical tissues, have been proposed during the past two decades. Today, ultrasound-guided percutaneous ethanol injection and thermal ablation with laser or radiofrequency have been thoroughly evaluated, and are accessible procedures in specialized centres. In clinical practice, relapsing thyroid cysts are effectively managed with percutaneous ethanol injection treatment, which should be considered therapy of choice. In solid non-functioning thyroid nodules that grow or become symptomatic, trained operators may safely induce, with a single session of laser ablation treatment or radiofrequency ablation, a 50% volume decrease and, in parallel, improve local symptoms. In contrast, hyperfunctioning nodules remain best treated with radioactive iodine, which results in a better control of hyperthyroidism, also in the long-term, and fewer side-effects. Currently, minimally invasive treatment is also investigated for achieving local control of small size neck recurrences of papillary thyroid carcinoma in patients who are poor candidates for repeat cervical lymph node dissection. This particular use should still be considered experimental.

Evaluation of thermal injury to liver, pancreas and kidney during irreversible electroporation in an in vivo experimental model

BACKGROUND

Irreversible electroporation (IRE) is a new technique for tumour cell ablation that is reported to involve non-thermal-based energy using high voltage at short microsecond pulse lengths. In vivo assessment of the thermal energy generated during IRE has not been performed. Thermal injury can be predicted using a critical temperature model. The aim of this study was to assess the potential for thermal injury during IRE in an in vivo porcine model.

METHODS

In vivo continuous temperature assessments of 86 different IRE procedures were performed on porcine liver, pancreas, kidney and retroperitoneal tissue. Tissue temperature was measured continuously throughout IRE by means of two thermocouples placed at set distances (0·5 cm or less, and 1 cm) from the IRE probes within the treatment field. Thermal injury was defined as a tissue temperature of 54°C lasting at least 10 s. Tissue type, pulse length, probe exposure length, number of probes and retreatment were evaluated for associations with thermal injury. In addition, IRE ablation was performed with metal clips or metal stents within the ablation field to determine their effect on thermal injury.

RESULTS

An increase in tissue temperature above the animals' baseline temperature (median 36·0°C) was generated during IRE in all tissues studied, with the greatest increase found at the thermocouple placed within 0·5 cm in all instances. On univariable and multivariable analysis, ablation in kidney tissue (maximum temperature 62·8°C), ablation with a pulse length setting of 100 µs (maximum 54·7°C), probe exposure of at least 3·0 cm (maximum 52·0°C) and ablation with metal within the ablation field (maximum 65·3°C) were all associated with a significant risk of thermal injury.

CONCLUSION

IRE can generate thermal energy, and even thermal injury, based on tissue type, probe exposure lengths, pulse lengths and proximity to metal. Awareness of probe placement regarding proximity to critical structures as well as probe exposure length and pulse length are necessary to ensure safety and prevent thermal injury. A probe exposure of 2·5 cm or less for liver IRE, and 1·5 cm or less for pancreas, with maximum pulse length of 90 µs will result in safe and non-thermal energy delivery with spacing of 1·5-2·3 cm between probe pairs.

Heat stress induced cell death mechanisms in hepatocytes and hepatocellular carcinoma: in vitro and in vivo study

BACKGROUND AND OBJECTIVE

The aims of the present study were to investigate the thermal-dose dependent effect of heat stress on hepatocyte and HCC cell death mechanisms using clinically relevant experimental heat stress conditions in vitro and to investigate apoptotic cell death induced by laser thermal ablation in vivo.

STUDY DESIGN/MATERIALS AND METHODS

Institutional Animal Care and Use Committee approved all studies. Hepatocyte and HCC cell lines were heat stressed from 37 to 60°C for 2 or 10 minutes and assessed for viability, cytotoxicity and caspase-3/7 activity at 6 and/or 24 hours post-treatment (N = 3). Viability experiments were repeated with the RIPK1 inhibitor Necrostatin-1 to block necroptosis (N = 3). Rats with orthotopic HCC tumors stably expressing luciferase (N1S1luc2) were randomized to US-guided laser ablation (3W-45s for an intentional partial ablation; N = 6) or sham (N = 6) and followed by post-ablation caspase-3/7 bioluminescence imaging at 6 and 24 hours and cleaved caspase-3 immunostaining. P < 0.05 was considered statistically significant.

RESULTS

Heat-stress induced apoptosis and necrosis in hepatocytes and HCC cells in a thermal dose and cell-type dependent manner. Inhibition of RIPIK1-mediated necroptosis induced a significant, differential increase in HCC cell viability under physiologic and hyperthermic heat stress (P < 0.001). Intentional partial laser thermal ablation induced a significant increase in caspase-3/7 activity in the laser versus sham ablation groups at both 6 hours (10.1-fold, P < 0.01) and 24 hours (16.7-fold, P < 0.02). Immunohistochemistry confirmed increased cleaved caspase-3 staining at the tumor ablation margin 24 hours post-ablation.

CONCLUSIONS

Both regulated and non-regulated cell death mechanisms mediate heat stress-induced HCC cell killing and vary between hepatocytes and HCC subtypes. Apoptosis is a significant mechanism of cell death at the HCC tumor ablation margin.

Cytotoxicity of hepatocellular carcinoma cells to hyperthermic and ablative temperature exposures: in vitro studies and mathematical modelling

PURPOSE

Image-guided ablative therapies use temperatures greater than 45 °C to kill abnormal cells. There is limited published data of cell survival after ablative temperature exposures, which is of importance to predict ablation zone dimensions. The objective of this study was to determine and mathematically model survival of hepatocellular carcinoma cells following ablative temperature exposures (45-60 °C).

MATERIALS AND METHODS

Hepatocellular carcinoma (HCC) cell lines were plated in 96-well plates, and heated between 45 and 60 °C for 0-32 min. Heating was applied by a rapid media exchange with heated Hank's balanced salt solution (HBSS) in a temperature-controlled water bath. Cell viability was determined by MTS assay. Survival data was modelled by the Arrhenius model, and the thermal isoeffective dose (TID) model where kinetic parameters were determined via non-linear optimisation.

RESULTS

Results suggest that the thermal dose based on cumulative equivalent minutes and parameters as used for hyperthermia exposures (<43 °C) is not applicable for ablative exposures. We found R = 0.72 for temperatures between 45-60°C for the TID model. The Arrhenius parameters were frequency factor A = 3.25E43 1/s, and activation energy Ea = 281 kJ/mol. These parameters correlate well with a prior study in the same cell line, and with threshold temperatures for necrosis from in vivo studies.

CONCLUSIONS

Our results suggest that standard TID model kinetic parameters based on hyperthermia studies, often also used at ablation temperatures, are not applicable at these higher temperatures for HCC cells.

Mechanisms of cryoablation: clinical consequences on malignant tumors

While the destructive actions of a cryoablative freeze cycle are long recognized, more recent evidence has revealed a complex set of molecular responses that provides a path for optimization. The importance of optimization relates to the observation that the cryosurgical treatment of tumors yields success only equivalent to alternative therapies. This is also true of all existing therapies of cancer, which while applied with curative intent; provide only disease suppression for periods ranging from months to years. Recent research has led to an important new understanding of the nature of cancer, which has implications for primary therapies, including cryosurgical treatment. We now recognize that a cancer is a highly organized tissue dependent on other supporting cells for its establishment, growth and invasion. Further, cancer stem cells are now recognized as an origin of disease and prove resistant to many treatment modalities. Growth is dependent on endothelial cells essential to blood vessel formation, fibroblasts production of growth factors, and protective functions of cells of the immune system. This review discusses the biology of cancer, which has profound implications for the diverse therapies of the disease, including cryosurgery. We also describe the cryosurgical treatment of diverse cancers, citing results, types of adjunctive therapy intended to improve clinical outcomes, and comment briefly on other energy-based ablative therapies. With an expanded view of tumor complexity we identify those elements key to effective cryoablation and strategies designed to optimize cancer cell mortality with a consideration of the now recognized hallmarks of cancer.

Image-guided percutaneous ablation therapies for local recurrences of thyroid tumors

The incidence of thyroid carcinoma has increased steadily over the last few decades. Most differentiated thyroid carcinomas (DTC) are cured thanks to the initial treatment with surgery and radioiodine therapy. Nevertheless, neck lymph node metastases are found in a few of these patients during their long-term clinical and ultrasound follow-up. In some of these cases radioiodine treatment may not be effective in eradicating nodal metastases due to scant 131-I uptake. Additionally, a few of these patients undergo repeated neck explorations and/or resections. Based on these considerations and on the frequently indolent course of DTC neck metastases, a non-surgical therapeutic approach should be considered to control small local foci of DTC. There is increasing interest in mini-invasive image-guided procedures that can be performed under local anesthesia which do not affect the performance status of the patient. Image-guided minimally invasive ablative therapies delivered by using needle-like applicators include both thermal and non-thermal source techniques. Over the past 25 years, these therapies have gained widespread attention and, in many cases, broad clinical acceptance as methods for treating focal malignancies. In an attempt to overcome the limitations of treating certain unresectable tumor types not amenable to a further surgical treatment, a few investigators have reported successfully combining percutaneous therapies with other oncologic treatment strategies (combined treatments). In this review, we reported mini-invasive techniques more commonly employed in selected cases to ameliorate local compressive symptoms, control hormonal production, and reduce the volume of neoplastic tissue prior to traditional palliative treatment.

Combination therapy of radiofrequency ablation and bevacizumab monitored with power Doppler ultrasound in a murine model of hepatocellular carcinoma

PURPOSE

The purpose of this study was to monitor tumour blood flow with power Doppler ultrasound following antiangiogenic therapy with bevacizumab in order to optimally time the application of radiofrequency (RF) ablation to increase ablation diameter.

MATERIALS AND METHODS

Athymic nude mice bearing human hepatocellular carcinoma xenografts were treated with bevacizumab and imaged daily with power Doppler ultrasound to quantify tumour blood flow. Mice were treated with RF ablation alone or in combination with bevacizumab at the optimal time, as determined by ultrasound. Ablation diameter was measured with histology and tumour microvascular density was calculated with immunohistochemistry. A computational thermal model of RF ablation was used to estimate ablation volume.

RESULTS

A maximum reduction of 27.8 ± 8.6% in tumour blood flow occurred on day 2 following antiangiogenic therapy, while control tumours increased 29.3 ± 17.1% (p < 0.05). Tumour microvascular density was similarly reduced by 45.1 ± 5.9% on day 2 following antiangiogenic therapy. Histology demonstrated a 13.6 ± 5.6% increase in ablation diameter (40 ± 21% increase in volume) consistent with a computational model.

CONCLUSION

Quantitative power Doppler ultrasound is a useful biomarker to monitor tumour blood flow following antiangiogenic treatment and to guide the application of RF ablation as a drug plus device combination therapy.

Principles of and advances in percutaneous ablation

Image-guided tumor ablation with both thermal and nonthermal sources has received substantial attention for the treatment of many focal malignancies. Increasing interest has been accompanied by continual advances in energy delivery, application technique, and therapeutic combinations with the intent to improve the efficacy and/or specificity of ablative therapies. This review outlines clinical percutaneous tumor ablation technology, detailing the science, devices, techniques, technical obstacles, current trends, and future goals in percutaneous tumor ablation. Methods such as chemical ablation, cryoablation, high-temperature ablation (radiofrequency, microwave, laser, and ultrasound), and irreversible electroporation will be discussed. Advances in technique will also be covered, including combination therapies, tissue property modulation, and the role of computer modeling for treatment optimization.

Direct measurement of the lethal isotherm for radiofrequency ablation of myocardial tissue

BACKGROUND

The lethal isotherm for radiofrequency catheter ablation of cardiac myocardium is widely accepted to be 50°C, but this has not been directly measured. The purpose of this study was to directly measure the tissue temperature at the edge of radiofrequency lesions in real time using infrared thermal imaging.

METHODS AND RESULTS

Fifteen radiofrequency lesions of 6 to 240 seconds in duration were applied to the left ventricular surface of isolated perfused pig hearts. At the end of radiofrequency delivery, a thermal image of the tissue surface was acquired with an infrared camera. The lesion was then stained and an optical image of the lesion was obtained. The thermal and optical images were electronically merged to allow determination of the tissue temperature at the edge of the lesion at the end of radiofrequency delivery. By adjusting the temperature overlay display to conform with the edge of the radiofrequency lesion, the lethal isotherm was measured to be 60.6°C (interquartile ranges, 59.7° to 62.4°C; range, 58.1° to 64.2°C). The areas encompassed by the lesion border in the optical image and the lethal isotherm in the thermal image were statistically similar and highly correlated (Spearman ρ=0.99, P<0.001). The lethal isotherm temperature was not related to the duration of radiofrequency delivery or to lesion size (both P>0.64). The areas circumscribed by 50°C isotherms were significantly larger than the areas of the lesions on optical imaging (P=0.002).

CONCLUSIONS

By direct measurement, the lethal isotherm for cardiac myocardium is near 61°C for radiofrequency energy deliveries <240 seconds in duration. A 50°C isotherm overestimates lesion size. Accurate knowledge of the lethal isotherm for radiofrequency ablation is important to clinical practice as well as mathematical modeling of radiofrequency lesions.

Imaging findings after radiofrequency ablation of adrenal tumors

OBJECTIVE

The purpose of this study was to describe the imaging findings after radiofrequency ablation of adrenal tumors.

MATERIALS AND METHODS

We retrospectively reviewed the imaging findings of all patients with adrenal tumors treated with radiofrequency ablation in our department from January 2001 through August 2009. The studies were reviewed in consensus by two attending abdominal imaging radiologists and an abdominal imaging fellow. Imaging findings before, immediately after, and at short- and long-term follow-up after ablation were recorded.

RESULTS

Fourteen patients (seven men, seven women; mean age, 56 ± 8.4 years) underwent radiofrequency ablation of adrenal tumors. One case of small pneumothorax and one case of small hemothorax were the only minor complications (complication rate, 14%). The expected side effects of radiofrequency ablation were found in 35% of patients: in two patients adjacent liver parenchyma was ablated, in two patients the diaphragmatic crus was injured, and in two patients local hematoma occurred (in one patient, both adjacent liver and diaphragmatic crus were ablated). Immediate soft-tissue findings after radiofrequency ablation included air bubbles in 12 patients (86%) and fat stranding around the adrenal gland in 13 patients (93%). A fat rim sign was found in 60% of patients at long-term follow-up. The attenuation of the tumor immediately after the procedure increased an average of 7 HU (median, 5 HU; range, -2 to 18 HU) and tended to decrease in long-term follow-up. At long-term follow-up, most (75%) of the tumors had decreased in size and attenuation.

CONCLUSION

Air bubbles and fat stranding are frequently seen immediately after radiofrequency ablation of adrenal tumors. A fat rim sign is a common finding at long-term follow-up. Attenuation of the ablated zone increases immediately after the procedure and decreases in long-term follow-up. The volume of the ablated zone has a variable size response, suggesting the need for baseline imaging.

Interstitial microwave transition from hyperthermia to ablation: historical perspectives and current trends in thermal therapy

This work reviews the transition from hyperthermia to ablation for cancer treatment with interstitial microwave (MW) antennas. Early work utilising MW energy for thermal treatment of cancer tissue began in the late 1970s using single antennas applied interstitially or the use of multiple interstitial antennas driven with the same phase and equal power at 915 or 2450 MHz. The original antenna designs utilised monopole or dipole configurations. Early work in thermal therapy in the hyperthermia field eventually led to utilisation of these antennas and methods for MW ablation of tumours. Efforts to boost the radiated MW power levels while decreasing antenna shaft temperatures led to incorporation of internally cooled antennas for ablation. To address larger tumours, MW treatment utilised arrays that were simultaneously activated by either non-synchronous or synchronous phase operation, benefiting both hyperthermia and ablation strategies. Numerical modelling was used to provide treatment planning guidance for hyperthermia treatments and is expected to provide a similar benefit for ablation therapy. Although this is primarily a review paper, some new data are included. These new data show that three antennas with 2.5 cm spacing at 45 W/channel and 10 min resulted in a volume of 89.8 cm(3) when operated synchronously, but only 53.4 cm(3) non-synchronously. Efficiency was 1.1 (synchronous) versus 0.7 (non-synchronous). MW systems, treatment planning, and image guidance continue to evolve to provide better tools and options for clinicians and patients in order to provide better approach and targeting optimisation with the goal of improved treatment for the patient.

Algorithm optimization for multitined radiofrequency ablation: comparative study in ex vivo and in vivo bovine liver

PURPOSE

To prospectively optimize multistep algorithms for largest available multitined radiofrequency (RF) electrode system in ex vivo and in vivo tissues, to determine best energy parameters to achieve large predictable target sizes of coagulation, and to compare these algorithms with manufacturer's recommended algorithms.

MATERIALS AND METHODS

Institutional animal care and use committee approval was obtained for the in vivo portion of this study. Ablation (n = 473) was performed in ex vivo bovine liver; final tine extension was 5-7 cm. Variables in stepped-deployment RF algorithm were interrogated and included initial current ramping to 105 degrees C (1 degrees C/0.5-5.0 sec), the number of sequential tine extensions (2-7 cm), and duration of application (4-12 minutes) for final two to three tine extensions. Optimal parameters to achieve 5-7 cm of coagulation were compared with recommended algorithms. Optimal settings for 5- and 6-cm final tine extensions were confirmed in in vivo perfused bovine liver (n = 14). Multivariate analysis of variance and/or paired t tests were used.

RESULTS

Mean RF ablation zones of 5.1 cm +/- 0.2 (standard deviation), 6.3 cm +/- 0.4, and 7 cm +/- 0.3 were achieved with 5-, 6-, and 7-cm final tine extensions in a mean of 19.5 min +/- 0.5, 27.9 min +/- 6, and 37.1 min +/- 2.3, respectively, at optimal settings. With these algorithms, size of ablation at 6- and 7-cm tine extension significantly increased from mean of 5.4 cm +/- 0.4 and 6.1 cm +/- 0.6 (manufacturer's algorithms) (P <.05, both comparisons); two recommended tine extensions were eliminated. In vivo confirmation produced mean diameter in specified time: 5.5 cm +/- 0.4 in 18.5 min +/- 0.5 (5-cm extensions) and 5.7 cm +/- 0.2 in 21.2 min +/- 0.6 (6-cm extensions).

CONCLUSION

Large zones of coagulation of 5-7 cm can be created with optimized RF algorithms that help reduce number of tine extensions compared with manufacturer's recommendations. Such algorithms are likely to facilitate the utility of these devices for RF ablation of focal tumors in clinical practice.

Liposomal doxorubicin increases radiofrequency ablation-induced tumor destruction by increasing cellular oxidative and nitrative stress and accelerating apoptotic pathways

PURPOSE

To determine if oxidative and nitrative stress and/or apoptosis contribute to increased coagulation when combining radiofrequency (RF) ablation with liposomal doxorubicin.

MATERIALS AND METHODS

Animal care committee approval was obtained. R3230 mammary adenocarcinomas in Fischer rats were treated with either RF ablation (n = 43), 1 mg of intravenously injected liposomal doxorubicin (n = 26), or combined therapy (n = 30) and were compared with control subjects (n = 11). A subset of animals receiving combination therapy (n = 24) were treated in the presence or absence of N-acetylcysteine (NAC) administered 24 hours and 1 hour before RF ablation. Tumors were analyzed 2 minutes to 72 hours after treatment to determine the temporal range of response by using immunohistochemical staining of the apoptosis marker cleaved caspase-3, phosphorylated gammaH2AX, and HSP70 and of markers of oxidative and nitrative stress (8-hydroxydeoxyguanosine [8-OHdG], 4-hydroxynonenal [4-HNE]-modified proteins, and nitrotyrosine [NT]). Statistical analyses, including t tests and analysis of variance for comparisons where appropriate, were performed.

RESULTS

By 4 hours after RF ablation alone, a 0.48-mm +/- 0.13 (standard deviation) peripheral band with 57.0% +/- 7.3 cleaved caspase-3 positive cells was noted at the ablation margin, whereas a 0.73-mm +/- 0.18 band with 77.7% +/- 6.3 positivity was seen for combination therapy (P < .03 for both comparisons). Combination therapy caused increased and earlier staining for 4-HNE-modified proteins, 8-OHdG, NT, and gammaH2AX with colocalization to cleaved caspase-3 staining. A rim of increased HSP70 was identified peripheral to the area of cleaved caspase-3. Parameters of oxidative and nitrative stress were significantly inhibited by NAC 1 hour following RF ablation, resulting in decreased cleaved caspase-3 positivity (0.28-mm +/- 0.09 band of 25.9% +/- 7.4 positivity vs 0.59-mm +/- 0.11 band of 62.9% +/- 6.0 positivity, P < .001 for both comparisons).

CONCLUSION

Combining RF ablation with liposomal doxorubicin increases cell injury and apoptosis in the zone of increased coagulation by using a mechanism that involves oxidative and nitrative stress that leads to accelerated apoptosis.

Thermal ablation a comparison of thermal dose required for radiofrequency-, microwave-, and laser-induced coagulation in an ex vivo bovine liver model

RATIONALE AND OBJECTIVES

To compare thermal dosimetry metrics for specified diameters of coagulation achieved using three different ablation energy sources.

MATERIALS AND METHODS

204 ablations measuring 20, 30, or 40 +/- 2 mm were created in an ex-vivo bovine liver model using 1) 2.5 cm cluster RF electrodes (n = 114), 2) 3 cm microwave antennas (n = 45), and 3) 3 cm laser diffusing fibers (n = 45). Continuous temperature monitoring was performed 5-20 mm from the applicators to calculate: a) the area under the curve (AUC), b) cumulative equivalent minutes at 43 degrees C (CEM43), and c) Arrhenius damage integral (Omega) for the critical ablation margin (DOC), with results compared by multivariate analysis of variance and regression analysis.

RESULTS

The end temperatures at the margin of coagulation varied, and was lowest for the RF cluster electrode (33-58 degrees C), higher for laser (52-72 degrees C), and covered the widest range for microwave (42-95 degrees C). These end temperatures correlated with applied energy, as linear functions (r(2) = 0.74-0.96). The total heat needed to achieve ablation (AUC) varied with applied energy and coagulation diameter as negative exponential (RF and laser) or negative power (microwave) functions (r(2) = 0.82-0.98). Similarly, CEM43 values varied exponentially with energy and distance (r(2) = 0.52-0.76) over a wide range of values (10(12)). Likewise, Omega varied not only based upon energy source and DOC, but also as a positive linear correlation to applied energy and with sigmoid correlation to duration of ablation (r(2) = 0.85-0.97).

CONCLUSION

Our study demonstrates that the thermal dosimetry of ablation is not based solely on a fixed end temperature at the margin of the coagulation zone. Thermal dosimetry is not constant, but dependent on the type and amount of energy applied and distance suggesting the need to take into account the rate of heat transfer for ablation dosimetry.

Radiofrequency ablation induces antigen-presenting cell infiltration and amplification of weak tumor-induced immunity

PURPOSE

To evaluate the influence of subtotal radiofrequency (RF) ablation on a tumor-specific immune response in a murine tumor model and to explore the role of intratumoral dendritic cells (ITDCs) in mediating this effect.

MATERIALS AND METHODS

Animal work was performed according to an approved protocol and in compliance with the National Cancer Institute Animal Care and Use Committee guidelines and regulations. A murine urothelial carcinoma (MB49) model expressing the male minor histocompatibility (HY) antigen was inoculated subcutaneously in female mice. Fourteen days later, splenic T cells were analyzed with enzyme-linked immunosorbent spot for HY immune response (n = 57). In subsequent experiments, mice were randomized into control (n = 7), RF ablation, ITDC (n = 9), and RF ablation + ITDC (n = 9) groups and monitored for tumor growth. Eleven days after treatment, tumors were harvested for histologic and immunohistochemical analysis. Animals demonstrating complete tumor regression were rechallenged in the contralateral flank.

RESULTS

Animals treated with subtotal RF ablation showed significant increases in tumor-specific class I and II responses to HY antigens and tumor regression. RF ablation, ITDC, and combined groups demonstrated similar levels of antigen-presenting cell infiltration; all groups demonstrated greater levels of infiltration compared with untreated controls. ITDC injection also resulted in tumor regression. However, combination therapy did not enhance tumor regression when compared with either treatment alone. Rechallenged mice in RF ablation, ITDC, and combination groups demonstrated significant tumor growth inhibition compared with controls.

CONCLUSION

Subtotal RF ablation treatment results in enhanced systemic antitumor T-cell immune responses and tumor regression that is associated with increased dendritic cell infiltration. ITDC injection mimics the RF ablation effect but does not increase immune responses when injected immediately after RF ablation.

Radiofrequency ablation: the effect of distance and baseline temperature on thermal dose required for coagulation

PURPOSE

To determine the effects of applied current, distance from an RF electrode and baseline tissue temperature upon thermal dosimetry requirements to induce coagulation in ex vivo bovine liver and in vivo porcine muscle models.

MATERIALS AND METHODS

RF ablation was performed in ex vivo liver at varying baseline temperatures-19-21 degrees C (n = 114), 8-10 degrees C (n = 27), and 27-28 degrees C (n = 27)-using a 3-cm tip electrode and systematically varied current 400-1,300 mA, to achieve defined diameters of coagulation (20, 30 and 40 +/- 2 mm), and in in vivo muscle (n = 18) to achieve 35 mm +/- 2 mm of coagulation. Thermal dose required for coagulation was calculated as the area under the curve and cumulative equivalent minutes at 43 degrees C.

RESULTS

Thermal dose correlated with current in a negative exponential fashion for all three diameters of coagulation in ex vivo experiments (p < 0.001). The temperatures at the end of RF heating at the ablation margin were not reproducible, but varied 38 degrees C-74.7 degrees C, for 30 mm coagulation in ex vivo liver, and 59.8 degrees C-68.4 degrees C in the in vivo experiment. CEM(43) correlated with current as a family of positive exponential functions (r(2) = 0.76). However, a very wide range of CEM(43) values (on the order of 10(15)) was noted. Although baseline temperatures in the ex vivo experiment did not change required thermal dose, the relationships between end temperature at the ablation margin and RF current were statistically different (p < 0.001) as analysed at the 400 mA intercept.

CONCLUSIONS

In both models, thermal dosimetry required to achieve coagulation was not constant, but current and distance dependent. Hence, other formulas for thermal dose equivalence may be needed to predict conditions for thermal ablation.

Computer modeling of the combined effects of perfusion, electrical conductivity, and thermal conductivity on tissue heating patterns in radiofrequency tumor ablation

PURPOSE

To use an established computer simulation model of radiofrequency (RF) ablation to characterize the combined effects of varying perfusion, and electrical and thermal conductivity on RF heating.

METHODS

Two-compartment computer simulation of RF heating using 2-D and 3-D finite element analysis (ETherm) was performed in three phases (n = 88 matrices, 144 data points each). In each phase, RF application was systematically modeled on a clinically relevant template of application parameters (i.e., varying tumor and surrounding tissue perfusion: 0-5 kg/m(3)-s) for internally cooled 3 cm single and 2.5 cm cluster electrodes for tumor diameters ranging from 2-5 cm, and RF application times (6-20 min). In the first phase, outer thermal conductivity was changed to reflect three common clinical scenarios: soft tissue, fat, and ascites (0.5, 0.23, and 0.7 W/m- degrees C, respectively). In the second phase, electrical conductivity was changed to reflect different tumor electrical conductivities (0.5 and 4.0 S/m, representing soft tissue and adjuvant saline injection, respectively) and background electrical conductivity representing soft tissue, lung, and kidney (0.5, 0.1, and 3.3 S/m, respectively). In the third phase, the best and worst combinations of electrical and thermal conductivity characteristics were modeled in combination. Tissue heating patterns and the time required to heat the entire tumor +/-a 5 mm margin to >50 degrees C were assessed.

RESULTS

Increasing background tissue thermal conductivity increases the time required to achieve a 50 degrees C isotherm for all tumor sizes and electrode types, but enabled ablation of a given tumor size at higher tissue perfusions. An inner thermal conductivity equivalent to soft tissue (0.5 W/m- degrees C) surrounded by fat (0.23 W/m- degrees C) permitted the greatest degree of tumor heating in the shortest time, while soft tissue surrounded by ascites (0.7 W/m- degrees C) took longer to achieve the 50 degrees C isotherm, and complete ablation could not be achieved at higher inner/outer perfusions (>4 kg/m(3)-s). For varied electrical conductivities in the setting of varied perfusion, greatest RF heating occurred for inner electrical conductivities simulating injection of saline around the electrode with an outer electrical conductivity of soft tissue, and the least amount of heating occurring while simulating renal cell carcinoma in normal kidney. Characterization of these scenarios demonstrated the role of electrical and thermal conductivity interactions, with the greatest differences in effect seen in the 3-4 cm tumor range, as almost all 2 cm tumors and almost no 5 cm tumors could be treated.

CONCLUSION

Optimal combinations of thermal and electrical conductivity can partially negate the effect of perfusion. For clinically relevant tumor sizes, thermal and electrical conductivity impact which tumors can be successfully ablated even in the setting of almost non-existent perfusion.

RF ablation with adjuvant therapy: comparison of external beam radiation and liposomal doxorubicin on ablation efficacy in an animal tumor model

PURPOSE

To determine the critical thermal dosimetry and relative efficacy for RF ablation combined with external beam radiation (XRT) or liposomal doxorubicin (LD), in an animal tumor model.

MATERIALS AND METHODS

This study was performed in two phases, in 13-18 mm diameter R3230 tumors subcutaneously implanted into Fischer rats. In phase 1, tumors (n = 30) were randomized into six groups. RF energy (titrated to 70 degrees C tip temperature) was applied for either 2.5 or 5 min (n = 15, each group). For each duration, one of three adjuvant therapies was applied (n = 5, each): no therapy (control), LD (1 mg intravenously, 30 min post-RF), or XRT (20 Gy at 1 Gy min(-1), within 2 h post-RF), with sacrifice at 48 h for pathologic analysis. In phase 2, thermal mapping was performed in 20 tumors throughout RF application (70 degrees C; 5 min), at 1.5-7 mm distances from the active electrode tip. Temperature profiles throughout the tumor were constructed and were used to interpolate temperatures over time at the critical ablation margin, to derive maximum threshold temperature, AUC (area under the curve) and CEM(43) (cumulative equivalent minutes at 43 degrees C). Ablation sizes and all calculated values were compared within and across experimental groups using MANOVA statistics with pair-wise T-test for individual comparisons.

RESULTS

RF/XRT produced the largest coagulation (11.7 +/- 1.5 mm at 2.5 min, >or=15 +/- 0.7 mm at 5 min), followed by RF/LD, and then RF alone (p < 0.001 for all comparisons). RF/XRT demonstrated temperature threshold decreases from RF alone of 11.7 +/- 0.01 degrees C and 12.7 +/- 0.38 degrees C at 2.5 and 5 min respectively (with absolute thresholds of 42 degrees C for XRT compared to 52 degrees C for RF alone). RF/LD had decreases of 4.0 degrees C at 2.5 min and 4.4 degrees C at 5 min. Thermal dose requirements (AUC) decreased by 7.79% or 9.28% for RF/LD compared to >or=19.36% or 25.82% for RF/XRT at 2.5 and 5 min (p < 0.001). CEM(43) values followed similar patterns (p < 0.001), but with a reduction of 10(1) and 10(4) in magnitude for RF/LD and RF/XRT therapies at 5 min, respectively.

CONCLUSIONS

For a standardized RF dose, the combination of high dose XRT and RF increased ablation size compared to RF and liposomal doxorubicin or RF alone. Increased ablation size is more closely associated with decreased temperature threshold necessary to induce coagulation, rather than the total thermal dose.

Magnetic thermal ablation using ferrofluids: influence of administration mode on biological effect in different porcine tissues

The purpose of this study was to compare the effects of magnetic thermal ablation in different porcine tissues using either a singular injection or a continuous infusion of superparamagnetic iron oxide nanoparticles. In the first setting samples of three ferrofluids containing different amounts of iron (1:171, 2:192, and 3:214 mg/ml) were singularly interstitially injected into specimens of porcine liver, kidney, and muscle (n = 5). Then the specimens were exposed to an alternating magnetic field (2.86 kA/m, 190 kHz) generated by a circular coil for 5 min. In the second experimental setup ferrofluid samples were continuously interstitially infused into the tissue specimens during the exposure to the magnetic field. To measure the temperature increase two fiber-optic temperature probes with a fixed distance of 0.5 cm were inserted into the specimens along the puncture tract of the injection needle and the temperature was measured every 15 s. Finally, the specimens were dissected, the diameters of the created thermal lesions were measured, and the volumes were calculated and compared. Compared to continuous infusion, a single injection of ferrofluids resulted in smaller coagulation volumes in all tissues. Significant differences regarding coagulation volume were found in kidney and muscle specimens. The continuous infusion technique led to more elliptically shaped coagulation volumes due to larger diameters along the puncture tract. Our data show the feasibility of magnetic thermal ablation using either a single interstitial injection or continuous infusion for therapy of lesions in muscle, kidney, and liver. Continuous infusion of ferrofluids results in larger zones of necrosis compared to a single injection technique.