Radiofrequency capacitive hyperthermia for deep-seated tumors. I. Studies on thermometry

The thermometry results of radiofrequency (RF) capacitive hyperthermia for 60 deep-seated tumors in 59 patients are reported. Hyperthermia was administered regionally using two RF capacitive heating equipments which the authors have developed in cooperation with Yamamoto Vinyter Company Ltd., (Osaka, Japan). Intratumor temperatures were measured by thermocouples inserted through angiocatheters which were placed 5 cm to 12 cm deep into the tissues. Tumor center temperatures were measured for 307 treatments in all tumors; thermal distributions within tumors and surrounding normal tissues were obtained for 266 treatments of 53 tumors by microthermocouples. Thermometry results obtained were summarized as follows. A maximum tumor center temperature greater than 43 degrees C and 42 degrees C to 43 degrees C was obtained in 23 (38%) and 14 (23%) of the 60 tumors respectively. The time required to reach 43 degrees C in the tumor center was within 20 minutes after the start of hyperthermia in 87% of tumors heated to more than 43 degrees C. Temperature variations within a tumor exceeded 2 degrees C in 81% of tumors heated to more than 43 degrees C. The lowest tumor temperature greater than 42 degrees C was achieved in six of the 53 tumors (11%). Of 42 tumors in which temperatures of the subcutaneous fat, surrounding normal tissues, and the tumor center were compared, 24 (57%) showed the highest temperature in the tumor center and ten (24%) in the subcutaneous fat. When the heating efficacy was assessed in terms of a maximum tumor center, it great deal depended on the treatment site, tumor size, thickness of subcutaneous fat, and tumor type. Tumors in the head and neck, thorax, lower abdomen, and pelvis could be heated better than tumors in the upper abdomen. Greater heating efficacy was shown in patients with large, hypovascular tumors, and with the subcutaneous fat measuring less than 15 mm thick. The predominant limiting factor for power elevation was pain associated with heating. Systemic signs including increases in pulse rate and body temperature were not serious and seldom became limiting factors for power elevation. Our thermometry results indicate that the advantages of deep RF capacitive heating are its applicability to various anatomic sites and negligible systemic effects. The disadvantages are that its primary usefulness is limited to patients with thin subcutaneous fat and with large or hypovascular tumors.     

Capacitive heating of phantom and human tumors with an 8 MHz radiofrequency applicator (Thermotron RF-8)

The thermal profile was investigated in agar phantoms and in human tumors heated capacitively with 8 MHz RF. Deep and homogeneous heating could be achieved in a large homogeneous phantom of 25 cm diameter and 24 cm thick when heated with a pair of 25 cm diameter electrodes, coupled to both bases of the phantom. When the size of the two electrodes was not the same, the region near the smaller electrode was preferentially heated. It was, therefore, possible to control the depth of heating by choosing properly sized electrodes. Therapeutic temperature (greater than 42 degrees C) could be obtained in 7 out of 9 small, as well as, bulky superficial human tumors as large as 8 X 8 X 10 cm. Indications are that heating of some deep-seated human tumors might be achieved by the capacitive method, provided that subcutaneous fat layer is cooled by temperature controlled bolus and large electrodes are used. The effect of the anatomical structure on the power deposition in the human body during capacitive heating should be further investigated.     

Precooling prevents overheating of subcutaneous fat in the use of RF capacitive heating

The usefulness of precooling subcutaneous tissue in the hyperthermic treatment of deep-seated human tumors with capacitive application of radio-frequency (RF) was investigated. A capacitive hyperthermia unit operated at 8 MHz of radiofrequency was used to heat deep-seated human tumors. The electrode surfaces were covered with flexible vinyl sheets and the space between the electrode and the vinyl sheets perfused with 0.4% saline. The temperature of the saline was controlled by circulating the saline through built-in heat exchangers. The depth of heating was controlled by pairing electrodes of different diameters. When subcutaneous fat was less than 1 cm thick, various deep-seated tumors could be heated to a therapeutic temperature without significant discomfort in the subcutaneous tissue as long as the skin was properly cooled with the cold saline bolus. However, the same cooling method could not prevent the occurrence of pain in subcutaneous tissue in obese patients. The pain usually developed when the temperature of subcutaneous fat 1-2 cm below the skin surface exceeded 42 degrees C. We observed that cooling the human skin surface with 10 degrees C bolus for 20 min could substantially lower the temperature of fat as deep as 2.0-2.5 cm. Therefore, when the subcutaneous tissue was precooled with 10 degrees C saline for 20 min or longer before heating, it was possible to prevent successfully the overheating of subcutaneous fat as thick as 2.0-2.5 cm and to raise the temperature of deep-seated tumors to therapeutic levels.     

OMRON RF hyperthermia treatment system HEH-500 C

The RF capacitive type hyperthermia system "HEH-500 C" enables regional heating for superficial and deep seated tumors. It consists of a high frequency generator (frequency; 13.56 MHz, out put power; 500 watts), a control unit, a cooling unit for the applicators, an I/F unit (communicate the thermometer to RF generator and plotter printer), a thermometer (thermocouple thermometer or fiber fluorothermometer) and a plotter printer. Heating profile of HEH-500 C was presented with thermographic picture on TX-150 muscle equivalent phantom. Effects of bone and fat layer on heating profile of phantom were also examined. Points to be solved on thermometry, electromagnetic field environment and clinical use were also discussed.     

Clinical evaluation of hyperthermia equipment: the University of Arizona Institutional Report for the NCI Hyperthermia Equipment Evaluation Contract

Two-hundred and fifteen independent sites in 203 patients were treated with hyperthermia at the University of Arizona from 10/81 through 3/86 under the auspices of this contract. In the head and neck, a site dominated by superficial tumors, air-coupled and water-coupled microwave applicators yielded the best results. Similarly in the thorax, also dominated by superficial tumors, water-coupled microwave applicators were best. In the abdomen and pelvis, sites dominated by deep tumors, only interstitial radiofrequency (RF) heating, an invasive technique useful only in selected cases, was capable of consistently producing therapeutic temperatures. Toxicity appeared to be site-related, and treatment discomfort was especially common in abdominal and pelvic sites. In conclusion, while superficial sites were readily heated using propagative electromagnetic devices, these devices were ineffective and poorly tolerated at deeper sites. Effective deep hyperthermia was best produced with interstitial techniques, and further development of these techniques using RF electrodes, implantable microwave antennas and thermoregulating ferromagnetic seeds, as well as scanned, focussed-ultrasound techniques, holds promise for effective heating of deep visceral sites.     

A comparison of the heating characteristics of capacitive and radiative superficial hyperthermia

Background: Superficial hyperthermia is applied in combination with radiotherapy for e.g. melanoma and recurrent breast cancer, using both capacitive and radiative systems. In this paper, numerical simulations are applied to address the question which technique yields the most favourable heating characteristics.

Methods: A 434?MHz contact flexible microstrip applicator (CFMA type 4H, size 19.6?×?19.6?cm²) and a capacitive system consisting of two circular electrodes with diameter 15 and 25?cm were modelled. The water bolus of the CFMA was filled with deionised water and for capacitive heating both saline and deionised water were modelled. Specific absorption rate (SAR) and temperature simulations were performed for a perfused muscle-equivalent phantom and phantoms with a 1?cm thick superficial fat layer, assuming cylindrical target regions. Subsequently, a real patient model with a chest wall recurrence was studied with the target assumed to have muscle-like properties, fat properties or heterogeneous properties as derived from the CT Hounsfield Units.

Results: Phantom simulations showed that high SAR peaks occur around the bolus edges with capacitive heating. Power absorption below the fat layer is substantially higher for radiative heating and unless the target region is limited to the fat layer, radiative heating yields better target coverage in terms of SAR and temperature. Patient simulations showed that the T₉₀ for radiative heating was 0.4–1.1?°C higher compared with capacitive heating.

Conclusions: Radiative heating yields more favourable SAR and temperature distributions for superficial tumours, compared with capacitive heating, especially within heterogeneous tissues. Higher tumour temperatures are achieved without occurrence of treatment limiting hot spots.     

Radiofrequency capacitive heating of deep-seated tumours using pre-cooling of the subcutaneous tissues: results on thermometry in Dutch patients.

The capacity of a radiofrequency, 13.56 MHz, capacitive hyperthermia system using extensive pre-cooling of the subcutaneous tissue to induce locoregional deep heating has been investigated in 11 patients. Tumour location was presacral in nine--and eccentric towards the lateral side of the pelvis in two patients. For thermometry multiple catheters (mean 2.7) were inserted into the treatment volume. The mean numbers of temperature measuring points per treatment were 9.4 in tumour, 5.5 in muscle and 7.2 in subcutaneous fat. RF energy was applied after 30 min of cooling through two flexible boli perfused with saline water at 5-10 degrees C. Patient tolerance to pre-cooling was very good and after some initial discomfort the patient became rapidly accustomed to the cold water boli. For some patients better temperatures were achieved when the conventional anterior-posterior applicator set-up was replaced by a set-up with an applicator on each lateral side of the patient. As patients can tolerate temperatures within the fat tissue as high as 45.5 degrees C without complaining it appears important to monitor the temperature at the transition of fat to muscle tissue to prevent subcutaneous burns. The study shows that pre-cooling cannot avoid preferential heating at the interface from fat to muscle tissue. In this patient group the quality of the hyperthermia treatment appeared to be rather poor: 60% of the measured tumour temperatures were below 40 degrees D.     

Effects of fat thickness on heating patterns of the microwave applicator MA-151 at 631 and 915 MHz

Previous studies showed that the surface heating patterns of the MA-151 applicator on a 2 cm fat and 10 cm thick muscle phantom had center heating at 581 and 930 MHz and two hot spots near the edges of the applicator at 657 and 779 MHz. The hot spots at 657 MHz were consistent with two blisters on a patient's thigh. Since the heating patterns on muscle only showed good center elliptical heating at all frequencies, in this study we have investigated the effects of fat thickness on the heating patterns. Thermograms of fat and muscle surfaces were taken on phantoms with 0, 0.25, 0.5, 1, and 2 cm thick fat exposed to 631 or 915 MHz energy. The 631 MHz was selected to provide reasonable energy coupling for all phantoms. At 631 MHz, two hot spots were evident on all fat surfaces. The pattern on the muscle surface under the 0.25 cm fat did not show two hot spots, but the heating was elongated in the E-field direction. At 915 MHz, the heating was elongated on the surface of the 0.25 and 2 cm fat, and two hot spots were observed on the 0.5 and 1 cm fat surfaces. However, the muscle heating was elliptical in all cases. The ratio of muscle to fat heating decreased as the fat thickness increased. At 0.5 cm fat the ratio was about 1. These results indicate that fat thickness influences heating in muscle. During treatment with this applicator, surface temperature probes should be placed over potential hot spots. Surface cooling is desirable for heating tumors beneath the fat.     

Pathophysiological changes after local heating of rat liver

Changes in blood flow in rat liver by local heating of the left lateral lobe were studied. A small portion of the left lateral lobe was heated by capacitive application of 8 MHz RF with a 2 cm diameter electrode on the ventral surface and a 7 cm diameter electrode on the dorsal surface of upper body of rats. A circular area with diameter of about 1 cm in the left lateral lobe could be heated to relatively homogeneous temperatures. The temperature in the remaining part of the left lateral lobe as well as in the rest of liver tissue also increased to varying degrees during the local heating of the left lateral lobe. When heated at 39 degrees or 41 degrees C for 45 min, the arterial blood flow, as measured with the radioactive microsphere method, in the heated area, and that in the rest of the left lateral lobe as well as in the whole liver initially increased during the first 30 min and then started to decline. During a 45 min heating at 43 degrees C, the arterial blood flow in the heated area slightly increased for 15 min and then significantly decreased thereafter. The arterial blood flow in the rest of the liver tissue also increased during the first 15 min of a 43 degrees C heating and began to decline. Histological examination of liver heated at 43 degrees C for 30 min showed wedge-shaped infarctions early on, which appeared to be organized into scar tissue by 7 days after heating. The concentrations of glutamic oxaloacetic transaminase (GOT), glutamic pyruvic transaminase (GPT), and lactate dehydrogenase (LDH) in the serum significantly increased 2 hr after heating the small portion of the left lateral lobe at 43 degrees C for 30 min and began to decrease thereafter although the serum level of GOT and GPT was still greater than the control value 24 hr after heating.     

The size and distance of the opposite flat applicator change the SAR and thermal distributions of RF capacitive intracavitary hyperthermia.

The variations of the specific absorption rate (SAR) and thermal distributions in the JSHO QA phantom were investigated by using the radiofrequency (RF) capacitive intracavitary hyperthermia (ICHT) applicator (AP-T01, Omron Electric Co., Kyoto, Japan) and the opposite flat applicators of different sizes (AP-75E: 7.5 cm in diameter, AP-100E: 10 cm in diameter, and AP-150E: 15 cm in diameter). The influences of the distance between both applicators were also investigated. Heating of the region between both applicators became weaker with the increase in size of the opposite flat applicator, and it became stronger with the decrease of the distance between both applicators. Heating near the flat applicator became weaker with the increase in size of the flat applicators, and it showed no apparent difference with the increase of the distance between both applicators. The normalized SAR values between AP-T01 and the opposite flat applicator became smaller and its slope became steeper with the increase in size of the opposite flat applicator and in the distance between both applicators. These results suggest that the variability of the specific absorption rate (SAR) and thermal distributions of the region between both applicators may show the potentiality of usefulness for heating the tumours of various sizes, shapes and location.     

The size and distance of the opposite flat applicator change the SAR and thermal distributions of RF capacitive intracavitary hyperthermia

The variations of the specific absorption rate (SAR) and thermal distributions in the JSHO QA phantom were investigated by using the radiofrequency (RF) capacitive intracavitary hyperthermia (ICHT) applicator (AP-T01, Omron Electric Co., Kyoto, Japan) and the opposite flat applicators of different sizes (AP-75E: 7.5cm in diameter, AP-100E: 10cm in diameter, and AP-150E: 15cm in diameter). The influences of the distance between both applicators were also investigated. Heating of the region between both applicators became weaker with the increase in size of the opposite flat applicator, and it became stronger with the decrease of the distance between both applicators. Heating near the flat applicator became weaker with the increase in size of the flat applicators, and it showed no apparent difference with the increase of the distance between both applicators. The normalized SAR values between AP-T01 and the opposite flat applicator became smaller and its slope became steeper with the increase in size of the opposite flat applicator and in the distance between both applicators. These results suggest that the variability of the specific absorption rate (SAR) and thermal distributions of the region between both applicators may show the potentiality of usefulness for heating the tumours of various sizes, shapes and location.     

Locoregional hyperthermia of deep-seated tumours applied with capacitive and radiative systems: a simulation study

Background: Locoregional hyperthermia is applied to deep-seated tumours in the pelvic region. Two very different heating techniques are often applied: capacitive and radiative heating. In this paper, numerical simulations are applied to compare the performance of both techniques in heating of deep-seated tumours.

Methods: Phantom simulations were performed for small (30?×?20?×?50?cm³) and large (45?×?30?×?50?cm³), homogeneous fatless and inhomogeneous fat-muscle, tissue-equivalent phantoms with a central or eccentric target region. Radiative heating was simulated with the 70?MHz AMC-4 system and capacitive heating was simulated at 13.56?MHz. Simulations were performed for small fatless, small (i.e. fat layer typically <2?cm) and large (i.e. fat layer typically >3?cm) patients with cervix, prostate, bladder and rectum cancer. Temperature distributions were simulated using constant hyperthermic-level perfusion values with tissue constraints of 44?°C and compared for both heating techniques.

Results: For the small homogeneous phantom, similar target heating was predicted with radiative and capacitive heating. For the large homogeneous phantom, most effective target heating was predicted with capacitive heating. For inhomogeneous phantoms, hot spots in the fat layer limit adequate capacitive heating, and simulated target temperatures with radiative heating were 2–4?°C higher. Patient simulations predicted therapeutic target temperatures with capacitive heating for fatless patients, but radiative heating was more robust for all tumour sites and patient sizes, yielding target temperatures 1–3?°C higher than those predicted for capacitive heating.

Conclusion: Generally, radiative locoregional heating yields more favourable simulated temperature distributions for deep-seated pelvic tumours, compared with capacitive heating. Therapeutic temperatures are predicted for capacitive heating in patients with (almost) no fat.     

Multi-institutional studies on hyperthermia using an 8-MHz radiofrequency capacitive heating device (Thermotron RF-8) in combination with radiation for cancer therapy

A joint clinical trial of hyperthermia using a newly developed 8-MHz radiofrequency (RF) capacitive heating device (Thermotron RF-8; developed in cooperation with Yamamoto Vinyter Co. of Osaka) was performed under collaboration of seven institutions. Radiation with 4 Gy twice a week for a total of 40 Gy or 2 Gy five times a week for a total of 50 Gy was delivered. After irradiation, hyperthermia at 42.5 degrees C +/- 0.5 degree C for 40 to 60 minutes was given twice a week for a total of 10 times. Tumors examined in this trial were located in various depths in the body, and included those which were considered refractory to conventional treatments or radioresistant such as malignant melanoma and soft tissue tumors. Of the 63 tumors treated, 52.4% showed complete regression (CR); 19.0% more than 80% regression (PRa); 20.6%, 80% to 50% regression (PRb); and 8.0% no regression (NR). Our joint clinical trial demonstrated that hyperthermia with the use of the Thermotron RF-8 is safe and effective in the treatment of radioresistant tumors located in superficial, subsurface, and in some cases deep regions, if the surface cooling is properly managed by the temperature-controlled saline pad and electrodes of appropriate size are paired.     

Theoretical and experimental investigations of a newly developed intracavitary applicator system for the radiothermotherapy of gynaecological tumours

This paper describes a new type of applicator system for the intracavitary radiothermotherapy of gynaecological tumours. Hyperthermia and radiation can be delivered by one and the same applicator. The hyperthermia is delivered by a modified capacitive heating technique (27–12 MHz) combined with cooling systems, the radiation by the numerically controlled oscillation of a gamma source on the inside of the main applicator, the RF gamma applicator (HDR afterloading technique). Isotherms and isodoses can be adapted to the anatomical-pathological situation. Interactions between the applicator system and the surrounding tissue were investigated in a theoretical model (FEM). Complex two-dimensional SAR calculations as well as three-dimensional temperature calculations were carried out. The RF gamma applicator was also examined by thermography; the thermography proved the theoretical modelling to be correct. The applicators were also tested in animal experiments.     

Preliminary technical, experimental and clinical results of the use of the HPLR 27 system for the treatment of deep-seated tumours by hyperthermia

Based on the theoretical deep penetration of radio frequencies, we have developed several 27.12 MHz ridged wave guides, with large apertures, and filled with circulating deionized water. Different low impedance matching networks have been tested and a 0-800 W power generator has been designed for this particular clinical use. This can be driven easily by a microcomputer which is used to regulate the temperature at depth by varying the sequencing time and/or by adjusting the RF power. The complete system, known as HPRL 27, is manufactured by Sairem Ets. Studies of the spatial power density distributions show a relatively homogeneous RF field for an aperture greater than 30 X 14 cm. Preliminary heating patterns studied in phantoms indicate the possibility of treating volumes greater than 2000 cm3 within the 50 per cent isotherm. Maximum heating is observed between 2 and 5 cm deep with passive cooling, and between 3 and 6 cm deep with active cooling, in which case the 50 per cent isotherm is situated between 2 and 10 cm deep. Preliminary phase I clinical results on 20 patients with pelvic or thoracic carcinomas show the possibility of heating deep-seated tumours to above 42 degrees C with a good patient tolerance. Temperature is controlled by implantable catheters or in the case of pelvic heating, by inserting catheters into the vagina, the rectum and the bladder. This new device is well adapted for the treatment of pelvic tumours.     

Nanoparticle-mediated radiofrequency capacitive hyperthermia: A phantom study with magnetic resonance thermometry

Abstract

In hyperthermia, focusing heat generation on tumour tissues and precisely monitoring the temperature around the tumour region is important. To focus heat generation in radiofrequency (RF) capacitive heating, magnetic nanoparticles suspended in sodium carboxymethyl cellulose (CMC) solution were used, based on the hypothesis that the nanoparticle suspension would elevate electrical conductivity and RF current density at the nanoparticle-populated region. A tissue-mimicking phantom with compartments with and without nanoparticles was made for RF capacitive heating experiments. An FDTD model of the phantom was developed to simulate temperature increases at the phantom. To monitor temperature inside the phantom, MR thermometry was performed intermittently during RF heating inside a 3Tesla MRI magnet bore. FDTD simulation on the phantom model was performed in two steps: electromagnetic simulation to compute specific absorption rate and thermal simulation to compute temperature changes. Experimental temperature maps were similar to simulated temperature maps, demonstrating that nanoparticle-populated regions drew more heat than background regions. Nanoparticle-mediated RF heating could mitigate concerns about normal tissue death during RF capacitive hyperthermia.     

Relevance of a New Impedance Matching, or Subtrap, Method for the Reduction of Pain During Hyperthermia

Capacitive heating is widely used in hyperthermic treatment of human malignancies. However, the pain on the body surface or thermoesthesia in the subcutaneous fatty layer may prevent an elevation of temperature in the tumors. Impedance matching is improved by a subtrap method entailing the application of two copper plates (10×850×0.06 mm) as a subtrap circuit to each of two capacitive electrodes. In a clinical trial the Tmax, Tave, Tmin for the subtrap method were all higher in comparison with those for the conventional technique (42.5±0.7°C, 41.9±1.0°C, 41.3±1.1°C vs. 41.1±1.5°C, 40.6±1.3°C, 40.0±1.3°C). Although the maximal radiofrequency (RF) power applied to patients was higher with the subtrap method (875±189 W vs. 763±200 W), the incidence of surface pain was reduced dramatically. It is concluded that the subtrap method substantially improves the RF capacitive heating of deep-seated tumors.     

Hyperthermia combined with radiation therapy for primarily unresectable and recurrent colorectal cancer.

The value of adjuvant hyperthermia to radiotherapy in the treatment of locally advanced colorectal cancers was investigated. Between 1981 and 1989, 71 primarily unresectable or recurrent colorectal tumors were treated with radiotherapy at the Department of Radiology, Kyoto University Hospital. Of the 71 tumors, 35 were treated with radiotherapy plus hyperthermia (group I), while 36 tumors (group II) were unsuitable for hyperthermia mainly because of difficulties with the insertion of temperature probes or the thickness of the patient's subcutaneous fat (greater than 2 cm). The mean total radiation dose was 58 Gy and 57 Gy for groups I and II, respectively. Thirty deep-seated pelvic tumors were treated with an 8 MHz radiofrequency capacitive heating device, and five subsurface tumors were treated with a 430 MHz microwave hyperthermia system. Hyperthermia was given following radiotherapy for 30-60 min for a total of 2-14 sessions (mean 5.7). In 32 of the 35 tumors heated, direct measurement of tumor temperature was performed. For the five tumors treated with the microwave heating device, the means of the mean maximum, average, and minimum measured intratumoral temperatures were 45.4 degrees C, 43.3 degrees C, and 40.6 degrees C, respectively. The corresponding values were 42.2 degrees C, 41.3 degrees C, and 40.3 degrees C for the 27 tumors treated with the capacitive heating device. Effective heating of deep-seated pelvic tumors was more difficult than heating of abdominal wall or perineal tumors. The local control rate at 6 months after the treatment, which was defined as absence of local progression of the tumors, was 59% (17/29) and 37% (11/30) for groups I and II, respectively. The objective tumor response rate (complete regression plus partial response) evaluated by computed tomography was 54% (19/35) in group I, whereas it was 36% (10/28) in group II. A better response rate of 67% was obtained in the 15 tumors with a mean average tumor temperature of greater than 42 degrees C. Although limitation of our current heating devices exist, the combination of hyperthermia with radiotherapy is a promising treatment modality in the treatment of locally advanced colorectal cancer.     

Characterization of the RF ablation-induced 'oven effect': the importance of background tissue thermal conductivity on tissue heating.

PURPOSE: To determine the effect of background tissue thermal conductivity on RF ablation heating using ex vivo agar phantoms and computer modelling. METHOD: Two-compartment cylindrical agar phantom models (5% agar, 5% NaCl, 3% sucrose) were constructed. These included a standardized inner compartment (2 cm diameter, 4 cm length, 0.25% agar) representing a tumour, surrounded by an outer compartment representing background tissue. The thermal conductivity of the outer compartment was varied from 0.48 W m-1 degrees Celsius (normal liver) to 0.23 W m-1 degrees Celsius (fat) by adding a fat-saturated oil-based solute (10-90%) to the agar. RF ablation was applied at 2000 mA current for 2 min. Temperatures were recorded up to 4 cm from the electrode tip at 1 cm intervals. Subsequently, a 2-D finite element computer model was used to simulate RF ablation of 2-24 min duration for tumours measuring 2-4 cm in diameter surrounded by tissues of different thermal conductivity with the presence or absence of perfusion (0-5 kg m-3 s-1) (n = 44). A comparison of results was performed. RESULTS: In agar phantoms, the amount of fat in the background tissue correlated with thermal conductivity as a negative exponential function (r2 = 0.98). Significantly increased temperatures were observed at the edge of the inner compartment (1 cm from the electrode tip) as the fat content of the outer compartment increased (p < 0.01). Thus, temperatures at 2 min measured 31.5 +/- 2.2 degrees Celsius vs 45.1 +/- 3.1 degrees Celsius for thermal conductivities of 0.46 W m-1 degrees Celsius (10% fat) and 0.23 W m-1 degrees Celsius (90% fat), respectively. On the other hand, higher levels of fat led to lower temperature increases in the background compartment (0.2 +/- 0.3 degrees Celsius for 90% fat vs. 1.1 +/- 0.05 degrees Celsius for 10% fat, p < 0.05). Phantom thermal heating patterns correlated extremely well with computer modelling (r2 = 0.93), demonstrating that background tissues with low thermal conductivity increase heating within the central tumour, particularly for longer durations of RF ablation and in smaller tumours. Furthermore, computer modelling demonstrated that increases in temperature at the tumour margin for background tissues of lower thermal conductivity persisted in the presence of perfusion, with a clinically relevant 4.5 degrees Celsius difference between background thermal conductivities of fat and soft tissue for a 3 cm tumour with perfusion of 2 kg m-3 s-1, treated for 12 min. CONCLUSION: Lower thermal conductivity of background tissues significantly increases temperatures within a defined ablation target. These findings provide insight into the 'oven effect' (i.e. increased heating efficacy for tumours surrounded by cirrhotic liver or fat) and highlight the importance of both the tumour and the surrounding tissue characteristics when contemplating RF ablation efficacy.     

Effect of functional magnetic particles on radiofrequency capacitive heating: an in vivo study.

Specific heating of magnetic particles in radiofrequency (RF) capacitive hyperthermia and its hyperthermic effect were investigated in an in vivo study. Magnetite cationic liposomes (MCLs) were injected into a rat tumor on the femur and 8 MHz-RF capacitive heating was applied to the rat under "mild heating" conditions. Although the input power of RF capacitive heating was low under the same power conditions, the MCLs-injected tumor was heated over 43 degrees C, whereas it was only heated to 41 degrees C in the case of the rats not injected with MCLs. A necrotic area in the tumor was observed in the heated rats. From the results of histological observation of the removed tissue, the necrotic area in the MCLs-injected tumor was wider than that in MCLs-free tumor. Complete tumor suppression was observed in 71% (5 / 7) of MCLs-injected rats, and the hyperthermic effect was greatly improved by the MCLs.