Droop: a rapidly computable descriptor of local minimum tissue temperature during conductive interstitial hyperthermia

Although the goal of local hyperthermia therapy for cancer is to elevate the temperature of a tumour to cytotoxic levels, without the presence of 'cold spots', varying blood flow has made the achievement of consistent, therapeutic temperature distributions extraordinarily difficult. The paper presents a novel approach to estimating local minimum tumour temperatures during conductive interstitial hyperthermia which facilitates identification and elimination of cold spots. Conductive interstitial hyperthermia is modelled mathematically for a parallel array of implanted, electrically heated catheters which warms the treated tissue by thermal conduction and blood perfusion. Computer simulations employing the bioheat transfer equation reveal a predictive relationship between implanted catheter temperature, catheter power, implantation geometry and local minimum tumour temperature. Formulation of this relationship in terms of a parameter named 'droop' allows estimation of local minimum intratumoural temperatures from individual catheter temperature and power. Computer simulations are also performed to determine the sensitivity of the droop-based estimator to variations in properties of the tissue and catheters. Generally, variations in geometry or thermal properties of about 10 per cent cause estimation errors of less than 1 degree C in magnitude. These results suggest that online estimates of thermal 'droop' may provide a practical route to more consistent control of intratumoural minimum temperature during conductive interstitial heat therapy.     

Helical antenna arrays for interstitial microwave thermal therapy for prostate cancer: tissue phantom testing and simulations for treatment

Interstitial microwave therapy is an experimental treatment for prostate cancer. The objective of this work was to measure the power deposition (specific absorption rate, SAR) patterns of helical microwave antennae both individually and in array patterns that would be useful for clinical treatment protocols. Commercial helical antenna 3D SAR patterns were measured in muscle equivalent phantoms using a thermographic technique. Two array patterns were tested: a 'square' and a 'crescent' array, both surrounding the urethra. To assess the feasibility of pre-treatment planning, the measured SAR patterns were input to a treatment planning computer simulation program based on a series of trans-rectal ultrasound images from a prostate cancer patient. The simulation solved the Pennes linear bioheat heat transfer equation in prostate tissue, with the aim of achieving a target of 55 degrees C at the prostate periphery while not allowing normal surrounding tissues (bladder, urethra, rectum) to rise above 42 C. These criteria could not be met with the square array but they could be met with the crescent array, provided that the prostate was first dissected away from the rectum. This can be done with a procedure such as 'hydrodissection', where sterile saline is injected to separate the prostate and rectum. The results of these SAR measurements and heat transfer simulations indicate that arrays of helical antennae could be used for safe and effective thermal therapy for prostate cancer.     

Computer controlled microwave system for clinical hyperthermia

A hyperthermia system which has been developed for clinical use is described. Different direct-contact applicators working at the ISM microwave frequency of 915 MHz are used for inducing heat in superficial tumours to a depth of about 3 cm. The system is a further development of an earlier prototype which worked at 2450 MHz. The microwave generator is controlled by a microcomputer governed by temperature measurements with thermistor probes inserted into the tissue. To minimise the influence of the interaction between the microwaves and the temperature probes, a pulsed irradiation technique is used. Up to July 1984, 80 superficial tumours had been treated with radiotherapy combined with local hyperthermia applied using this system.     

Design optimization of a robust sleeve antenna for hepatic microwave ablation

We describe the application of a Bayesian variable-number sample-path (VNSP) optimization algorithm to yield a robust design for a floating sleeve antenna for hepatic microwave ablation. Finite element models are used to generate the electromagnetic (EM) field and thermal distribution in liver given a particular design. Dielectric properties of the tissue are assumed to vary within +/- 10% of average properties to simulate the variation among individuals. The Bayesian VNSP algorithm yields an optimal design that is a 14.3% improvement over the original design and is more robust in terms of lesion size, shape and efficiency. Moreover, the Bayesian VNSP algorithm finds an optimal solution saving 68.2% simulation of the evaluations compared to the standard sample-path optimization method.     

Development of a novel loosely wound helical coil for interstitial radiofrequency thermal therapy

We have developed a novel, radiofrequency thermal therapy device designed to improve local control of large solid tumours using heat in the range 55-90 degrees C. The device is a solenoid or helical coil designed to be loosely wound inside a tumour and excited with radiofrequency energy. Typically, we associate a uniform axially directed magnetic field with a solenoid coil, which when time varying, results in an electric field inside the coil, which lies mainly in the circumferential direction. In addition to this magnetically induced electric field, there exists a less familiar axially directed electric field inside the coil. Previous investigators have demonstrated the presence of this secondary axial electric field both experimentally and theoretically. Our design exploits the size and uniformity of these electric fields, for heating and coagulating a large tissue volume with a single applicator. The loosely wound solenoid is constructed from Nitinol, an electrically conductive shape memory alloy that permits the minimally invasive percutaneous insertion of the coil through a single cannulating delivery needle. To demonstrate the potential of this device and to determine the optimal frequency of operation, phantom tissue models and finite-element calculation models using COMSOL 3.2 were used to characterize frequency- and geometry-dependent trends in absorption rate density (ARD), which is proportional to electric field intensity. Radial and axial ARD profiles were measured, calculated and evaluated to determine the frequency and geometry best suited for producing large, homogenous coagulation volumes. Based on the trade-off between radial and axial uniformities of the ARD profiles, a 2 cm diameter coil with a 4 cm length and 1 cm pitch, operated at 27.12 MHz, produced the optimal heating pattern, as determined using tissue-mimicking phantom models.     

The feasibility of interstitial ultrasound hyperthermia.

One of the most promising ways to increase the efficacy of brachytherapy is to combine it with hyperthermia. In this paper, the feasibility of using ultrasound transducers as interstitial hyperthermia sources was investigated. The ultrasound output of eight cylindrical transducers (diameter 1 mm and length 25 mm) was studied. It was found that many of these transducers were able to generate between 2 and 3 W of acoustic energy at the frequency of 9.5 MHz. The ultrasound field emitted radially was well collimated and extended the full length of the transducer. In vitro perfused liver and kidney experiments showed that an array of four transducers placed in brachytherapy catheters up to a maximum spacing of 20 mm in a square pattern could induce therapeutic temperatures. Also, the effect of flow rate into the organs and catheter cooling were investigated. These results showed that interstitial ultrasound sources are potentially the most promising way of generating therapeutic temperatures through standard interstitial radiation therapy catheters.     

Design of a microwave array hyperthermia applicator with a semicircular reflector

The design of a hyperthermia applicator for heating biological tissues is presented in which the applicator consists of an array antenna surrounded by a perfect electrically conducting reflector. The heat hazard to superficial tissues is reduced by the introduction of a dielectric protecting layer over them. A method of moments formulation is applied to approximate the electric field within the biological medium and a closed form expression is presented for the electromagnetic coupling problem, which enables an optimisation procedure to be performed. The applicator enhances both penetration and focusing: deep tumours, close to the bone region, are heated and the percentage of biologically healthy tissue exposed to a specific absorption rate (SAR) hazard level diminishes by 53.8%.     

Design of a microwave array hyperthermia applicator with a semicircular reflector

The design of a hyperthermia applicator for heating biological tissues is presented in which the applicator consists of an array antenna surrounded by a perfect electrically conducting reflector. The heat hazard to superficial tissues is reduced by the introduction of a dielectric protecting layer over them. A method of moments formulation is applied to approximate the electric field within the biological medium and a closed form expression is presented for the electromagnetic coupling problem, which enables an optimisation procedure to be performed. The applicator enhances both penetration and focusing: deep tumours, close to the bone region, are heated and the percentage of biologically healthy tissue exposed to a specific absorption rate (SAR) hazard level diminishes by 53.8%.     

Non-contact respiratory monitoring system using a ceiling-attached microwave antenna

Using a microwave antenna attached to the room ceiling, we conducted non-contact monitoring of respiratory chest wall motions of subjects in bed and covered by a soft comfortable bedding, to measure the vital signs of patients under nursing care in a welfare institution. Long-term vital sign monitoring using electrodes places a heavy burden on monitored individuals. Our non-contact respiratory monitoring system comprises a 1,215 MHz-microwave radar (LDR-1), antenna box attached to the ceiling, and personal computer with analyzing software. The system was tested on eight healthy volunteers (mean age, 25 years; range, 21–44 years) and eight elderly volunteers with some disorders (mean age, 69 years; range, 66–75 years). Respiratory rates of subjects measured using this system correlated with rates measured using respiration sensors (r = 0.97, P < 0.001 for healthy volunteers, r = 0.98, P < 0.0001 for elderly volunteers). The system could monitor subtle changes in respiratory rate, and monitoring respiratory rate increases caused by disorders such as pneumonia will be possible.     

Computer-aided design and evaluation of novel catheters for conductive interstitial hyperthermia

Conductive interstitial heating is a modality in which heating elements are implanted directly into the treated tissue. One implementation of such therapy employs electrically heated catheters that are implanted in staggered, parallel rows. To explore strategies for maximising the uniformity of tissue temperature distributions achieved with heated catheters, a two-dimensional computer model with cylindrical co-ordinates was used to evaluate radially and longitudinally the temperature distributions produced by a typical interior catheter surrounded by other similar catheters. Insights from the computer model led to new designs for catheters containing multiple heating elements that produced more uniform thermal distributions, eliminating previous ‘cold spots’ within the treatment volume located near the ends of the catheter. The new catheter designs also include compartments for the optional placement of radioactive seeds for simultaneous thermoradiotherapy.     

Heating characteristics of antenna arrays used in microwave ablation: A theoretical parametric study

A numerical study of the performance of antenna arrays used in microwave ablation (MWA) is carried out. Double-slot coaxial antennas in triangular and square configurations are studied. Clinical (healthy vs. malignant) and experimental (in vs. ex vivo) scenarios for hepatic cancer treatment are modeled, and further application in bone and lung tissue is examined. It is found that triangular arrays can create spherical ablation zones, while square configurations result in flatter ones. Thresholds in power and application times for creating continuous ablation zones are calculated, and the characteristics of the latter are quantified.     

Effect of insertion depth on helical antenna performance in a muscle-equivalent phantom

Barrett's oesophagus is considered to increase the risk of cancer 30 fold. A set of helical microwave antennas was designed to investigate their potential use in the thermal therapy of Barrett's oesophagus. For treatment, a balloon filled with muscle-equivalent material encapsulates the antenna. The effects of insertion depth and coil-spacing on the thermal distribution produced by the antennas (20-35 mm) were characterized. The 35 mm helical antenna, with a coil-spacing of 3.6 mm resulted in uniform heating for an insertion depth of 40 mm. It was observed that the resultant temperature distribution produced, by the antennas, was dependent on the insertion depth within the phantom. For all antennas studied, deeper insertion resulted in two high intensity regions, approximately 1/4 and 3/4 along the antenna length. In contrast, shallow insertion resulted in predominant tip heating with undesirable heating at the phantom entry point. However, by manipulating the coil-spacing of the helix, uniform temperature profiles were achieved for a range of insertion depths.     

A review of coaxial-based interstitial antennas for hepatic microwave ablation

Although surgical resection remains the gold standard for treatment of liver cancer, there is a growing need for alternative therapies. Microwave ablation (MWA) is an experimental procedure that has shown great promise for the treatment of unresectable tumors and exhibits many advantages over other alternatives to resection, such as radiofrequency ablation and cryoablation. However, the antennas used to deliver microwave power largely govern the effectiveness of MWA. Research has focused on coaxial-based interstitial antennas that can be classified as one of three types (dipole, slot, or monopole). Choked versions of these antennas have also been developed, which can produce localized power deposition in tissue and are ideal for the treatment of deep-seated hepatic tumors.     

The use of a dispersive ground electrode with a loosely wound helical coil for interstitial radiofrequency thermal therapy

We have developed a novel, thermal therapy device designed to improve local control of large solid tumours using heat in the range 55-90 degrees C. The device is a helical coil designed to be loosely wound inside a tumour and excited with radiofrequency energy at 27.12 MHz. This design exploits the size and uniformity of the electric fields generated by magnetic induction inside this solenoidal geometry for heating and coagulating a large target volume. The use of the electrically conductive shape memory alloy Nitinol for the coil and an external ground plane permit the minimally invasive percutaneous insertion of the coil through a single cannulating delivery needle. To demonstrate the feasibility of this device, phantom models and finite-element models using COMSOL 3.2 were used to characterize uniformity of the radial and axial ARD (absorption rate density) profiles of different monopolar coil geometries. COMSOL 3.2 was also used to calculate temperature profiles and distributions produced by these coils in a non-perfused tissue-mimicking domain following a 10 min heating period. ARD results showed that optimum radial and axial uniformities were achieved with a 0.75 cm pitch and 3 cm length for a 1.5 cm diameter coil, and a 1.4 cm pitch and 4.2 cm length for a 2 cm diameter coil. These coils were able to produce lesions in excised bovine liver of 4 cm x 4.5 cm and 3.5 cm x 6.5 cm, respectively. Predicted temperature profiles showed similar profile sizes and shapes in a non-perfused domain, with the absolute temperature rise determined by the source input to the coil. These results demonstrate the potential of this interstitial, monopolar induction coil device for heating large tumours using a single applicator delivered through a single needle insertion.     

Optimization of a beam shaping bolus for superficial microwave hyperthermia waveguide applicators using a finite element method

Temperature inhomogeneity in hyperthermia treatments often limits the total thermal dose that can be delivered to the tumour region. To reduce such inhomogeneities, a prototype dynamically modifiable square array of saline-filled patches which attenuate microwave energy was developed for superficial treatments that use external microwave applicators. The array was situated inside the coupling water bolus that is often used with external applicators. The prototype has been previously tested clinically with promising results. A more complete theoretical analysis of the performance of this new bolus design and improvements to its design by modelling are presented here. The analysis was performed by performing five iterative simulations of the SAR pattern produced inside a tissue structure by a waveguide applicator with a water bolus containing the dynamic patch array attached. Between iterations the patch array configuration was modified in an attempt to improve the ability of the bolus to confine heating to an 'L'-shaped tumour region. These simulations were performed using the finite element method. The steady-state temperature profile was then computed using a finite element method based simulation of heat transfer that assumed a given applicator power level and water bolus temperature. Several iterations of these heat transfer simulations were performed with varying applicator power level and water bolus temperature to improve the confinement of heating to the target region. The analysis showed that the dynamic patch array should be capable of conforming heating to an 'L'-shaped target tumour region while limiting the heating to the surrounding normal tissue to an acceptable level.     

Radiation characteristics of helical tomotherapy

Helical tomotherapy is a dedicated intensity modulated radiation therapy (IMRT) system with on-board imaging capability (MVCT) and therefore differs from conventional treatment units. Different design goals resulted in some distinctive radiation field characteristics. The most significant differences in the design are the lack of flattening filter, increased shielding of the collimators, treatment and imaging operation modes and narrow fan beam delivery. Radiation characteristics of the helical tomotherapy system, sensitivity studies of various incident electron beam parameters and radiation safety analyses are presented here. It was determined that the photon beam energy spectrum of helical tomotherapy is similar to that of more conventional radiation treatment units. The two operational modes of the system result in different nominal energies of the incident electron beam with approximately 6 MeV and 3.5 MeV in the treatment and imaging modes, respectively. The off-axis mean energy dependence is much lower than in conventional radiotherapy units with less than 5% variation across the field, which is the consequence of the absent flattening filter. For the same reason the transverse profile exhibits the characteristic conical shape resulting in a 2-fold increase of the beam intensity in the center. The radiation leakage outside the field was found to be negligible at less than 0.05% because of the increased shielding of the collimators. At this level the in-field scattering is a dominant source of the radiation outside the field and thus a narrow field treatment does not result in the increased leakage. The sensitivity studies showed increased sensitivity on the incident electron position because of the narrow fan beam delivery and high sensitivity on the incident electron energy, as common to other treatment systems. All in all, it was determined that helical tomotherapy is a system with some unique radiation characteristics, which have been to a large extent optimized for intensity modulated delivery.     

Localized heating characteristics of hyperthermia using a reentrant cavity

An applicator based on a reentrant cavity with excellent localized heating characteristics is proposed in order to treat deep tumours localized in the head and neck. Numerical and experimental analyses of the applicator, which produces an electromagnetic field distribution required for this type of localized heating, are performed. A simple and clear procedure for achieving localized heating requires applicator miniaturization. However, miniaturization causes an increase in the resonant frequency and leads to the degradation of the characteristics for heating deep tissue. Therefore, it is proposed that dielectrics be inserted into the applicator and the resonant frequency be reduced. From the results of the numerical analysis and experimental examination, it is shown that a deep region of 100–120 mm around the centre of a phantom can be heated.