A theoretical model for input impedance of interstitial microwave antennas with choke.

PURPOSE: Two important characteristics for interstitial microwave antennas used in clinical hyperthermia are: (1) a good impedance match to minimize reflected power; and (2) a good power deposition pattern which is independent of insertion depth. A major problem of the miniature coaxial dipole antennas used for interstitial hyperthermia is the fact that the impedance and power deposition patterns of these antennas change with insertion depth. One possible solution is the addition of a coaxial choke. A theoretical model for calculating the input impedance of interstitial microwave antennas having a coaxial choke is presented, which may serve as the first step in the design of such antennas. METHODS AND MATERIALS: A theoretical model for calculating the input impedance of coaxial microwave antennas with and without a choke is presented using insulated antenna theory. The theoretical model was used to calculate the input impedance of several prototype antennas having various choke and feedline dimensions, and comparison was made with experimentally measured impedance measurements in tissue-equivalent phantom. RESULTS: The choke section of the antenna is not ideal if conventional plastic insulation is used as the choke dielectric, because the desired radiating length of the antenna is significantly shorter than the quarter-wavelength in the choke dielectric. Impedance calculations based on the theoretical model correlate reasonably well with experimentally measured impedance. Based on these calculations, the effect of parameters such as choke layer thickness and choke dielectric constant are discussed for a 915 MHz antenna with choke. CONCLUSION: The theoretical model can serve as a design aid for optimizing choked microwave antenna designs, as well as predicting the impedance match of a given antenna design at a given insertion depth. The model allows the effect of some variables not accessible experimentally such as termination impedance to be studied, which may also be useful in the understanding of these antennas. Calculations are easily performed on a desktop computer.

A coaxial microwave applicator for transurethral hyperthermia of the prostate.

Benign prostatic hyperplasia (BPH) is a common disease of elderly men. The current definitive treatment for urinary obstruction caused by this disease is surgery (transurethral resection of the prostate, or TURP). Recent evidence suggests that hyperthermia may be a useful nonsurgical alternative for treatment of symptomatic BPH. A transurethral microwave applicator has been designed around a Foley catheter for delivery of local hyperthermia to the prostate. The Foley balloon is used to maintain the antenna position within the prostatic urethra. The Foley catheter also features an antenna choke to confine power deposition to the intended region. The antenna is a coaxial dipole designed to operate at 915 MHz. Qualitative and quantitative specific absorption rate (SAR) patterns are shown for this antenna. In vivo experiments in dog prostate demonstrate that temperatures > 42 degrees C can be obtained > 1 cm away from the catheter, while maintaining a maximum urethral temperature of 47 degrees C to 48 degrees C. Histology obtained acutely after the hyperthermia treatments showed minimal damage to the periurethral tissues. We conclude from these studies that this microwave applicator is capable of providing local hyperthermia to the prostatic tissues with a predictable and well-circumscribed thermal distribution.

Interstitial microwave-induced hyperthermia and iridium brachytherapy for the treatment of obstructing biliary carcinomas.

In a phase I clinical study, 10 patients with obstructive biliary carcinomas were treated with single-antenna interstitial microwave hyperthermia and iridium-192 brachytherapy. For each patient a standard biliary drainage catheter was implanted percutaneously through the obstructed common bile duct. This catheter accommodated a single microwave antenna which operated at 915 MHz, and one or two fibreoptic thermometry probes for temperature measurement. Under fluoroscopic guidance the microwave antenna and temperature probes were positioned in the CT-determined tumour mass. The 60-min heat treatment achieved a central tumour temperature of 45-55 degrees C while keeping temperatures at the proximal and distal margins at 43 degrees C. Immediately following the hyperthermia treatment the microwave antenna and temperature probes were removed, and a single strand of iridium-192 double-strength seeds was inserted to irradiate the tumour length. A dose of 5500-7900 cGy calculated at 0.5 cm radially from the catheter was administered over 5-7 days. Upon removal of the iridium a second hyperthermia treatment was performed. A total of 18 hyperthermia treatments were administered to the 10 patients. In two cases the second hyperthermia treatment after brachytherapy was not possible due to a kink in the catheter, or bile precipitation in the catheter. All patients tolerated the procedure well, and there were no acute complications. To evaluate the volumetric heating potential of this hyperthermia method, specific absorption rate (SAR) values were measured at 182 planar points in muscle phantom. Insulated and non-insulated antenna performance was tested at 915 MHz in a biliary catheter filled with air, saline, or bile to mimic clinical treatments. The insulated antenna exhibited the best performance. Differences between antenna performance in saline and bile were also noted. In summary, this technique may have potential for tumours which obstruct biliary drainage and are accessible to percutaneous decompression using standard diagnostic radiological procedures.

Transient finite element analysis of thermal methods used to estimate SAR and blood flow in homogeneously and nonhomogeneously perfused tumour models.

A two-dimensional time-dependent finite element model was developed to evaluate thermal techniques for estimating blood flow and specific absorption rate (SAR). In these computer simulations, homogeneously and nonhomogeneously perfused tumour models were heated by a 915 MHz interstitial microwave antenna array. Representative blood flow values were assigned within the tumour, and the applied SAR distribution was based on a previously developed antenna theory. SAR values were estimated from the power-on transient temperatures, and blood flow values were estimated from thermal clearance data after power was discontinued. These estimated parameters were then compared to the known 'true' blood flow and SAR values throughout the treatment region. SAR values could be predicted with reasonable accuracy throughout most of the heated region independent of local blood flow. For a homogeneous model, thermal clearance was found to yield reasonably accurate blood flow estimates at high perfusion rates and less accurate estimates at lower perfusion rates. However, for the inhomogeneous model, the blood perfusion estimates were generally poor, and an average blood flow value for the tumour was obtained with little ability to resolve the differences in perfusion between regions. Using temperatures observed early in the cool-down curve resulted in improved spatial resolution, but increased the contribution of thermal conduction to the blood flow estimates. A single time-constant exponential thermal decay curve was found to be a necessary but not sufficient condition for reliable blood flow estimates using this technique.

Interstitial hyperthermia and iridium brachytherapy in treatment of malignant glioma. A phase I clinical trial.

An oncolytic effect of hyperthermia in the 42 degrees to 43 degrees C range has been previously demonstrated in cell culture and animal models. To apply this modality clinically, an interstitial microwave antenna array system has been developed for the delivery of controlled hyperthermia to an intracranial tumor volume, and a Phase I clinical trial involving six patients with malignant gliomas was undertaken. The protocol to study technical feasibility and patient tolerance combined interstitial iridium-192 irradiation and interstitial hyperthermia with 60-minute hyperthermia sessions immediately before and after brachytherapy. After-loading catheters suitable for both treatment modalities were implanted using a computerized tomography-assisted technique. Thermometry data confirmed the ability of a microwave antenna system to achieve reliable temperature distributions, and reasonable patient tolerance was documented.

Intraoperative interstitial microwave-induced hyperthermia and brachytherapy.

Intra-operative placement of 11-gauge nylon catheters into deep-seated unresectable tumors for interstitial brachytherapy permits localized heating of tumors (hyperthermia) using microwave (915 MHz) antennas which are inserted into these catheters. Four preliminary cases are described where epithelial tumors at various sites were implanted with an antenna array and heated for 1 hour, both before and after the iridium-192 brachytherapy. Temperatures were monitored in catheters required for the appropriate radiation dosimetry but not required for the interstitial microwave antenna array hyperthermia (IMAAH) system. Additional thermometry was obtained using nonperturbed fiberoptic thermometry probes inserted into the catheters' housing antennas. No significant complications, such as bleeding or infection, were observed. This approach to cancer therapy is shown to be feasible and it produces controlled, localized hyperthermia, with temperatures of 50 degrees C or more in tumors. This technique may offer a therapeutic option for pelvic, intra-abdominal and head and neck tumors.