Microwave Urethral Catheter Development for Cancer Treatment, BPH and Balloon Angioplasty: US Patents 4,612,940 and 4,700,716.
A multiple patented microwave urethral catheter involves a miniature, directional antenna array inside a 22F catheter shaft for directing the microwave energy to a specific volume of prostatic tissue to be treated by microwave thermotherapy. BPH treatment by thermal means generally requires temperatures in the 60 – 70 C range, well above hyperthermic temperatures. The critical requirements of proper dosimetry and a precision temperature pattern within the tissue volume or prostatic lobe will be met at reduced power levels as compared to the competition. Power directionality is insured by controlling the sidelobe and backlobe energy using parasitic antenna elements in close proximity to the radiating element connected to the generator. Most of the radiated power will therefore occur in the main beam direction. Measured power and dosimetry requirements are in the 15-25 watt range of transmitter power and 15-30 minutes of application as compared to the published 60 watts for 60 minutes of microwave thermotherapy for BPH treatment. This new directional antenna array design represents a main beam spread of microwave heating power in azimuth of approximately 60 degrees or less as compared to the usual 360 degree spread in azimuth created by conventional magnetic or electric dipoles
The patented technology provides lower transmitter power, precision in establishing heating pattern boundaries without the need for additional cooling requirements greater safety, and lower equipment cost [i.e., smaller] microwave generators. Finally, microwave leakage along the cable connecting the array to the generator is eliminated by a patented choke system built into the antenna. (Systems with microwave leakage make it difficult to define the location of the heating pattern.) Conventional fiber optics and reflected power measurements are employed for temperature sensing and signal feedback to the generator system for power control to insure appropriate prostate temperatures for optimum treatment protocols.
Overview of Recently Proposed Technology
A miniature and flexible microwave antenna system as energy source with diagnostic capability for precise tissue heating, monitoring, and control is proposed. The basic antenna components are the result of years of development of miniature radiating elements in catheters by Kasevich in applications such as BPH and microwave balloon angioplasty and other non-medical applications using radio frequencies. The shape and volume of the tissue to be heated is provided by the design of the antenna system with specific material properties to control the thermal pattern shape, depth of energy penetration, and temperature distributions at a given frequency. Control of volume temperature close to the energy source below 41 C is provided by separated but relatively localized energy sources. They electromagnetically focus separately on the target volume to achieve the desired temperature difference between the targeted volume and boundaries to insure no tissue damage to surrounding tissue.
The proposed miniature system will provide the required target temperature and energy delivery time and be capable of either non-invasive or minimally invasive application without damage to the surrounding tissue. Impedance spectroscopy methods will be employed for real time display and monitoring the volume tissue electrical impedance vs. frequency which depends on the target volume size and temperature. Any measured volume or unwanted temperature deviation during treatment is immediately discernible based on a previously calibration of the tissue impedance properties related to antenna design and tissue electrical conductivity and dielectric permittivity (dielectric constant).
A fiber optic temperature probe will be an integral part of the antenna system to provide real time measurement of temperature between the target volume and energy source. The input impedance of the energy source target (antenna) will change with time and frequency and can be monitored and displayed by a portable network analyzer as contours of impedance vs. frequency which is the basis for impedance spectroscopy.