Contrasting Heating Patterns and Efficiency of the Prostatron and Targis Microwave Antennae for Thermal Treatment of Benign Prostatic Hyperplasia

Abstract

Objectives. To determine the design and performance characteristics of two microwave antennae for use in thermal treatment of benign prostatic hyperplasia. Methods. Prostatron and Targis antennae were subjected to detailed physical examination and measurement. The heating patterns generated by these two types of antennae were characterized in detail using tissue-equivalent phantoms. Measurements of return loss as a function of frequency were conducted to evaluate the capacity of the antennae for impedance matching. Percent reflected power was calculated from the return loss results to provide a relative measure of potential for efficient delivery of thermal energy. Results. The Prostatron antenna was found to be a monopole design consisting of a coaxial cable with a 3.3- cm length of inner conductor exposed at the tip. The Targis antenna was observed to be a dipole design with a 2.8-cm helical coil attached through a ground connection and a tap point to a coaxial cable. The heating pattern of the Targis antenna was symmetric; that of the Prostatron was asymmetric with substantial back heating along the catheter axis in the direction of the microwave power source. The mean extension of the 30°C isotherm in the direction of the power source with the Prostatron antenna (71.5 mm; 95% confidence interval [CI], 63.4 to 79.6 mm) was 55% greater ( P <0.0005) than that with the Targis antenna (46.0 mm; 95% CI, 38.2 to 53.8 mm). Return loss with the Targis antenna declined sharply to a relative minimum value of −32.9 dB (95% CI, −73.8 to 8.0 dB) at 915 MHz, providing evidence of this antenna’s capacity for impedance matching; little change was observed with the Prostatron in return loss over a frequency range 100 MHz above and below this antenna’s standard operating frequency of 1296 MHz. The mean reflected power of the Targis antenna (0.4%; 95% CI, 0.0% to 1.4%) was lower by more than 20-fold ( P = 0.036) than that of the Prostatron antenna (11.0%; 95% CI, 3.4% to 18.7%); thus, the potential for efficient operation was greater with the Targis than the Prostatron antenna. Conclusions. The Targis microwave antenna was found to provide a more targeted heating pattern and have a capacity for more efficient thermal energy delivery than the Prostatron antenna. These differences observed in vitro could potentially translate into clinical advantages in vivo, such as improved tolerability of microwave treatment, reduced risk of complications, greater thermoablative efficacy, and scalability.