CONTRASTING HEATING PATTERNS AND EFFICIENCY OF THE PROSTATRON AND TARGIS MICROWAVE ANTENNAE FOR THERMAL-TREATMENT OF BENIGN PROSTATIC HYPERPLASIA

Objectives. To determine the design and performance characteristics of two microwave antennae for use in thermal treatment of benign prostat ic hyperplasia. Methods. Prostatron and Targis antennae were subjected to detailed physical examination and measurement. The heating pattern s generated by these two types of antennae were characterized in detai l using tissue-equivalent phantoms. Measurements of return loss as a f unction of frequency were conducted to evaluate the capacity of the an tennae for impedance matching. Percent reflected power was calculated from the return loss results to provide a relative measure of potentia l for efficient delivery of thermal energy. Results. The Prostatron an tenna 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 cab le. The heating pattern of the Targis antenna was symmetric; that of t he Prostatron was asymmetric with substantial back heating along the c atheter axis in the direction of the microwave power source. The mean extension of the 30 degrees C isotherm in the direction of the power s ource with the Prostatron antenna (71.5 mm; 95% confidence interval [C I], 63.4 to 79.6 mm) was 55% greater (P < 0.0005) than that with the T argis 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 an tenna's capacity for impedance matching; little change was observed wi th 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 Prostat ron antenna (11.0%; 95% CI, 3.4% to 18.7%); thus, the potential for ef ficient operation was greater with the Targis than the Prostatron ante nna. Conclusions. The Targis microwave antenna was found to provide a more targeted heating pattern and have a capacity for more efficient t hermal 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, reduce d risk of complications, greater thermoablative efficacy, and scalabil ity. (C) 1998, Elsevier Science Inc. All rights reserved.