Complications resulting from spurious fields produced by a microwave applicator used for hyperthermia.

Microwave hyperthermia applicators are generally characterized at two or three frequencies. These frequencies usually are at the extremes of the applicator's operating range. However, such applicators are often used at arbitrary frequencies within their range. The most common reason for choice of frequency is to achieve acceptable coupling in the clinical configuration with a given patient. Occasionally a spurious transmission pattern will result from the frequency chosen that can lead to undetected high power densities well away from the target volume. In the present report, such a situation is discussed. The transmission pattern resembled "horns" reported at the field edge of certain accelerators. This pattern resulted in the inability to heat the target volume and the production of thermal blisters outside that volume. Such an occurrence underscores the need to characterize the transmission characteristics of a microwave hyperthermia applicator at all the frequencies used.