The efficiency of clinical microwave applicators measured by a calorimetric method.

When inducing localized hyperthermia for superficial cancer therapy with microwaves there has often been question about the total power output from the applicator. Although specific absorption rates and thermograms are used to obtain localized power distributions and heating patterns, these provide, at best, only an approximation of the total power applied to tissues or phantoms. In this paper a calorimetric technique for obtaining total microwave output power from applicators is described. An experimental apparatus was constructed and it was found to be accurate to approximately +/- 5 W. The power output from four clinical microwave applicators as a function of applied electric power was measured and the efficiency was found to be 40% in average. Along with enhancing quality assurance, the areas of hyperthermia research which may benefit the most from this calorimetric technique are computer modeling and patient treatment planning.