Optimizing transurethral microwave thermotherapy: a model for studying power, blood flow, temperature variations and tissue destruction.

OBJECTIVE To examine the role of microwave power and blood flow on temperature variations and tissue destruction in the prostate, using a theoretical model of transurethral microwave thermotherapy (TUMT), and thus compare fixed-energy TUMT with no intraprostatic temperature monitoring (constant microwave power applied over a fixed period) with 'feedback' TUMT in which the microwave power is adjusted according to the monitored intraprostatic temperature. MATERIALS AND METHODS The temperature distribution in the prostate was modelled for a typical TUMT catheter at various blood flow rates. The volume of tissue destroyed was simultaneously calculated from cell survival data after thermal exposure. The calculated quantity of tissue destroyed at the different microwave power levels and blood flow rates was used to describe qualitatively the simulated treatments. RESULTS Treatment monitoring and consistency were better during feedback TUMT than fixed-energy TUMT, in that the former compensated for variations in blood flow rate. The modelled values agreed with observations during real TUMT. CONCLUSIONS Blood flow rate is a key factor in the outcome of TUMT. Only by measuring intraprostatic temperature is it possible to compensate for the large variations in prostatic blood flow and obtain consistent treatment results. Repeated interruptions prompted by high rectal temperatures should be minimized and preferably avoided, as the quantity of tissue destroyed is then greatly reduced, and in extreme cases the treatment is totally ineffective.