Thermal Dose Expression in Clinical Hyperthermia and Correlation with Tumor Response/Control1

Thermal dose has been identified as one of the most important factors which influence the efficacy of hyperthermia. Adequate temperature must be delivered for an appropriate period of time to the entire tumor volume in order to achieve optimal therapeutic results. Present clinical thermometry systems provide coarse temperature readings, since only selected tumor or normal tissue temperatures are monitored. Experimental in vitro and in vivo data suggest that the minimal temperature observed in the tumor determines therapeutic effectiveness. Unfortunately, at the pres ent time, clinical data documenting these observations are scarce. The inhomogeneity of temperature distribution through out the tumor volume makes difficult accurate correlations with tumor response and subsequent tumor control. Several mathematical models have been offered to express the time-temperature equivalency in relation to a reference tem perature (43°equivalent). Factors such as step-down heating, fractionated hyperthermia, thermal adaptation, and combination with irradiation, in addition to physiological parameters such as blood flow, play a major role in the expression of thermal dose. In order to meaningfully express thermal dose in clinical hyper thermia, several procedures are recommended, such as static phantom studies of specific absorption rate distributions for heat delivery equipment, detailed thermal mapping in hyperthermia sessions, development of reliable predictive biomathematical models to express temperature-time equivalency, and the fos tering of research in 3-dimensional noninvasive clinical thermom etry.