Correlations of thermal washout rate, steady state temperatures, and tissue type in deep seated recurrent or metastatic tumors.

The rates of cooling ("thermal washout") in selected sites in tumor and adjacent normal tissues following the completion of clinical hyperthermia sessions were analyzed in ten patients treated with combined radiation and hyperthermia for deep seated recurrent or metastatic tumors. The temperatures were recorded at 10 second intervals for at least 2 minutes after the cessation of microwave power at the end of the 30-60 minute duration hyperthermia treatments. These thermal washouts were characterized by the slope of a log-linear relation between temperature elevation above the oral baseline temperature and time. Washout rates (expressed as a perfusion rate in ml/100g-min) significantly correlated with tissue categories as noted on CAT scan (i.e., tumor, normal tissue, tumor/normal tissue interface, hypodense tumor areas). Relationships between thermal washout rate and steady-state temperature elevation were tested and also showed significant correlations in general and for some specific tissue categories. The implications of these findings in explaining inhomogeneities in heating patterns, and in hyperthermia treatment modeling will be presented.