Regional power deposition for hyperthermia: theoretical approaches and considerations.

Physical analysis of electromagnetic and ultrasonic fields is useful to define systems capable of depositing power in "deep" tumors extending more than a few cm from the skin surface. Several recent analyses of electromagnetic field configurations for treating such tumors are reviewed here. Most electromagnetic systems result in exposure of substantial normal tissue volumes or regions to unfocused power absorption, and safely achieving intratumoral temperature elevation to greater than or equal to 42 degrees relies in part upon higher blood flow rates existing in normal tissue than in tumor. Optimizing therapeutic approaches to deep tumors with devices producing regional power deposition may thus require improved knowledge and control of regional blood flow distributions, along with further development of energy sources appropriate for specific deep sites. Multiple electric dipole sources with separate phase and amplitude control appear to be the most general noninvasive electromagnetic solution at present for deep heating. In specific locations, focused ultrasound is also theoretically capable of producing advantageous power deposition at depth.