A theoretical comparison of the temperature distributions produced by three interstitial hyperthermia systems

Three interstitial hyperthermia systems were compared on the basis of their abilities to heat tumors: ferromagnetic seeds, microwave antennas, and rf electrode needles. The theoretical calculations were performed using two-dimensional tumor models containing static tissue and electrical properties and various blood flow patterns. Basic assumptions were kept the same to permit a meaningful comparison of the different systems. Power deposition patterns were calculated based on the appropriate theoretical expressions. The bioheat transfer equation was solved using a finite element method. Temperature distributions were calculated for four 1 or 2 mm diameter implants on the corners of 1, 2, and 3 cm squares. The tumor was assumed to be circular with a diameter 1 cm longer than the diagonal of each square. Three blood flow models were considered: homogeneous, nonhomogeneous, and concentric annulus perfusion models. The blood flow ranged from 0 to 100 ml/100 g/min with the ratio of tumor/normal tissue blood flow ranging from 0 to 40. A Hyperthermia Equipment Performance (HEP) rating was used as a criterion for comparing the temperature distributions from the various cases examined. As expected, the higher HEP ratings were generally produced by decreasing the spacing between the implants and/or lower blood perfusion rates. Under the conditions of this study, the microwave antennas were able to adequately heat a larger number of cases to therapeutic temperatures than either of the other two modalities.