Large stationary microstrip arrays for superficial microwave hyperthermia at 433 MHz: SAR analysis and clinical data

Superficial hyperthermia with present day applicators provides a challenge when tumours exceed several cm in diameter. Unless microstrip applicators are scanned, the usable heat region often falls short of treating the entire region with 50% power or specific absorption rate (SAR). New microstrip applicator designs were evaluated through SAR analysis and compared to the traditional microstrip applicators used in the clinic at Dartmouth over the past six years. The new designs included fabricating thin archimedean spirals (1.0 mm strip width) incorporating dielectric substrate (epsilon = 5.3-10.8). The designs were optimized at 433 MHz for an arm length of 59 cm. Measurements in a plane 1.0 cm from the surface showed that thin spirals outperformed traditional designs by increasing the 50% SAR area by a factor of 2.5, while maintaining the same physical size. Arrays of four elements were fabricated from thin spirals, although SAR evaluation showed only 10-20% SAR between elements. Since this was deemed unacceptable and the design goal was to fabricate a stationary applicator that had at least 50% SAR between elements, dual element designs were created with gradually overlapped elements. It was found that overlapping three coils of the spiral created a large region that equalled or exceeded 50% SAR that could not be matched by single applicators. Coherent operation of the dual spiral array resulted in more central power deposition and incoherent operation resulted in more peripheral power deposition. SAR measurements at the fat/muscle interface showed an elongated heating pattern in hydroxyethylcellulose muscle equivalent phantom. Power deposition 1.0 cm deep in muscle retained the same basic size and shape with or without the fat layer. Patient treatments for chestwall tumours confirmed that the dual overlapping applicator heated a larger region without the sharp temperature peak associated with single applicators.