The impact of the waveguide aperture size of the 3D 70 MHz AMC-8 locoregional hyperthermia system on tumour coverage

The 70 MHz AMC-4 system, with one ring of four waveguides, provides 2D power steering. The newly developed AMC-8 system enables 3D steering, using two rings of four 70 MHz waveguides. The current waveguide aperture size is 20.2 × 34.3 cm2. Waveguides and water boluses cover a large area of the body, which is not ideal for short patients. The aim of this study is investigating the impact of smaller waveguides on tumour coverage, using treatment planning. Finite-difference time-domain simulations were performed at 2.5 × 2.5 × 5 mm3 resolution. Virtual AMC-8 systems with waveguide aperture sizes of 20.5 × 34.25, 17.5 × 34.25, 14.5 × 34.25, 11.5 × 34.25, 8.5 × 34.25 cm2 and the AMC-4 system were modelled. Simulations were performed for elliptical (36 × 24 × 100 cm3) tissue-equivalent phantoms and for five cervical cancer patients. For the phantoms Sratio (SARmax_border/SARtarget) was evaluated for standard and optimized settings. For the patients, temperature distributions were evaluated after optimization of tumour temperature, while limiting normal tissue temperatures to 45 °C. Phantom simulations showed a favourable Sratio for all two-ring systems, compared to the AMC-4 system, for optimized phase-amplitude settings. Patient simulations demonstrated that the improvement in T90 for the operational AMC-8 system was ∼0.5 °C. This improvement was independent of the aperture size. The average number of imminent hot spots and their total volume was almost comparable for 8.5 and 20.5 cm wide apertures, but the locations were different. Two-ring waveguide systems with eight antennas and aperture sizes in the range from 20.5 × 34.25 cm2 to 8.5 × 34.25 cm2 showed a stable gain in tumour temperature compared to a single-ring four-antenna system with 20.5 × 34.25 cm2 wide apertures.