Improved power steering with double and triple ring waveguide systems: The impact of the operating frequency

Introduction: Regional hyperthermia systems with 3D power steering have been introduced to improve tumour temperatures. The 3D 70-MHz AMC-8 system has two rings of four waveguides. The aim of this study is to evaluate whether T90 will improve by using a higher operating frequency and whether further improvement is possible by adding a third ring. Methods: Optimised specific absorption rate (SAR) distributions were evaluated for a centrally located target in tissue-equivalent phantoms, and temperature optimisation was performed for five cervical carcinoma patients with constraints to normal tissue temperatures. The resulting T90 and the thermal iso-effect dose (i.e. the number of equivalent min at 43°C) were evaluated and compared to the 2D 70-MHz AMC-4 system with a single ring of four waveguides. FDTD simulations were performed at 2.5 × 2.5 × 5 mm3 resolution. The applied frequencies were 70, 100, 120, 130, 140 and 150 MHz. Results: Optimised SAR distributions in phantoms showed an optimal SAR distribution at 140 MHz. For the patient simulations, an optimal increase in T90 was observed at 130 MHz. For a two-ring system at 70 MHz the gain in T90 was about 0.5°C compared to the AMC-4 system, averaged over the five patients. At 130 MHz the average gain in T90 was ∼1.5°C and ∼2°C for a two and three-ring system, respectively. This implies an improvement of the thermal iso-effect dose with a factor ∼12 and ∼30, respectively. Conclusion: Simulations showed that a 130-MHz two-ring waveguide system yields significantly higher tumour temperatures compared to 70-MHz single-ring and double-ring waveguide systems. Temperatures were further improved with a 130-MHz triple-ring system.