Comparison of two different 70 MHz applicators for large extremity lesions: Simulation and application

Abstract

Introduction: Motivation for this research was a patient with large and bulky melanoma lesions on a leg, treated with hyperthermia in a special set-up with an open water bolus and two opposing applicators. Treatment planning was used to find the most suitable heating method, comparing 70 MHz capacitive contact flexible microstrip applicators (CFMAs) and 70 MHz waveguides.

Methods: The first three sessions were performed with CFMA applicators; the last session with waveguides. Power and water temperature were adjusted to achieve clinically relevant temperatures. Finite difference time domain (FDTD) simulations were performed for a CFMA and waveguide on a fat-muscle geometry to compare effective field size (EFS) and effective heating depth (EHD). A CT scan of the patient's leg was automatically segmented into muscle, fat and bone; tumour lesions were outlined manually. Patient simulations were performed to evaluate the 3D heating pattern and to compare CFMAs and waveguides for equal power and water temperature.

Results: Hyperthermia treatment was well tolerated. Temperature measurements indicated mainly superficial heating with CFMAs. Simulated EHD was 2.1 and 2.4 cm for CFMA and waveguide, respectively and EFS was 19.6 × 16.2 cm² and 19.4 × 16.3 cm². Simulation results showed a better tumour coverage using waveguides; absorbed power in the tumour was ∼75% higher with waveguides and absorption in fat was approximately twice as high with CFMAs. Simulations showed that a relatively high water temperature (∼42°C) improves the overall temperature distribution.

Conclusion: CFMAs and waveguides have a similar EFS and EHD, but for large extremity lesions, the performance of 70 MHz waveguides is favourable compared to 70 MHz CFMA applicators.

• Hyperthermia
• RF heating
• capacitive heating
• waveguides
• CFMA applicators
• hyperthermia treatment planning

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