A comparison of the heating characteristics of capacitive and radiative superficial hyperthermia

Superficial hyperthermia is applied in combination with radiotherapy for e.g. melanoma and recurrent breast cancer, using both capacitive and radiative systems. In this paper, numerical simulations are applied to address the question which technique yields the most favourable heating characteristics.

A 434 MHz contact flexible microstrip applicator (CFMA type 4H, size 19.6 × 19.6 cm²) and a capacitive system consisting of two circular electrodes with diameter 15 and 25 cm were modelled. The water bolus of the CFMA was filled with deionised water and for capacitive heating both saline and deionised water were modelled. Specific absorption rate (SAR) and temperature simulations were performed for a perfused muscle-equivalent phantom and phantoms with a 1 cm thick superficial fat layer, assuming cylindrical target regions. Subsequently, a real patient model with a chest wall recurrence was studied with the target assumed to have muscle-like properties, fat properties or heterogeneous properties as derived from the CT Hounsfield Units.

Phantom simulations showed that high SAR peaks occur around the bolus edges with capacitive heating. Power absorption below the fat layer is substantially higher for radiative heating and unless the target region is limited to the fat layer, radiative heating yields better target coverage in terms of SAR and temperature. Patient simulations showed that the T₉₀ for radiative heating was 0.4-1.1 °C higher compared with capacitive heating.

Radiative heating yields more favourable SAR and temperature distributions for superficial tumours, compared with capacitive heating, especially within heterogeneous tissues. Higher tumour temperatures are achieved without occurrence of treatment limiting hot spots.