Capacitive heating devices for therapeutic hyperthermia: a review of essential physical characteristics and challenges

Therapeutic hyperthermia is typically applied using radiative or capacitive heating devices. Capacitive heating applies electrode pairs positioned on the patient to increase the tumor temperature and is considered a user-friendly method for relatively easy application of hyperthermia to both superficial and deep-seated tumor sites. However, this heating technique also has some limitations that are mainly inherent to the physics of capacitively induced heating. This review provides an overview of the principles of capacitive heating devices and the factors influencing the power absorption and resulting temperature distribution in the patient. Device parameters that strongly influence the achieved temperature distribution include the electrode sizes, the water bolus temperature for skin cooling, the cooling medium, and the output power. These parameters vary in commercially available devices. Complete characterizations of most of the commercially available capacitive devices are still lacking. Sparse phantom measurements in the literature characterizing capacitive devices indicate that therapeutic heating is at least possible for superficial tumors and tumors at intermediate depth. The dominant E‑field orientation with capacitive heating induces preferential subcutaneous fat heating, which limits deep heating and makes it most effective for slender patients (i.e., fat layer thickness < 1.5-2 cm). Numerical simulations have been helpful in optimizing device design, particularly in terms of bolus cooling, and are increasingly used for patient-specific treatment planning. Based on the physical characteristics and present literature, it can be concluded that appropriate patient selection is important to ensure effective and responsible use of capacitive heating devices.