Heating technology for malignant tumors: a review

Abstract

The therapeutic application of heat is very effective in cancer treatment. Both hyperthermia, i.e., heating to 39-45 °C to induce sensitization to radiotherapy and chemotherapy, and thermal ablation, where temperatures beyond 50 °C destroy tumor cells directly are frequently applied in the clinic. Achievement of an effective treatment requires high quality heating equipment, precise thermal dosimetry, and adequate quality assurance. Several types of devices, antennas and heating or power delivery systems have been proposed and developed in recent decades. These vary considerably in technique, heating depth, ability to focus, and in the size of the heating focus. Clinically used heating techniques involve electromagnetic and ultrasonic heating, hyperthermic perfusion and conductive heating. Depending on clinical objectives and available technology, thermal therapies can be subdivided into three broad categories: local, locoregional, or whole body heating. Clinically used local heating techniques include interstitial hyperthermia and ablation, high intensity focused ultrasound (HIFU), scanned focused ultrasound (SFUS), electroporation, nanoparticle heating, intraluminal heating and superficial heating. Locoregional heating techniques include phased array systems, capacitive systems and isolated perfusion. Whole body techniques focus on prevention of heat loss supplemented with energy deposition in the body, e.g., by infrared radiation. This review presents an overview of clinical hyperthermia and ablation devices used for local, locoregional, and whole body therapy. Proven and experimental clinical applications of thermal ablation and hyperthermia are listed. Methods for temperature measurement and the role of treatment planning to control treatments are discussed briefly, as well as future perspectives for heating technology for the treatment of tumors.

Keywords: HIPEC; ablation; heating equipment; hyhperthermia; thermal therapy.

Figure 1:

A: Characteristics of the three main treatment approaches in terms of temperature level. B: Time to yield muscle fibrosis (continuous curve) or necrosis (dotted curve) at specific temperatures for hyperthermia and ablation (based on Dewhirst et al [20]).

Figure 2:

Schematic representation of the heating mechanisms of different clinical heating techniques: Electromagnetic (i.e. capacitive, radiofrequency, microwave, infrared and laser heating), focused ultrasound, hyperthermic perfusion and conductive heating.

Figure 3:

Left: schematic overview of the heating depth from the skin and the tumor/target size that can be heated with various non-invasive heating techniques. Right: Schematic overview of the tumor/target size that can be heated with various invasive heating techniques.

Figure 4:

Graphical representation of different heating techniques.

Figure 5:

Trends in heating technologies, represented by a rough estimate of the number of publications from 1995 till present on whole body hyperthermia (WBH), hyperthermia combined with radiotherapy (HT+RT), hyperthermia combined with chemotherapy (HT+CHT), hyperthermic intraperitoneal chemotherapy (HIPEC) and thermal ablation. Search terms used in Web of Science were: (WBH OR whole body hyperthermia; (hyperthermia AND radiotherapy) OR thermoradiotherapy; (hyperthermia AND chemotherapy) OR thermochemotherapy; HIPEC OR Hyperthermic Intraperitoneal Chemotherapy; thermal ablation OR RFA OR MWA). NB: Although this search did probably not retrieve all relevant publications and some overlap is likely between topics (e.g. HT+CHT and HIPEC), this graph is indicative for the trends in interest for different heating techniques.