Devices and techniques for the clinical application of concomitant heat and ionizing radiation by external means

For more than two decades it has been known that the concomitant application of heat and ionizing radiation results in an enhanced cell killing. Nevertheless, most of the clinical trials combining radiotherapy and hyperthermia have employed sequential delivery schemes of these two cancer therapy modalities. In other words, the hyperthermia treatments have usually been given either before or after the radiation treatments. The concomitant delivery of both therapies has been discouraged due to many logistical problems mostly related to the cost effective utilization of radiation treatment machines, and also due to the fact that hyperthermia systems have not yet been developed specifically for this purpose. Although many clinical trials have shown that sequential thermoradiotherapy is more beneficial than radiotherapy alone, extensive thermal dosimetry studies have clearly shown that tumors consistently failed to achieve therapeutic temperature elevations for adequate periods of time. This is considered one of the major problems in clinical hyperthermia today. Therefore, one way to maximize the therapeutic effects of hyperthermia in clinical practice is to develop new heating systems that both improve thermal coverage of tumors and that allow the concomitant delivery of ionizing radiation. This paper describes the devices and techniques that have been developed and that are under development at our institution to achieve both of these goals in the treatment of superficial cancerous tumors by external means. Three systems are described. The first system utilizes single waveguide (915 MHz) microwave applicators for superficial hyperthermia and a Cobalt-60 teletherapy unit. The second system consists of a modified multielement planar ultrasound applicator and the Cobalt-60 unit. This approach takes advantage of the properties of ultrasound to remove all nonuniformly perturbing parts out of the radiation beam path. Both of these systems have successfully been used clinically in a phase I/II trial, thus establishing both the technical and clinical feasibility of this combined therapy. The third system is a novel design that utilizes an ultrasound linear array and a scanning acoustic reflector to minimize the amount of water needed to transfer the sound waves from the ultrasound source to the target volume. The conceptual design of this system is described and its advantages in comparison with the two other systems are discussed. Numerical simulations of both acoustic power and temperature distributions are reported which show that the simplest (unoptimized) design of the scanning ultrasound reflector linear array system is at least as effective as the static ultrasound array applicators in current clinical use.