Dosimetry and techniques for simultaneous hyperthermia and external beam radiation therapy

An increased biological effect is realized when hyperthermia and radiation therapy are combined simultaneously. To take advantage of this effect, techniques have been developed that combine existing hyperthermia devices with a linear accelerator. This allows concomitant delivery of either ultrasound or microwave hyperthermia with photon radiation therapy. Two techniques have been used clinically: the orthogonal technique, in which the microwave or ultrasound beam and the radiation beam are orthogonal to one another, and the en face technique, in which the ultrasound or microwave beam and the radiation beam travel into the tumour through the same treatment window. The en face technique has necessitated the development of special attachments so that the hyperthermia device can be mounted to the linear accelerator and so that non-uniform portions of the hyperthermia device can be removed from the radiation beam. For microwave therapy, applicators are mounted onto the linear accelerator using the compensating filter tray holder. For ultrasound, special reflector devices are mounted to a frame that is mounted onto the compensating filter tray holder of the linear accelerator. Because the linear accelerator is an isocentric device, the height of the radiation source is fixed, and this has necessitated specially designed devices so that the ultrasound support system is compatible with the linear accelerator. The treatment setups for both the en face technique and the orthogonal technique require the interaction of both hyperthermia and radiation therapy personnel and equipment. The dosimetry and day-to-day operations for each technique are unique. The simulation for the en face technique is much different from the simulation of a normal radiation treatment and requires the presence of a hyperthermia physicist. Also, for the en face technique, the attenuation of the microwave applicator and the thickness and attenuation of the ultrasound reflector system are taken into account for radiation dosimetry. This paper presents details of the dosimetry and logistics of the techniques for simultaneous thermoradiotherapy based on 7 years of experience treating more than 50 patients.