Physical and Clinical Aspects of Supervoltage Rotational Therapy

Abstract

Two of the more significant trends in radiation therapy today are the utilization of x-rays produced at higher voltages and the more adequate and uniform administration of this radiation by rotational and multiportal methods. This paper will consider the physical and clinical aspects of 2-million-volt x-ray therapy combined with rotational administration, with particular reference to the treatment of tumors of the head and neck. Choice of X-Ray Voltage The trend toward higher voltages is based on the undisputed evidence that such radiation has improved qualities of penetration, freedom from scatter, and skin tolerance at the portal of entry. Our selection of 2-million-volt x-rays as the most suitable for the general therapy problem was made after conducting studies over the l-million to 4-million-volt range (1) and was based on fundamental and practical considerations, including the following: 1. X-rays produced at 2 million volts attain most of the desired physical properties of increased depth dose, high output intensity, uniform field illumination, and minimum beam scatter. With a v-cm.square field, 2-million-volt radiation gives more than twice the percentage depth dose attained with 200-kilovolt radiation at 10 cm., and three times the dose at 15 cm. depth, with far less side-scatter. These characteristics are quantitatively shown for a 5-cm.wide beam of 2-million-volt x-rays in the isodose curve of Figure 1. Although depth dose continues to increase with voltage, this increase is accompanied by developing nonuniformity in field illumination and increasing exit dose. It is our judgment that, for the general cancer problem, involving both large-and small-field irradiation and both bone and soft-tissue tumors, the optimum radiation qualities are attained broadly in the 2- to 5-million-volt range. Practical and clinical considerations, some of which are indicated below, determined our preference for 2 million volts. 2. Skin tolerance has reached a satisfactorily high level at 2 million volts (2). About 4,000 r delivered at normal incidence in daily doses of 300 r are required to elicit a threshold erythema on the skin at the portal of entry.