Some Factors Influencing the Distribution of Radiation Using a Two-oblique-field Technic

Abstract

IN the treatment of pelvic malignancies by means of x-radiation, there has been during the past decade a progressive increase in the target-skin distance, resulting in an increased intensity at depth. To-day 200 kv. x-ray treatments are given at target-skin distances of 80 centimeters or higher. Obviously the simplest technic is to use one large 20 × 20 cm. field anteriorly and posteriorly. The large field radiates the entire pelvis and due to the scattering from such a large volume of irradiated tissue, the depth intensity at a point 10 cm. below the skin may be as high as 50 per cent of the skin dose. Mattick (1) has published tables simplifying the procedures in calculating the dose by this technic. In cases in which still greater intensities at depth are desired four or even six fields may be applied. Following the suggestion of Weatherwax (2) long narrow rectangular fields should be used, two anteriorly and two posteriorly (and two lateral) and the radiation directed so as to overlap in the midpelvic region. This latter method was adopted at the State Institute several years ago with the angulation of the beams and the separation between the two anterior and two posterior ports predetermined. In view of the fact that the patient was treated at one sitting with only one, or at most two, ports per day over a period of from 40 to 50 days, a variation from day to day in the angulation or spacing of the ports could be very easily introduced. The question arose, therefore, as to how significant such slight daily variations might be in routine treatments. The present paper deals with ionization measurements made in a water phantom which was radiated by two simultaneous beams of radiation in order to arrive at some conclusions regarding variations in the set-up and the resulting errors. The physical set-up for these measurements was as follows: Two XPT type tubes excited simultaneously by one generator, were operated at 200 kv.p. and 10 ma. The intensity in air and the half-value layer of copper were determined for each tube separately with the radiation from one of the tubes cut off by means of a lead shutter. The output from each agreed with that of the other and, in addition, the quality of the beam was the same as with a single tube operated at the same voltage and 25 ma., namely, 1.0 mm. copper half value layer. Each tube could be rotated on its long axis so as to permit the adjustment of the angulation of the two beams. The radiation from both tubes was confined and directed by means of wood treatment cones lined with lead rubber for the entire treatment distance of 80 centimeters. The fields were 10 × 15 centimeters. With the long axes of the fields parallel the radiation was directed into the water phantom with the following angulation and spacing: 1. Each beam inclined 6° away from the perpendicular. 2. Each beam inclined 11° away from the perpendicular. 3. Each beam inclined 16° away from the perpendicular. A second adjustment was provided so that the phantom end of the treatment cones in each of the three angulations could be touching, separated 2 centimeters or 4 centimeters, giving, in all, nine combinations.