Radiation Dose Distribution in Water for 22.5-Mev-Peak Roentgen Rays

This paper describes an investigation of the distribution of dose produced in water by 22.5-Mev-peak x-rays, Central-axis depth doses and several representative isodose distributions for a fixed–position betatron have previously been published (1, 2). The Memorial Center betatron, however, which was used in this study, differs from others previously constructed in its shielding, compensation, collimation, and suspension system. It was designed to be flexible in position so as to permit rapid, accurate alignment of an x–ray or electron beam for therapy. Because it is the model for a number of units now being installed for therapeutic purposes, a detailed study of the collimation system and compensating x–ray filter was made to determine their effect upon dose distribution in water. Instrumentation The data were obtained with an isodose recorder (3) which automatically analyzes and plots the relative space distribution of dosage produced by a radiation beam in air or water. This automatic isodose plotter (Fig. 1) consists of four units: a dual channel D.C. amplifier, a self-balancing ratio unit, a servo-mechanism for controlling the position of a probe detector in a water phantom, and a recorder for plotting the position of the probe detector. The equipment used in the room with the betatron is illustrated in Figure 2. The wax-covered ionization chamber has a cylindrical air cavity 5 mm. in diameter and 5 mm. in length. This chamber is rigidly mounted on a preamplifier containing a Victoreen VX-5800 electrometer tube and a 1 X 1012 ohm input resistor. These two units are coated with wax so that they may be immersed in the water phantom. The entire detector assembly is attached to a carriage which can be moved along three perpendicular axes by means of a system of Selsyn motors. Figure 2 shows the equipment arranged for irradiation of the water phantom from above. This permits measurement of the radiation distribution from the surface of the water to a depth of 20 em. Measurements can also be made at greater depths if irradiation is done through the 5–mm. Lucite window on the side of the water phantom. A schematic diagram of the complete isodose recorder system is given in Figure 3. With this system, a direct measurement of the signal from the probe can be made. In order to obtain the space distribution of air cavity ionization in water, fluctuations of the incident x-ray intensity must be balanced out. This is accomplished by the use of a fixed reference chamber. The ratio of the amplified outputs of the probe and reference chamber is independent of variation in the x–ray intensity and can be read directly on the register of the “ratio unit.” This equipment can also be used for automatic plotting of isodoses.