A multiple-diaphragm system for a cesium 137 unit.

Abstract

The 540-rhm (roentgens per hour at 1 meter) commercial cesium-137 teletherapy unit which was purchased for use at the Radiation Center of Cook County Hospital, Chicago, Ill., was provided with a considerable number of cones and cone extension devices, almost all of which gave entirely unrealistic treatment areas. The square areas were either too small or too large, and the rectangular ones too long and too narrow. After attempts to get the manufacturer to provide usable sizes proved futile, we decided to build a set of suitable diaphragms in our shops. Figure 1 shows the general scheme. A brass frame, attached to the 27-cm. cone, serves as a holder for the diaphragm drawers. The drawers are brass boxes, 4 1/4 inches square, filled with lead, and with square and rectangular openings, which serve as the diaphragms. The walls of the openings are tapered to coincide with the divergence of the beam. The lead is 2.86 cm. thick, and transmits 5 per cent of the cesium-137 primary gamma radiation. The holes in the lead are of such a size that the areas of the projected beam at 50 cm. are those that are clinically desirable. The field sizes provided are 5×5 cm., 6×8 cm., 7×7 cm., 8×10 cm., 8×15 cm., 10 × 10 cm., 10 × 15 cm., 12 × 15 cm., 15 × 15 cm. The drawer with the smallest opening weighs only nine pounds. All other drawers weigh less. They slide in and out easily and present no handling problem to the female technicians. At a 50-cm. source-skin distance, the bottom of the drawer is 18 cm. from the patient. This distance keeps penumbra to a reasonable value, and also keeps the electron contamination from being of consequence. A small 12-volt automobile headlight bulb is mounted about halfway up on one inside wall of the cone, and is located in the central beam. Since the bulb is nearer the patient than the source, the visible light beam would be larger than the gamma radiation field. To overcome this, a thin aluminum plate is attached permanently to the bottom of each drawer. The plate has a hole, smaller but the same shape as the opening in the lead diaphragm. The hole in the aluminum is cut to such a size that the visible light and the gamma radiation fields correspond at 50 cm. distance. A pair of crossed wires cast shadows, and the intersection of the shadows indicates the center of the field. Since we sometimes use a 35-cm. distance, there are a series of very small holes drilled in the plate, the visible light from which indicates the gamma-ray field at 35 cm. distance. A visual distance indicator is provided by a simple range-finding device. A pencil light source, which projects a small visible “S,” is mounted on the outside of the cone. It is permanently directed so that when its “S” centers on the cross-hair shadows on the patient's skin, a distance of 50 cm. has been attained.