An automatic depth-dose acquisition system.

Abstract

Three years ago, the Department of Radiology of the University of Florida, Gainesville, initiated computer programs for the IBM 709 digital computer for automatic radiation treatment planning. At that time, the data for any radiation field size were available only in the form of iso-dose curves; a data presentation not immediately useful for computers. The computer requires data in digital form at discrete points rather than in a continuous form as presented by isodose curves. To prepare radiation field data for computer usage, we at first digitized the isodose curves by laying a sheet of translucentgraph paper over the curve and finding the dose value at each grid point on the graph. This method was time-consuming, and, in addition, it was inaccurate because of the many interpolations needed. To obtain the radiation field data quickly and accurately, we have devised a system which measures dose values at discrete points and punches them on IBM cards. Scanning System In general, an ionization-chamber-type detector is mechanically moved through a water phantom by an automatic scanning device. This scanner has previously been reported by Mauderli and Hazard (2). The scanning system was modified so that depth-dose data at points 5 mm apart could be recorded. Figure 1 is a block diagram of the system presently used to obtain depth-dose values. The first scan is made at the proper depth so that the maximum value is recorded. The second scan is made at a 1 cm depth, and all the following scans are made at depths increased by 1 cm. Once the scanning system is started, it will continue (scanning) automatically until a desired depth is reached, or it may be stopped and restarted during the scan without entering the radiation area. The signal from the probe is fed to an analog-to-digital converter and, via a serializer and translator network, registered on IBM cards. Detection System The detection system used for the scanner consists of a cylindrical ionization chamber on the end of a 20-cm stylus. The chamber, constructed from Bakelite, has an inside diameter of about 4.5 mm and a length of 26 mm. The central electrode is made from aluminum; this is connected directly to the input of a specially designed solid-state electrometer amplifier (1). The amplifier is inserted into an adapter attached to the end of the stylus. Figure 3 is a photograph of the complete probe assembly, consisting of the ionization chamber and amplifier housing. In routine use, the amplifier has demonstrated negligible drift (0.5 per cent of full scale per day) and a fast response. Figure 2 discloses that the response curves for scanning speeds of 0.45 cm per second and 2.0 cm per second are identical. In normal use, scanning speed is limited to 2 cm per second (4 data points recorded) by the card punch. Responses for higher speeds have been checked with a radiation-chopping lead wheel.