Automatic Dosimetry System for Cobalt-60 Teletherapy Unit

Abstract

Since the efficiency of a radiotherapy department can be improved by the use of versatile, rugged, and simply operated dosimetry equipment, an automatic dosimetry system has been developed and is presently being installed on our cobalt-60 teletherapy unit. The 5 mm depth dose in the beam axis is entered in a keyboard at the control console, as shown in Figure 1. The operator may select any dose from 1 to 999 roentgens. The signal from a monitoring chamber is integrated and fed to a modified digital voltmeter which is calibrated from 0 to 999 roentgens. When the voltmeter displays the dose that has been entered in the keyboard, the treatment is automatically terminated. A digital clock incorporated in the system determines the actual treatment time by displaying the elapsed time in minutes and seconds. Figure 2 is a photograph of the voltmeter and clock. To increase the convenience of this system, corrections for changes in the output for any combination of rectangular fields arc made automatically as the collimator opening is set for a particular field size. A resistance network incorporating a pair of function potentiometers coupled to the collimator-adjusting mechanism allows the correct dose to be displayed on the digital voltmeter. After the termination of the treatment, and before initiating another, the system must be reset. The reset circuit automatically clears the voltmeter, resets the clock, and clears the keyboard, thus preventing the operator from using the previous dose for the following patient.