An improved clinical dosimeter for the measurement of radiation.

Abstract

The efficiency of a therapy center which employs various forms of radiation is greatly increased by the use of a versatile, rugged, and simply operated dosimeter. The recent development of high-gain Raytheon electrometer tubes (CK 5886 and CK 5889) has made possible considerable simplification in d.c. amplifiers. The dosimeter to be described consists of only two hearing-aid battery-operated pentodes in a simple circuit. The device is being used for pelvic and esophageal measurements during x-ray and radium treatment. It is also useful in the construction of radium and cobalt molds, in the measurements of depth dose, etc. The dosimeter consists of a d.c. amplifier used in conjunction with interchangeable ionization chambers. These chambers are made in a variety of shapes and dimensions to suit the particular conditions under investigation. The amplifier is capable of measuring instantaneous dosage rates as low as 1 r/hr. with an ionization chamber volume of 0.1 c.c. Pre-Amplifier Design A schematic diagram of the pre-amplifier and the end of the typical ionization chamber is given in Figure 1. Details of the inner construction are essentially the same as those described by Johns, Epp, and Fedoruk (1). To increase the versatility of the dosimeter, however, the pre-amplifier of Figure 1 is arranged to contain two grid resistors so that the sensitivity may be quickly altered by a factor of 10. The method by which this is done is illustrated in the figure. The ends of the 1011 ohm and 1012 ohm resistors and the grid of the 5886 are connected to pins at the bottom of holes A, B, and C. The end of the ionization chamber is fitted with three protruding pins, D, E, and F, which may be inserted into A, B, and C. When this is done, the end of the 1011 ohm resistor is grounded, while the 1012 ohm resistor and the central electrode of the chamber are connected to the grid of the electrometer. To reduce the sensitivity by a factor of 10, the ionization chamber is merely rotated through 180 degrees and re-inserted to connect D to C and F to A. By slight re-arrangement in construction, more than two ranges can be incorporated. This pre-amplifier is connected to the d.c. amplifier with the required length of shielded multi-strand cable. Ionization Chamber Design In Figure 2 are shown the details of construction of three typical ionization chambers. Each of these can be connected to the pre-amplifier as indicated in Figure 1. Photographs of the chambers are given in Figure 4. Pelvic Chamber: The pelvic chamber has a cylindrical air cavity with a diameter of 5 mm. and a length of 6 mm., giving a volume of about 120 cu. mm. Since the chamber was designed to be placed in body cavities, it was made of a single piece of rigid aluminum tubing. It is used in the routine investigation of the dose received in the rectum and bladder during the course of intracavity radium treatment of carcinoma of the cervix uteri.