Photoconductance crystals for intracavitary dosimetry.

Abstract

A supplementary method for the detection and measurement of radiation is the use of photoconductance crystals such as cadmium sulfide and cadmium selenide, which are employed commercially in photoelectric operating assemblies, such as door openers, counters, burglar alarms, automobile headlight dimmers, etc. These crystals are grouped with the semiconductors of electricity. Exposure to light or radiation reduces the inherent resistance of the crystal to the passage of current so that, by comparatively simple circuitry, the changes in resistance can be detected and recorded (Fig. 1). This resistance change is directly related to the quantity and quality of the stimulating agent—light or radiation. The small size of the crystals (in millimeters), their photoelectric properties, and high sensitivity make their application to clinical use intriguing. While there are a number of reports (9, 10, 13) in the literature of experimental data concerning the use of the crystals as detectors of ionizing radiation, there are few studies in the medical field concerning practical clinical usage. In 1953, Mauldon and Martin (12) developed a cadmium sulfide crystal probe for measuring dose rates in the rectum following the insertion of radon for the treatment of carcinoma of the cervix. They have also reported (1955) on a needle probe using cadmium sulfide crystals for the interstitial measurement of the dose delivered to the axillary lymph nodes in patients with breast cancer. Stephens-Newsham and LaPalme (17) have used photoconductance crystals as a centering device in cobalt-beam therapy of the esophagus. The purpose of the present study is the correlation of the calculated dose delivered to a volume of tissue with the actual dose as determined by intracavitary detectors in the form of photoconductance crystals. Commercially available intracavitary ionization chambers are usually employed for the direct measurement of the depth dose. Cadmium sulfide crystals, however, present several advantages which are of value clinically. Due to the relatively low impedance of the crystal, no difficulties are encountered in respect to leakage of charge, humidity, or length of cable, such as are found with small intracavitary thimble chambers. The photoconductance probes are relatively inexpensive—the probes, cable, and ratemeter having been assembled for less than $100. Our studies were based on commercially available cadmium sulfide crystals, obtained as a Clairex Corp. C1-2 photocell, having overall dimensions of 14 mm. length and 5 mm. diameter. In use clinically in a light-proofed plastic shield, the probe measured 18 mm. in length and 7 mm. in diameter. The flexible coaxial type cable, to which the cell was attached, measured 4 mm. in diameter (Fig. 2). The unit is thus of such size that it can be readily passed into body cavities such as the nasopharynx, esophagus, uterine cervix, etc.