High-Energy Radiography (Cobalt 60 and Cesium 137) for Tumor Localization and Treatment Planning 1 Elliott B. Springer , M.D. , Leon Pape , M.S. , Fred Elsner , R.T. and Melville L. Jacobs , M.D. ↵ 1 From the Department of Radiology, City of Hope Medical Center, Duarte, Calif. Presented at the Forty-seventh Annual Meeting of the Radiological Society of North America, Chicago, Ill., Nov. 26–Dec. 1, 1961. Excerpt Teletherapy sources such as cobalt 60 and cesium 137 have been used for localizing studies in conjunction with radiation therapy for some time. In fact, one of the few references in the literature (1) points out that, under certain circumstances, radiography with these high-energy sources can provide diagnostic information beyond that provided by routine radiography. In view of the obvious advantages to using the teletherapy source for radiographic localization, it is surprising that so little has appeared concerning the procedure in the literature. This may well be due to the equally obvious limitation imposed on a radiographic system in which the energy of the radiation results in practically 100 per cent Compton interaction with the subject. Since Compton interactions are essentially independent of atomic number, the resulting differentials in absorption for the various structures in the human body are not very large and contrast, therefore, is at a minimum. In high-energy radiography, the proper filmis sandwiched between lead-intensifying screens. The physical phenomenon which occurs is not precisely understood. One recent paper (2) attributes the main intensifying action of the lead screens to the result of photoelectric interaction of the beam with the screens, while in another paper (1), the mechanism is held to result from Compton interaction. Without attempting to equivocate, it appears to us that the mechanism probably involves both processes. Comparison of the relative magnitude of cross section for absorption in either process, however, would indicate that the Compton process is the predominant one. In general, the problems associated with high-energy radiography are: ( a ) the relatively long exposure times required and ( b ) the lack of contrast. In our attempts to overcome these two problems, we recognized the inherent inability to increase differential absorption among the various tissues. This meant that the problem of contrast could be resolved favorably only by changing the energy of the incident beam or by amplifying the minimal contrast produced by the slightly different absorption characteristics. While change of the incident energy by multiple scattering is theoretically possible, it is basically impractical because each scattering also reduces the beam intensity markedly. This leaves in practice the approach of amplifying the contrast. Because, in photographic processes, changes in light intensity give nonlinear changes in film blackening, we were led to consider the possibility of changing x-ray photons to light photons. The simplest way is the standard application of intensifying fluorescent screens. Therefore, in addition to the lead-scattering screens, we have interposed, between the lead screens and the film, a set of high speed intensifying fluorescent screens. In our opinion, these serve both to increase the film blackening, thus reducing the required exposure time, and to increase the contrast. Copyrighted 1962 by The Radiological Society of North America, Inc.
Radiology – Radiological Society of North America, Inc.
Published: Feb 1, 1962
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