File:The American journal of roentgenology, radium therapy and nuclear medicine (1906) (14757320525).jpg

Identifier: americanjournroen09ameruoft (find matches)
Title: The American journal of roentgenology, radium therapy and nuclear medicine
Year: 1906 (1900s)
Authors: American Radium Society American Roentgen Ray Society
Subjects: Radiotherapy X-rays
Publisher: Springfield, Ill. C.C. Thomas
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: University of Toronto

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om the skin.* Then the thickness oftissue /. (Fig. 7) which affects the radiationleaching the back of the tumor is sub- If placed within the tumor the problem is evidently different, .IKS. Some Problems of Radiation Therapy 489 stantially the same for all rays enteringthe tumor, no matter how far the sourcemay be. Even the volume of radiatedtissue is nearly the same, if the properdiaphragm is used. Consequently thedecrease of radiation due to the interveningtissue, and the scattering effect are thesame for all practical distances of applica-tion (when the same quality of radiationis used). can be used to determine the differencein the amounts of radiation received bythe skin and tumor from the operation ofthe inverse square law. The decrease dueto the intervening tissue is additionaland quite independent of that due to therelative distances of skin and tumor fromthe source. In practice, economy of timeis an important factor to consider. There-fore such a combination of filter and dis-
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FlG. 7. The effect of absorption on the curve ofFigure 6 is simply to lower the 100 per centline and every point on the curve in thesame proportion. It amounts realty to a decrease of the scale of * ■ Suppose, for instance, that the radiation reaching thepoint B (including primary, scattered, andsecondary radiation) for any one positionof the source is 50 per cent of what itwould be if the tissue were not present.Then the maximum relative amount thatwe could ever get byr placing the source at a very long distance is ——lx. For any 100 other distance the value of I, can be obtained from the dotted curve whichapproaches as a limit the 50 per cent line. Thus for a value of - of 19, the value of ^ z lx is 45, which is again 90 per cent of thepossible maximum for this case, that is, - = 00 per cent. Hence for any given set of50 conditions (distance, depth of tumor, quality of radiation, etc.), the upper curve tance (as well as other factors) should beused as will give the desired amou

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