RBE and Quality of Electromagnetic Radiation at Depths in a Water Phantom

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The biological effectiveness of electromagnetic radiation in the Xand y-ray regions has been investigated quite extensively along with that of many other types of radiation (1, 2). These studies have resulted in the determination of an RBE for these radiations compared to a standard X-radiation. Although the RBE is known to depend on a great number of factors, it is strongly dependent on the linear energy transfer (LET); LET, in turn, varies with the type and quality of radiation being considered. The biological indicators used in most RBE investigations have been small animals or organisms or the surfaces of larger animals. As a result of thris, little o no attenuation and subsequent energy degradation of the radiation has taken place prior to the production of the biological effect. It is of great interest to know whether the biological effectiveness of a given primary type of radiation is affected by the degradation that takes place when this beam of radiation passes through material that overlies the site of biological interest. Very little information is available at the present time regarding the RBE of radiation in depth (3-5). There has, however, been an interest in the change in radiation quality which takes place when Xor -y-rays pass through tissue or tissue-equivalent material, and several papers have been written on this subject. In 1932 Quimby and McNattin (6) measured changes in radiation quality by measuring absorption within tissue-equivalent phantom material, using spherical ionization chambers with surrounding spherical copper filters. This absorption method of studying radiation quality was further pursued by Greening (7) in 1951. Another physical line of attack was initiated by Clarkson and Mayneord in 1939 (8). They used a pair of coaxial ionization chambers which were identical except for inner wall material, one chamber having a