HUMAN TOLERANCE TO THERMAL NEUTRONS 443 



charge. We know that neutrons of higli energy, from 0.1 or 0.2 Mev and up, 

 do have an extensive damaging eflfect on tissue. These neutrons are able to 

 disrupt molecular relationships and otherwise cause disturbance of the 

 normal life pattern. These efTects arise largely from the mechanical impact 

 of fast neutrons on atomic nuclei. It is the lack of such high levels of kinetic 

 energy which make the slow neutron innocuous in this respect. 



When neutrons of thermal energy, namely 0.025 ev, penetrate into tissue, 

 they do not directly cause damage by their passage ; rather, they pervade the 

 tissue as a gas pervades whatever space it is released into. However, with 

 increasing penetration their numbers significantly and predictably decrease 

 as they travel from the surface through solid tissues. The pattern of attenua- 

 tion is shown in Diagram I. Any effects may be expected to result only from 

 capture with atomic transformations, radioactivity resulting therefrom, or 

 both. 



Radiations coming from the induction of acti\ity in tissue components 

 ha\e to be recognized in assaying the total efTects of neutron exposure of 

 tissue. Chief among these reactions are the capture of slow neutrons by 

 hydrogen, resulting in an immediate gamma ray emission at over 2 Mev, the 

 capture of slow neutrons by nitrogen, giving an instantaneous proton of 

 energy 0.6 Mev, and the capture of slow neutrons by carbon, yielding gamma 

 rays of up to 8 Mev. Other constituents in tissue, particularly sodium and 

 chlorine, may contribute readily measurable additional radiation. 



In neutron capture therapy, a much greater intensity of reaction is in- 

 duced in the treated tissue by providing thermal neutrons in large numbers 

 at such a time after the target atom injection as we empirically have shown 

 to be efTccti\e in experimental tumor destruction and sparing of adjacent 

 normal tissues. The mechanisms of the cytocidal effect are under intensive 

 study, as no explanation is conveniently provided by chemical analyses of 

 target atom species for the differently affected tissues. 



In the experimental in\estigation of neutron capture therapy at Brook- 

 ha\en, two reactors ha\e been used as sources of neutrons. The work was 

 begun at the large graphite research reactor when it first reached its full 

 operating power in 1951. A part of its biologic shielding was modified to 

 provide a hollow cone delivering neutrons through a treatment aperture. 

 Subsequent improvements in this arrangement provided a shutter and a 

 higher flux of neutrons. Some 40 experimental procedures with patients, 

 together with studies on mice, rabbits, and swine at the graphite research 

 reactor, gave experience and background for the design and construction of 

 the medical research reactor, a reactor entirely oriented toward studies in 

 man. The details have been published pre\iously ( Farr, 1959). 



Dosimetry of neutron capture-induced gamma radiations in tissue is not 

 a straightforward measurement, since there is no detector which, in a "amma 



