HUMAN TOLERANCE TO THERMAL NEUTRONS 457 



difficulty in calculating the rad equivalent of thermal neutron exposure, we 

 have had sufficient experience with measurement of thermal neutrons by 

 foil activation to place a high degree of confidence in the total thermal 

 neutron exposure indicated. Moreover, the observations, as made, are de- 

 pendent on their calculations only for further interpretative elucidation and 

 not for primary validity. Preliminary observations on animals and patients, in 

 which the maximum intensity reported in these studies has been exceeded, 

 are pointed-to only to emphasize that beyond the exposures discussed here 

 there still remains a margin of safety. We have been surprised that none of 

 the structures, neuronal, supporting, or vascular, have shown any changes 

 under the exposures noted. To achieve greater exposures, further change in 

 the moderator-reflector system of our reactor must be accomplished, so that 

 gammas resulting from materials capture of wayward neutrons may be 

 further diminished in a significant manner. 



Since we have observed no evidence of a limit to the tolerance to thermal 

 neutrons, we cannot speculate on these data. They are presented as observa- 

 tions made repeatedly (in animals) to establish their reproducibility. The 

 practical significance of this may be considerable in the consideration of a 

 reactor for diagnostic purposes, but until limits are set, this must remain 

 largely an area for discussion only. 



It must be emphasized that our observations were not limited to an histo- 

 logic examination of exposed tissue. Functional observations were carried out 

 on all patients receiving neutron capture therapy. These included specific 

 visual acuity and perception tests, audiometric examinations, and complete 

 neurologic examinations at frequent intervals. Perhaps the most significant 

 was day to day observation of the patient, his general behavior, and his 

 reaction to situations frequently new to him. By all these criteria, no dele- 

 terious effects were seen. Complex movements were unafTected, and both 

 logical and emotional responses showed no changes. Patients were observed 

 closely up to I/2 years, which probably is adequate time to permit the de- 

 velopment of usual postradiation sequelae. In regard to visual acuity, it 

 should be pointed out that frequently the retina itself received considerable 

 exposure to thermal neutrons, but no changes in function resulted. 



Summary 



In the histologic studies of serial sections of the brains of 17 patients 

 treated by neutron capture therapy for intracranial neoplasms, no changes 

 consistent with and attributable to exposure with thermal neutrons were 

 found in the nervous tissue. A maximum thermal neutron exposure of 

 1.73 X 10^^ thermal neutrons per square cm occurred over a 200 sec interval. 

 One additional patient receiving only thermal neutron exposure (approxi- 



