HUMAN TOLERANCE TO THERMAL NEUTRONS 445 



field, is completely sensiti\e only to specific particle radiations resulting from 

 the reaction. Consequently, we must rely largely on calculations with such 

 experimental measurements and obser\ations as can be appropriately carried 

 out to establish the \alidity of our conclusions. Since the particle energy 

 released is proportional to the concentration of tarcjet atoms and total 

 number of neutrons, our measurements are directed to these ends. The con- 

 centration of the neutron captiue target element is assayed by chemical 

 means, while the total number of neutrons passing a plane is determined by 

 activation of gold foils or wires inserted into the neutron penetrated area. 

 Gamma ray levels have been calculated and roughly measured by ordinary 

 ionization chambers coxered by neutron filters and, experimentally, by 

 chemical dosimeters, glass needles, and solid state ionization devices. Fast 

 and intermediate energy neutrons in various regions are cxaluated insofar as 

 possible by characteristic energy activation foils and fission chamber tech- 

 niques. Measurements gi\en in this paper were derived by one or more of 

 these procedures. 



Results 



Efiects on nontimiorous neiuonal structmes were studied in 16 patients 

 receiving neutron capture therapy for gliomas and sarcomas, with a 20-40 

 minute exposure to thermal neutrons. The material discussed is being 

 reported in detail elsewhere Farr et ai, to be published). 



Whole brain sections were prepared from 16 cases in which the total 

 surface neutron exposure varied from 0.44 X 10^" to 6.31 X 10^" per 

 square cm and in which the total dose of tetraborate and pentoborate salts of 

 sodium, ranged from 26-50 mg per kg body weight when calculated as B^'^ 

 per dose. Experimental exidence has firmly established a correspondence 

 between tissue concentration and total close for immediate distribution. 

 Borate salts appear to distribute imilormly in body water, eventually reach- 

 ing a single equilibrium concentration. In 3 cases of slioblastoma multiforme 

 tumor, necroses appeared to be present in the region of the emergent neutron 

 port. In a 4th case of cerebellar angiosarcoma and ependymoma, there may 

 have been an irradiation effect, but the presence of a fimgal lesion did not 

 lead to a clear-cut evaluation. In a 5th case of sarcoma at the site of 



Di.\GRAM L On the left is shown the thermal neutron approximate iso-flu.x con- 

 tours in neutrons per square cm with the maximum intensity exposure from the neu- 

 trons cloud at the port of entry. The direction of movement of the neutron fog is 

 shown, and the fall off in neutron intensity from the port of entry to the midbrain. 

 To the right is a graph indicating effects on thermal neutron distribution in tissue 

 with different B'" concentrations. Surprisingly little effect is noted therefrom in these 

 phantom studies. 



