PATHOLOGIC CHANGES IN BRAIN CAUSED BY ALPHA PARTICLES 385 



ionization chamber connected, Brao-^ cui\es could be measured and con- 

 verted to tissue equi\alent \alues. Depth-dose distribution in tissue was 

 inferred from the measured surface dose by use of a Bragg cur\e derived in 

 this manner (see Fig. 6). It was found that the effective range in tissue of 

 the full energy protons and alpha particles (after allowance was made for 

 the various absorbers, Al foils, and air gaps, which are permanently in the 

 beam during exposure) was about 118 mg per cm-, or 1.180 /x, if a tissue 

 density of 1 is assumed. Homogeneity of the particle flux over the target 

 area was checked from densitometer readings of exposed films or from "burn 

 patterns" obtained on exposed ozalid paper. When the flux distribution was 

 unsatisfactory, adjustments were made with the focusing magnets and, if 

 necessary, in the entire alignment of the experimental setup. Details of 

 dosimetric procedure and of particle properties are described elsewhere 

 (BrUstad ct al, 1960; Birge et al, 1956). 



The dose rate used dining the exposure of the rats was approximately 

 10,000 rad per minute surface close, which corresponded to about 4 x 10'' 

 particles per cm- per sec. Due to difficulties in the accelerator operation, a 

 constant dose rate was sometimes difficult to maintain: fluctuations o\er a 

 factor of 2 or more occurred. 



Observations 



The brain-surface doses of alpha particles and protons and the times of 

 sacrifice of the animals are gi\en in Table L The brains were exposed to 

 alpha particles at surface doses of 50 to 6,000 rad and to protons at a sur- 

 face dose of 6,000 rad. 



Whether produced by alpha particles or by protons, the basic lesion tol- 

 lowino irradiation consisted initially of a zone of cell damatic which stretched 

 horizontally across the cerebellar and cerebral cortex. The lower border of 

 the zone was sharp: the upper border tended to be indistinct. Within the 

 deepest ])art of the zone of damage a ner\e-cell-poor or ner\e-cell-free hand 

 eventually appeared. The width of the zone and of the band \aried with 

 the radiation dosage. In the text which follows, the terms "'zone"" and ■"band" 

 are used in the sense indicated. 



Width of the Lesions in Terms of the Energy Given Off 

 Along the Bragg Curve 



Measurement of the width of the zone and the band of damage following 

 exposin^e to 6,000 rad surface radiation dose was carried out in practically 

 all 211 brains < Table I i. The average maximal width of the zone and of the 

 band, both in the cerebellum and the cerebrum, was \irtually the same in 

 the brains exposed to alpha particles as those exposed to protons (Table II) . 



