Female 61 to 63 - day - old Sprague-Dawley rats were exposed once to a single dose of either 0.43 - MeV
neutrons or 250 - kVX - rays . For neutrons 23 rats were exposed in plastic tubes rotated around and 31 c m from
a water-cooled tritium impregnated target bombarded with 2.45 - MeV protons from a V a n de Graaff generator.
The mean kerma was measured at the rat location by integrating the response of a rat - sized homogeneous
tissue equivalent ionization chamber of minimum mass. The ratio between absorbed dose and kerma is under
investigation and is anticipated to be approximately 0.7. A compensated GM gamma-ray dosimeter indicated
that the gamma - ray doses were 3.5% of the total dose. All rats were examined weekly for the presence
of breast tumours and these were removed, fixed, stained and verified histologically as mammary neoplasms.
At 10 months after exposure 98<7ο of the rats were a live . The neutron kerma, the per cent of rats with
mammary neoplasia, and the number of rats were, respectively: 0.125 rads, 8.2°}o, 182; 0.5 rads, 9.0^0,
89; 2 rads, 20. 6,68; and 8 rads, 31.1%, 45. The X - ray results were: 30 R, 1.4% 95; 60 R, 27. l°Io, 48;
and 90 R, 35.4%, 48. A 3. O^o incidence was found in 167 control rats. At 10 months after exposure the
mammary neoplastic response after 8 rads of neutrons corresponds approximately to that after 60 - 90 R of
X - rays . Similarly, the response after 2 rads of neutrons was intermediate between 30 and 60 R of X - rays and
the response after 0 . 125 and 0.5 rads of neutrons was similar to that after 30 R of X - rays . This demonstrates
that the RBE for 0.43 - MeV neutrons is much lower at high doses than at low doses. Determination of the
confidence limits for the dose-RBE dependence and dose-incidence relationship will be determined as additional
data are collected
