Pathologic Findings in Canine Brain Irradiated with Fractionated Fast Neutrons or Photons

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Abstract
Adult male purebred beagles (37) received total doses of 1333, 2000, 3000, or 4500 rad of fast neutrons (15 MeV av) in 4 fractions per wk for 7 wk to the entire brain. Nineteen dogs received 4000, 6000 or 9000 rad of photons (60Co) in an identical fractionation pattern. Dogs that received 4500, 3000 and 2000 rad of neutrons and 9000 rad of photons developed neurologic signs and died or were euthanatized when moribund 32-98, 47-96, 62-720 and 22-187 days, respectively, following irradiation. CSF contained excess protein and erythrocytes during and sometimes before the generally brief course. The onset of neurologic symptoms was usually followed by a moribund state in < 48 h. The relative biological effectiveness (RBE) as measured by onset of neurologic symptoms and mortality was > 4.5 (9000 rad photons/2000 rad neutrons). Gross changes included hemorrhage, edema and malacia primarily in the white matter, especially the corona radiata, cerebellar white matter, corpus callosum and corpus fornicis. Lesions were generally more extensive in dogs with the longest latent periods, but were not modality specific. One beagle developed a brain tumor and died 1207 days following 1333 rad neutron irradiation. [Fast neutrons may provide a therapeutic advantage in those forms of cancer not well controlled by low linear energy transfer radiation. Unacceptable side effects may, however, limit its use.].

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