The effects of fractionated doses of fast neutrons or photons on the canine brain: Evaluation by computerized tomography and evoked response recording

Abstract

The use of fast neutrons in the treatment of cancer necessitates a knowledge of the normal tissue responses. This study was designed to compare the late effects of fractionated doses of fast neutrons with fractionated doses of photons on canine brains by evoked response recording and viewing computerized tomograms (CT). Adult male beagles were irradiated to the entire brain (four fractions per week) with fast neutrons to total doses of 13.33 Gy (1333 rad), 20 Gy (2000 rad), 30 Gy (3000 rad) or 45 Gy (4500 rad) or with photons (four fractions per week) to total doses of 40 Gy (4000 rad), 60 Gy (6000 rad) or 90 Gy (9000 rad). A relative biological effectiveness (RBE) of 4 was obtained for normal brain tissue assessed by mortality and onset of neurologic symptoms. Every three months post-irradiation, visual and sensory evoked responses were recorded. Changes over time appeared to be minimal; however, computerized tomographs showed marked brain shrinkage. A method of quantitating cerebrospinal fluid and parenchyma) volumes from scans is described and future use of these CT ratios to generate dose response curves and RBE values is postulated.