does not predict its response to prolonged ' biologically equivalent ' fractionated doses

Purpose : To determine if the brain ’ s response to single doses predicts its response to ‘ biologically equivalent ’ fractionated doses. Methods : Young adult male Fischer 344 rats were whole-brain irradiated with either single 11, 14, or 16.5 Gy doses of 137 137 Cs γ rays or their ‘ biologically equivalent ’ 20, 30, or 40 Gy fractionated doses (fWBI) delivered in 5 Gy fractions, twice/week for 2, 3, or 4 weeks, respectively. At 2 months post-irradiation, cellular markers of infl ammation (total, activated, and newborn microglia) and neurogenesis (newborn neurons) were measured in 40 μ m sections of the dentate gyrus (DG). Results : Although the total number of microglia in the DG/ hilus was not signifi cantly diff erent ( p 0.7) in unirradiated, single dose, and fWBI rats, single doses produced a signifi cant ( p 0.003) increase in the percent-activated microglia; fWBI did not ( p 0.1). Additionally, single doses produced a signifi cant ( p 0.002) dose-dependent increase in surviving newborn microglia; fWBI did not ( p 0.8). Although total proliferation in the DG was reduced equally by single and fWBI doses, single doses produced a signifi cant dose-dependent ( p 0.02) decrease in surviving newborn neurons; fWBI did not ( p 0.6). Conclusions : These data demonstrate that the rat brain ’ s cellular response to single doses often does not predict its cellular response to ‘ biologically equivalent ’ fWBI doses.