Iatrogenic cognitive deficits are a major concern in brain cancer radiotherapy, especially in pediatric cases. Compared to X-ray treatment modalities, proton and ion beam therapies show improved sparing of cognitive faculties. Dendritic branching and spine density are crucial for cognitive function. Previous studies found losses of dendritic complexity in irradiated mature neurons with established connectivity; however, pediatric brains exhibit continuous neurogenesis and network formation. We compare effects of carbon ion and X-ray irradiation of cultured immature hippocampal neurons from 16-day embryonic mice on the complexity of their dendrites throughout a 3-week maturation period. Cell preparation, irradiation, and imaging were conducted at Gunma University in 2017; data were analyzed after a hiatus. Fluorescence images were acquired with MAP2 labeling the dendritic cytoskeleton, DAPI labeling the nucleus, and drebrin, an actin binding protein localized in dendritic spines, to facilitate tracing and morphometric analysis in Imaris (Oxford Instruments). Cohorts irradiated with 225 kV X-rays to 0.5 Gy and 1 Gy dose reached adulthood with significantly shorter (86.5% and 39.0% total length; p < 0.05) and less branched (83.0% and 20.8% of branch points; p < 0.05) dendrites than unirradiated controls. Similar trends were observed in cohorts irradiated with 290 MeV/nucleon carbon ions to equal doses, but effects were less pronounced and appeared to saturate with increasing dose (85.0% and 81.8% total length; 69.4% and 69.2% of branch points). Spine density per unit shaft length fell in direct proportion to dose for both types of radiation, although the log-normal shape of the distribution was preserved. These findings may support the hypothesis that particle beam radiotherapy spares pediatric cognitive function by preserving the capacity of immature neurons to develop sufficiently complex dendrites and numerous synapses, whereas even low-dose X-ray irradiation significantly disrupts these outcomes.
Andersen et al. (Sun,) studied this question.