RADIATION therapy targeted at a person’s head, also known as prophylactic cranial irradiation, is routinely used to treat brain tumours. However, the treatment may progressively result in damage to one’s neurological functions later in life, even causing debilitating cognitive dysfunction. But the exact mechanisms underlying the impairment have not been clear.
In experiments with mice, Vipan Parihar and Charles Limoli, oncologists at the University of California, Irvine, observed significant changes in the neural architecture. The findings were reported in the latest issue of Proceedings of the National Academy of Sciences. The researchers found that there was a reduction in the complexity of dendrites—the branch-like structures on neurons that receive input from other neurons—following treatment with a low dose of radiation. The experiments involved subjecting mice to a cranial irradiation dose of 1-10 gray and studying a range of micromorphometric parameters in 10 days and 30 days following the exposure.
While a typical X-ray dose is of the order of milligrays, targeted curative radiotherapy doses for cancers are a few tens of grays; 1 Gy is the typical dose given to children for cancer therapy. The researchers found that there was more than 50 per cent decline in dendritic branching, length, and area in a dose-dependent manner. The exposure at these doses was found to result in 20-35 per cent reduction in the number of neurons in the hippocampus and 40-70 per cent reduction in the density of dendritic spines, small membranous protrusions that make synaptic connections with axons of other neurons.
Immature dendritic spines in developing neurons were found to show the greatest sensitivity to irradiation. Following a high dose, their number was found to have reduced by 40 per cent after 10 days. After 30 days, the number had decreased by 43 per cent for a 1 Gy dose and 73 per cent for a 10 Gy dose. In contrast, more mature spines were relatively insensitive to the radiation. Such changes are similar to what are found in many other neurodegenerative conditions.
The researchers, however, add that further research is needed before one can extrapolate to determine the effects of radiation on the architecture of neurons in human patients.
“That does not mean that one should demonise the radiation,” the newspaper Der Tagesspiegel reported, quoting Thorsten Langer, an oncologist at the University Hospital for Children and Adolescents in Erlangen, Germany. “We need it almost always to save the children.”
R. Ramachandran
