Radiation-induced neuroinflammation and radiation somnolence syndrome

Cranial irradiation remains a standard treatment for malignant and benign brain diseases. Although this procedure helps to lengthen the life expectancy of the patient, the appearance of adverse effects related to radiation-induced injury is inevitable. Radiation somnolence syndrome (RSS) has been described as a delayed effect observed mainly after whole-brain radiotherapy in children. The RSS was first linked to demyelination, but more recently it has been proposed that the inflammatory response plays a primary role in the aforementioned syndrome. To evaluate the feasibility of this hypothesis, we explored previous work about RSS and reviewed published research that included measurements of the inflammatory response in models of brain exposure to ionizing radiation. Pro-inflammatory cytokines such as interleukin-1β, tumor necrosis factor-α, interleukin-6 and interleukin-18 as well as other inflammatory markers such as cyclooxygenase-2, prostaglandin E₂, glial fibrillary acid protein, intercellular adhesion molecule-1 and nuclear factor-κB appear to be involved in the brain's response to radiation. However, certain publications have described the somnogenic effects of these cytokines and inflammatory markers. Although the radiation response is a complex phenomenon that involves several molecular and cellular processes, we propose that inflammation may be closely related to the adverse effects of brain irradiation and therefore to the etiology of RSS.