Evaluation of the effect of radiotherapy on neuronal integrity in rats using [¹¹C]flumazenil PET

Aim: Whole brain radiotherapy is frequently used in the treatment of primary and metastatic brain tumors. Unfortunately, radiotherapy also damages normal brain tissue, which can lead to serious and debilitating cognitive dysfunction. The exact mechanisms behind the radiotherapy-induced cognitive dysfunction remain elusive, but it may, in part, be explained by neuronal damage and neuronal loss. The aim of this study was therefore to evaluate if whole brain radiotherapy induced neuronal loss in healthy rats, using PET with the GABAA receptor antagonist [11C]flumazenil as a marker for neuronal loss.Material and methods: Male Wistar rats (n=24, age 8-9 weeks) were divided in 3 groups of 8 rats: control, sham irradiated, and whole brain irradiated. Whole brain irradiation was performed by the delivery of 25 Gy of X-rays in a single dose, under anesthesia, using an in-house developed rat brain collimator so that only the brain was exposed. Sham irradiated rats underwent the same procedure, but were not irradiated. [11C]Flumazenil PET scans were performed 3 months after (sham-)irradiation. Control rats were scanned at an age of 8- 9 weeks to assess the effect of aging on [11C]flumazenil PET. [11C]Flumazenil was injected intravenously at a constant flow of 1 ml/min (62±26MBq) and a dynamic PET scan of 60 minutes was acquired. The PET scan was accompanied by arterial blood sampling and metabolite analysis. One-tissue compartment modeling was used to calculate the volume of distribution (VT) in different brain regions, using blood and metabolite corrected plasma as input.Results: In control rats, the highest VT was found in the frontal cortex (12.0±2.0) and hippocampus (11.0±1.8) and the lowest in the pons (4.9±0.8) and medulla (4.2±0.7). No statistical significant differences in the VT were found between control and sham irradiated rats, showing that aging did not affect [11C]flumazenil binding. However, the VT in rats irradiated with 25 Gy was statistically significantly lower in all brain regions (p<0.05), including frontal cortex (33%), hippocampus (32%), pons (33%), and medulla (35%), when compared to sham irradiated rats.Conclusion: Whole brain irradiation resulted in statistically significantly decreased binding of [11C]flumazenil to GABAA receptor. This finding suggests that radiotherapy-induced cognitive dysfunction could be related to neuronal loss