A Potential Novel Organ At Risk: Vitreous Humour

Radiation therapy for the ocular and periocular areas can cause vision-threatening complications. There is no data in the literature about the possible contribution of the vitreous in the emergence of radiation-induced orbital complications. For this purpose, an experimental animal model was developed to reveal ionizing radiation-induced changes in the vitreous humour. Six white New Zealand rabbits were used in this experimental study. High dose (60 Gy in 3 fractions) ionizing radiation was delivered to the right globe of all rabbits. Left globes were the control group. Animals were kept alive for 12 weeks after radiation exposure. During the 12 weeks period, the animals and their eyes were examined weekly (including fundus examination). At the end of 12 weeks, animals were sacrificed, eyes were enucleated, and vitrectomy was performed. Collected vitreous fluids were assayed for viscosity, type II, V/XI, IX collagen levels, and its metabolomic profile. In the weekly examinations, no changes were observed in the retina of the animals. Compared to the control group (left eyes of each animal), collagen type 2 levels of the study group were found to be significantly decreased (P<0.05). No significant difference was determined between groups in terms of collagen type 5,9 and 11 content. In 4 animals, the viscosity of the samples was found to be decreased by the effect of radiation. As the number of animals included in the study is limited, this difference could not be confirmed by statistical analysis. Comparative metabolomic analysis of vitreous humour samples from rabbits was carried out based on the GC-MS metabolomic profiling. After deconvolution and aligned of the chromatograms, 401 mass spectral features have been detected, and 126 of them were annotated using retention index libraries. These identified metabolites belong to the Krebs cycle, amino acid, and fatty acid metabolism. The multivariate analysis shows a clear separation between control and irritated vitreous humour samples. At metabolomics level, phosphocolamine, N-ethylglycine, cholic acid, and citric acid level were significantly altered (p<0.05, paired test, n = 6). To our knowledge, this is the first study to demonstrate that ionizing radiation affects vitreous humour and significantly changes its viscosity, type II collagen levels, and metabolomics content. Serious pathologies such as retinal detachment may occur as a result of changes in the vitreous humor with aging. Changes in the vitreous that occur with aging are similar to the radiotherapy effect. In this experiment, the changes in the vitreous without any change in retina suggest that the pathologies which occur in the retina may be secondary to the pathologies in the vitreous.