Researchers at the University of California, Los Angeles (UCLA; USA) examined several vitreous substitutes, including silicone oil, heavy oil, and perfluorocarbon (PFC) for their radiation attenuation properties from iodine 125, using both empirical measurement and theoretical calculation. In vitro and ex vivo cadaveric dosimetry measurements were obtained with thermo-luminescent dosimeters to demonstrate the attenuation effect during simulated plaque brachytherapy. The results showed that radiation attenuation using the silicone oil at distances corresponding to vital ocular structures achieved the best results, with a projected attenuation reaching as high as 57%.

Based on these results, the researchers developed a new surgical technique, which takes place immediately before the patient's eye is exposed to the radiation. The patients first undergo an exam to measure baseline vision before treatment. The surgeon next removes the vitreous gel, which supports the interior shape of the eye; the gel is replaced with silicon oil, a substance that is often used to hold the retina in place during surgery to repair retinal detachment. The surgeons then treat the ocular melanoma by stitching a gold plaque containing radioactive seeds to the sclera of the eye, removing it a few days later; after removing the radiation plaque from the treated eye, the oil is flushed away with saline, which is eventually replaced by the patient's natural fluids. The study was published in the July 2010 issue of Archives of Ophthalmology.

“Until recently, physicians focused on killing the tumor and considered vision loss secondary. Our results suggest that silicon oil offers a safe tool for protecting the patient's vision during radiation,” said lead author Tara McCannel, M.D., an assistant professor of ophthalmology and director of the UCLA ophthalmic oncology center. “The substance acts like a physical shield, reducing the amount of radioactive rays that reach the back and sides of the eye. We hope that silicon oil's ability to block radiation translates into better vision for patients down the road.”

While radiation treatment for ocular melanoma kills the cancer cells, it also causes irreversible injury to the optic nerve fibers and macula, the section of the retina responsible for central vision; the radiation damage to the macula also weakens the blood vessels supplying the eye. Even when the radiation treatment is successful, the eye structures remain extremely fragile and prone to atrophy, which can lead to subsequent vision loss.

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