Radioactive Plaque Brachytherapy
Owing to slower cell turnover, melanomas are less radiosensitive compared with some other tumors, but most melanomas regress satisfactorily after radiotherapy.[15–21] Plaque brachytherapy is the most common conservative treatment used in the management of choroidal melanomas followed by charged-particle radiotherapy.[19–24] Currently, I-125 and Ruthenium-106 (Ru-106) sources are widely used worldwide.[25–31]
Ru-106 is a β-ray emitter with a half-life of 373.59 days. Ru-106 plaques can be prescribed to tumors with less than 5 mm thickness as there is a rapid dose fall off because of low penetration of electrons. The usual tumoricidal dose is 80–100 Gy at the tumor apex, whereas with Ru-106 applicators, the dose rate is 60 mGy/min at 3 mm depth, falling to 30 mGy/min at 5 mm depth. It is recommended that a 1500-Gy scleral dose should not be exceeded. Larger and thicker (<10 mm) tumors can be more safely treated with I-125 plaques. The COMS trial considered enucleation as the only option for patients with large melanomas, whereas Puusaari et al. reported that the eye was successfully conserved at 8 years in 80% of patients with large melanomas. In certain eyes, plaque radiotherapy may provide better chances of saving vision, especially for the patients who are unwilling to undergo primary enucleation. The I-125 sources emit gamma irradiation with deeper penetration and have a half-life of 59.4 days. The COMS study compared enucleation to I-125 brachytherapy in medium-size tumors. A medium-sized tumor was defined as a tumor with an apical height of 2.5–10.0 mm and a basal diameter of 16.0 mm or less. For patients in both treatment arms, 5-, 10- and 12-year all-cause mortality rates were 19, 35 and approximately 40%, respectively. In the I-125 brachytherapy group, rates of melanoma-associated deaths at 5, 10 and 12 years were 10, 18 and 21%, respectively, and in the enucleation group were 11, 17 and 17%, respectively. There was no difference in melanoma-associated or all-cause mortality between the two treatment modalities.
The selection of radiotherapy modalities is one of the factors that influences local control rate. Wilson et al. reported 5-year melanoma recurrence rates of 11, 4 and 5% for Ru-106, I-125 and proton-beam therapy, respectively. However, patients treated with Ru-106 showed fewer side effects and better visual results. The tumor thickness limitations of the Ru-106 treated plaque may account for the slightly higher recurrence rates, particularly for larger tumors. The recurrence rate following Ru-106 brachytherapy was as low as 3–4% in some studies and as high as 11 and 16% in other studies.[34–38]
Ru-106 plaque brachytherapy is usually associated with less severe radiation-related complications compared with I-125 plaques and charged-particle radiotherapy, depending on the location of the tumor.[27,39–42] Cataract formation is the most common complication and the 5-year probability is 37%. The cumulative 5-year probability of vitreous hemorrhage is 24% following treatment. Dry eye, keratitis, radiation-induced iris neovascularization, neovascular glaucoma, radiation-induced retinopathy, radiation-induced optic neuropathy, episcleral deposits, scleral necrosis and extraocular muscle alterations are the other ocular complications reported after plaque brachytherapy.[43–50] Inraoperative echographic localization of radioactive plaques can be used if required. This method enhances treatment success and may reduce the rates of postradiation complications. These complications were reported less frequently during the follow-up period.
Radiation retinopathy has been reported to occur in 10–63% of patients treated with plaque brachytherapy, and the mean time from treatment to the development of maculopathy was found to be 25.6 months. Treatments modalities for radiation retinopathy include intravitreal injections of triamcinolone and bevacizumab, laser photocoagulation, hyperbaric oxygen treatment, PDT and oral pentoxyphylline. Finger et al. treated patients with palladium-103-induced retinopathy with multiple intravitreal injections of bevacizumab. Visual acuity was improved or stable in 86% of patients and 14% regained two or more Snellen lines of vision.[52,53]
Expert Rev Ophthalmol. 2012;7(2):189-195. © 2012 Expert Reviews Ltd.