Background/Aims: Ruthenium plaques are used for the treatment of ocular tumors. There is, however, a controversy regarding the maximum treatable tumor height. Some advocate eccentric plaque placement, without a posterior safety margin, to avoid collateral damage to the fovea and optic disc, but this has raised concerns about marginal tumor recurrence. There is a need for quantitative information on the spatial absorbed dose distribution in the tumor and adjacent tissues. We have overcome this obstacle using an approach based on Monte Carlo simulation of radiation transport. Methods: CCA and CCB 106Ru plaques were modeled and their geometry embedded in a computerized tomography scan of the eye of a patient. Different tumor sizes and locations were simulated with the general-purpose Monte Carlo code PENELOPE. Results: Cumulative dose-volume histograms were obtained for the tumors and the tissues at risk considered. Plots of isodose lines for both plaques were obtained in a computerized tomography study. Conclusions: Ruthenium eye plaques are an adequate treatment option for tumors up to around 5 mm in height. According to our results, assuming a correct placement of the plaque, a tumor of 6.5 mm apical height is about the maximum size that can be treated safely with the large CCB plaque.