Technical Note: Monte Carlo study of 106Ru/106Rh ophthalmic plaques including the 106Rh gamma spectrum

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PurposeTo assess the influence of the 106Rh gamma spectrum on the Monte Carlo simulation of 106Ru/106Rh ophthalmic plaques, which has been neglected without a quantitative estimation in all previous publications.MethodsSimulations were run with the penelope 2014 Monte Carlo code for radiation transport. Depth–dose distributions in water were simulated for the plaque models CCA, CCC, CCX and CIA. In addition to the 106Rh beta spectrum, all gamma components from the 106Rh gamma spectrum were included in the simulations. Depth–dose curves were compared with those obtained without considering the 106Rh gamma spectrum. Moreover, half–value (HVL) and tenth–value layers (TVL) were estimated for the 106Rh gamma spectrum in water, PMMA, stainless steel and lead. Some practical radiation protection applications were discussed. Parallel computing was implemented to reduce computing time.ResultsThe contribution of the 106Rh gamma spectrum on the depth–dose curves is negligible at depths of clinical interest. The HVL and TVL of the 106Rh gamma spectrum were found to be similar to those of 137Cs. The air–kerma rate at 1 m for a CCA plaque in typical clinical conditions was about Symbol, resulting in equivalent doses at that point elow 0.05 mSv during a treatment. The air–kerma rate would be underestimated by a factor of 5 if the 106Rh gamma spectrum were not considered. Also, a freely available software tool was developed to ease parallelization of penelope 2014 simulations that use penmain as steering main program.ConclusionsThe influence of the 106Rh gamma spectrum is not relevant for clinical purposes, thus validating the common assumption from the literature. However, for simulations at large distances from the plaques, such as for radiation shielding assessment and estimation of dose to personnel, the gamma spectrum from 106Rh must be taken into account to obtain accurate results.

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