Monte Carlo calculation of the dose distributions of two ¹⁰⁶Ru eye applicators

Abstract 

Background and purpose: Beta emitting ¹⁰⁶Ru applicators are widely used to treat choroidal melanoma. In view of the importance of clinical applications of this radioisotope and the relative lack of knowledge of the dose distributions, three-dimensional dose maps of two concave applicators were calculated by means of Monte Carlo simulation.

Materials and methods: Simulations of small CCA and CCB concave applicators manufactured by Bebig were performed using the Monte Carlo code PENELOPE, which allows the description of the structure (geometry and materials) of the applicator in detail. Electrons are emitted from the ¹⁰⁶Ru nuclei isotropically, with initial energy randomly sampled from the corresponding Fermi spectra and with initial positions uniformly distributed on the radioactive layer. Primary electrons, as well as the produced δ-rays, are assumed to be absorbed in the medium when they slow down to an energy of 70 keV. Bremsstrahlung photons with energies larger than 7 keV are also simulated. The simulation code has been run on a 166 MHz PENTIUM PC.

Results: Three-dimensional dose distributions produced by the CCA and CCB applicators in a water sphere, concentric with the applicator, were evaluated. To minimize the magnitude of statistical uncertainties, advantage has been taken of the cylindrical symmetry of the problem. The relative depth-dose (along the symmetry axis of the applicator) was also evaluated from the applicator surface up to distances larger than 1 cm, with statistical uncertainties of a few percent. Results compare well with data supplied by the manufacturer.

Conclusions: We have performed accurate Monte Carlo calculations of three-dimensional dose distributions from CCA and CCB ¹⁰⁶Ru applicators. The results, presented in the form of two-dimensional maps, depth-dose distributions along the symmetry axis and lateral dose profiles, provide a detailed description of the dose delivered in treatments of choroidal melanoma.

Keywords: Monte Carlo, Dosimetry, Brachytherapy, Eye melanoma, Ruthenium-106