Assessment of ocular beta radiation dose distribution due to 106Ru/106Rh brachytherapy applicators using MCNPX Monte Carlo code

Nilseia Barbosa, Luiz da Rosa, Artur Menezes, Juraci Reis, Alessandro Facure, Delson Braz
2014 International Journal of Cancer Therapy and Oncology  
Purpose: Melanoma at the choroid region is the most common primary cancer that affects the eye in adult patients. Concave ophthalmic applicators with 106 Ru/ 106 Rh beta sources are the more used for treatment of these eye lesions, mainly lesions with small and medium dimensions. The available treatment planning system for 106 Ru applicators is based on dose distributions on a homogeneous water sphere eye model, resulting in a lack of data in the literature of dose distributions in the eye
more » ... ons in the eye radiosensitive structures, information that may be crucial to improve the treatment planning process, aiming the maintenance of visual acuity. Methods: The Monte Carlo code MCNPX was used to calculate the dose distribution in a complete mathematical model of the human eye containing a choroid melanoma; considering the eye actual dimensions and its various component structures, due to an ophthalmic brachytherapy treatment, using 106 Ru/ 106 Rh beta-ray sources. Two possibilities were analyzed; a simple water eye and a heterogeneous eye considering all its structures. Two concave applicators, CCA and CCB manufactured by BEBIG and a complete mathematical model of the human eye were modeled using the MCNPX code. Results and Conclusion: For both eye models, namely water model and heterogeneous model, mean dose values simulated for the same eye regions are, in general, very similar, excepting for regions very distant from the applicator, where mean dose values are very low, uncertainties are higher and relative differences may reach 20.4%. For the tumor base and the eye structures closest to the applicator, such as sclera, choroid and retina, the maximum difference observed was 4%, presenting the heterogeneous model higher mean dose values. For the other eye regions, the higher doses were obtained when the homogeneous water eye model is taken into consideration. Mean dose distributions determined for the homogeneous water eye model are similar to those obtained for the heterogeneous eye model, indicating that the homogeneous water eye model is a reasonable one. The determined isodose curves give a good visualization of dose distributions inside the eye structures, pointing out their most exposed volume. Relative depth dose and lateral dose profiles were tallied along the symmetry axes of the applicators, extending from the inner concave surface into the water sphere. The Figures 4, 5 and 6 show the relative depth dose and lateral profile dose distributions along distances perpendicular to the symmetry axis at different depths, simulated with MCNPX *F8 tally, for both CCA and CCB applicators compared with experimental data provided by the manufacturer, BEBIG, and with the MC-PENELOPE calculations carried out by Hermida-López 37 , whose results were well discussed and compared with those of other authors. 32, 33, 34 FIG. 4: Central axis depth dose curves for CCA applicator: Comparison between BEBIG GmbH manufacturer manual data and MCNPX (present work) results (left Figure) . Comparison between PENELOPE/Hermida-López 37 simulations and MCNPX (present work) results (right Figure) . The relative doses have been normalized to 100% at a depth of 2 mm (left figure) and 100% at a depth of 1 mm at the central axis (right Figure) . FIG. 5: Central axis depth dose curves for CCB applicator: Comparison between BEBIG GmbH manufacturer certificate data and MCNPX (present work) results (left Figure) . Comparison between PENELOPE/Hermida-López 37 simulations and MCNPX (present work) results (right Figure) . The relative doses have been normalized to 100% at a depth of 2 mm (left Figure) and 100% at a depth of 1 mm at the central axis (right Figure) . FIG. 6: Lateral dose distributions calculated using MCNPX at 1, 2, 3, 4, 6, and 8 mm depths for CCA (left figure) and CCB (right figure) applicators. The data are compared to PENELOPE/Hermida-López 37 results. The relative doses have been normalized to 100% at a depth of 1 mm at the central axis. www.ijcto.org
doi:10.14319/ijcto.0203.8 fatcat:2mw62pktsrgpfbyztkkxztw2i4