Monte Carlo estimation of absorbed dose distributions in water and anthropomorphic phantoms obtained from homogeneous and heterogeneous 106Ru eye plaques

Uveal melanoma and retinoblastoma are rare malignant tumors that, if not properly treated, can be life-threatening. Brachytherapy with 106Ru eye plaques is an effective treatment in preserving the ocular globe and maintaining visual acuity to some extent. Although these plaques have been used for decades, there is a lack of precise knowledge about the absorbed dose distribution in the tumor volume and the surrounding structures at risk, introducing large variability in the clinical outcome as it is reflected in the retrospective published studies. The aim of this thesis is to improve, using the general-purpose Monte Carlo radiation transport code PENELOPE, the current knowledge on the dose distribution inside the eye produced by two actual 106Ru plaques taking into account the distribution of the radioactive substance in their emitting surface.

As a preliminary step, the absorbed dose in water produced by two generic CCA and CCB eye plaque models, which consider the emitter substance homogeneously distributed, is simulated. Computed depth doses along the symmetry axis of the plaques are compared with experimental data provided by the manufacturer of the plaques and previously published results. Later, using a measured emitter map of the actual CCA1364 and CCB1256 plaques, simulated depth doses are compared with experimental data and also with the homogeneous approximation. Comparison between lateral profiles and isodose lines are also computed for both plaques. Results show that the actual heterogeneous distribution reproduces better the data provided in the certificate of the plaques.

Dose distributions inside an eye are simulated using an anthropomorphic phantom. The main structures of the eye are segmented to determine the absorbed dose in them. The CCA eye plaque is simulated in an anterior, equatorial and posterior placement while the CCB, due to its larger size, is only simulated in an equatorial placement. To analyze the effect of the het- erogeneities on the absorbed dose, rotations around the symmetry axis of the plaques are also simulated. Results indicate that not considering the distribution of the emitter substance pro- duces an overdosage in all segmented volumes as occurs with the cornea when is irradiated anteriorly which receives 71% higher dose with respect to the homogeneous assumption.

Finally, tumor volumes are also segmented and the absorbed dose in them analyzed for different plaque placements and rotations. Eccentric placements of the plaques are also con- sidered. Results show that knowledge of the emitter map allows eccentric treatments and confirms an overdosage when considering the homogeneous assumption.


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