gms | German Medical Science

Background: Depending on size and position of a tumour in the eye radiation therapy can be administered either with protons or with plaques. Proton radiation therapy with its homogeneous dose distribution is usually used for tumours with a prominence of greater than 5 mm or if the tumors are located at the posterior pole of the eye. In most of the other cases Ruthenium-106 plaques are used. It is possible to achieve a good tumor control with both methods.

Methods: For the treatment planning intercomparison a choroidal melanoma with 2 mm distance to the fovea was chosen. The tumour base is 11.5×10.5 mm²; its prominence is 3.5 mm. The proton dose distribution is calculated by the treatment planning program OCTOPUS v4.4.9. A total dose of 60 CGE (Cobalt Gray Equivalent, 1 CGE = 1.1 Gy for protons) was prescribed. The dose calculation for the CCA-plaque of Bebig was performed by the Monte-Carlo Code MCNPX v2.6.0. Dose prescription to the tumour apex was 100 Gy and to the sclera at least 400 Gy.

Results: Proton beam irradiation allows a homogeneous irradiation of the tumour with 60 CGE. The dose of the plaque varies between 100 Gy (tumour apex) and 346 Gy (tumor base). The sclera contact dose is 426 Gy. In both cases, protons and plaque, 25% of the retina is surrounded by the 20 Gy isodose. The mean dose to the fovea administered by the plaque is 23.7 Gy. In the proton case the fovea gets no dose.

Conclusions: Both methods achieve irradiation of the tumour according to their prescriptions. Proton beam therapy has a homogeneous dose distribution with less radiation to the retina and especially to the fovea.