A PRELIMINARY COMPARATIVE TREATMENT PLANNING STUDY FOR RADIOTHERAPY OF AGE-RELATED MACULOPATHY

Purpose: We present a comparative planning of different approaches for external radiotherapy in age-related maculopathies. Materials and met hods: Calculated dose distributions and dose-volume histograms for (a) bilateral irradiation with 6 MV photons, (b) a single lateral-oblique beam using either photons, electrons or protons and (c) an anterior c ircular proton beam. Results: For lateral photon or electron beams the dose to the lens is usually lower than 10% of the dose to the macula. The entrance doses for bilateral photon beams are about 50% which inc rease up to 100% at the orbital bone. About 5 mm of optic nerves are; irradiated at the maximal dose while the optic chiasma is spared. A si ngle photon beam gives 50% of the dose to the fellow eye. The electron beam spares the fellow eye but gives a rather inhomogeneous dose to t he target volume. For a lateral proton beam, 4 mm of optic nerve recei ves 90% of the dose, the skin dose is at least 70% of the dose to the macula and the lens and the fellow eye are spared. An anterior proton beam gives 90% of the dose to 1 mm of optic nerve and the 50% isodose approaches the periphery of the lens. Conclusion: Doses to the critica l structures can be dramatically diminished for all the techniques by reducing the beam size, but only if very precise set-up techniques are used. Proton beams are an attractive solution, but the impact of such a choice on the use of proton facilities and on the national health s ystem should be carefully evaluated, as well as the risk of radio-indu ced secondary neoplasias. (C) 1998 Elsevier Science Ireland Ltd.