Development of micro-collimators for highly precise irradiation of ophthalmic tumours with I-125 brachytherapy eye applicators

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The aim of radiation therapy of ocular tumours is a complete tumour control combined with the best possible conservation of the eye functionality. All regions outside of the target volume have to be spared, especially structures at risk such as the optic nerve, blood vessels and the ciliary body. Since these structures are often located within a few millimetres from the target volume, or even adjacent to it, large dose gradients between these regions are essential for a successful therapy. Field gradients and geometry of conventional eye plaques with the low-energy photon emitter I-125 and the beta emitter Ru-106 fit these requirements only in a limited way. Thus, it is often tried to get better functional results of the eye by an expensive proton radiation therapy. To overcome this substantial disadvantage of ophthalmic brachytherapy we developed the novel concept of micro collimators consisting of metal- and plastic sheets, for usage with I-125 seeds. Due to their small size they could be inserted into typical eye plaques. In spite of their quite simple structure, micro collimators emit the radiation with exactly defined direction and angle, as measurements of prototypes showed. The dose gradient at the lateral field boundaries can reach more than one order of magnitude within 0.5 mm - a value even surpassing that of expensive proton beams. Thus, a more effective sparing of structures at risk can be reached. Hence, micro collimators offer the possibility of a highly precise conformal brachytherapy that can be adjusted to the situation of the individual patient. In order to determine the range of application and to prepare the technique for a usage in the clinical routine, extensive basic investigations are still necessary. The influence of constructive parameters of the micro collimators on their dose distribution and the optimisation of plaques with several collimators and their resulting radiation field have to be investigated. For this research, numerical simulations and highly precise dosimetric measurements with recently developed miniature plastic scintillators are mandatory. A sufficiently exact and reproducible way of the construction of micro collimators and applicators, with integrated positioning tools for the ophthalmologist in the operational theatre, and approaches for an effective therapy planning have to be developed, as well. The objective of the proposed research project is the immediate implementation of micro collimators into the radiation therapy of ocular tumours. An essential improvement of the patient's life quality by reduction of side effects is thereby intended, compared to the techniques used so far.