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Modern lateral external beam (lens sparing) radiotherapy for retinoblastoma.

A standard 6 MV linear accelerator X-ray beam has been adapted to produce a non-divergent and almost penumbra-free beam edge by a beam splitting and extended collimation system. Using a contact lens to provide the reference point on the front surface of the eye and an attached rod-and-scale measuring system that is linked to the sharp beam edge system, it has proved possible to place this field border with an exactitude of within 0.5 mm at a required distance behind the front surface of the eye. This system has been developed for the treatment of small retinoblastomas not amenable to focal treatment methods; the technique may have other applications. Data in this manuscript corroborate an earlier Dutch publication and extend the observations on the physics beam profile obtainable, the immobilization of the patient, the anaesthetic procedure, the contact lens system and the dose prescription. Both scanning densitometry and TLC measurements in an anthropomorphic head shell (with extractable eye) confirm the extreme precision and sharp beam profile obtainable by this technique.