Small-field irradiation of monkey eyes with protons and photons.

The relative biological efficiency (RBE) of a 7 mm diameter proton beam was compared to a 6.25 mm 2 MV photon beam by measuring their relative effects in owl monkey eyes. We chose as the endpoint the appearance of a circumscribed patch of chorioretinal edema 2 days after a single dose. Both unmodulated protons and modulated proton beams showed an RBE close to unity. Dose is critically dependent on the position of the Bragg peak relative to the retina and choroid. Irradiation with modulated protons in five fractions once per week produced a marked tissue-sparing effect; no clinical or histological effects were evident 1 yr later, until a total dose of 12,500 rad or more had been delivered. These experiments are of importance to the planning of small-beam therapy of eye tumors. Finally, they demonstrate that carefully collimated narrow beams can deliver a tissue-destroying dose to a localized area of retina and choroid, while causing minimal damage to adjacent normal tissue.