The influence of silicone oil used in ophthalmology on the proton therapy of uveal melanomas

In ocular proton therapy the important parameters range and modulation of the proton field are calculated by a simple geometric ray tracking. Due to the assumed homogeneous eye tissue, the relative-to-water proton stopping power is nearly constant within the radiation field. In cases when silicone oil tamponades are used, this will no longer hold. CT-data which are necessary to estimate the path length of protons within the oil show bigger attenuation of x-rays then in eye tissue. The use of a CT-calibration curve to deduce the relative-to-water electron density for proton stopping power calculation would be misleading giving a value of 1.05 and thus a higher proton stopping power. In reality the relative-to-water electron density is smaller, namely 0.95, leading to a proton range prolonged by about 11% of the path length within the silicone oil, if the demand for the range adjustment would not be recognized (missing information from ophthalmologists or no CT data). This finding is congruent in analytical and Monte Carlo calculations as well as in experimental investigations, being presented here. The misleading result of the CT calibration curve is explained and therapeutic consequences are demonstrated. Multiple Coulomb scattering in silicone oil does not affect the size of the planed irradiation field.