Backscatter radiation at bone-titanium interface from high-energy X and gamma rays.

Occasionally head and neck cancer patients treated with high-energy X rays and gamma rays have titanium metal dental implants in their maxillae or mandibles. The resulting effect of the bone-metal interface on the radiation dose is of interest. Ionization measurements for 60Co gamma rays and 6 MV and 25 MV X rays were made. A thin-window parallel-plate chamber was used to determine the magnitude of the dose enhancement that was due to the backscattered electrons from titanium. The results showed that for 60Co there is a 15% increase in dose to solid bone at the entrance side of the titanium. For higher energy X rays, the increase in dose was about the same or slightly lower than for 60Co. Monte Carlo calculations substantiated the measurements. This increase in dose fell off rapidly and became negligible at 1-2 mm from the interface. This backscattered dose should be taken into account when planning radiation therapy treatment for patients with dental implants.