Radiation absorbed doses at compact bone-titanium interfaces in diagnostic radiography: a Monte Carlo approach.

Objectives: The aim of this study was to estimate the radiation absorbed dose at cortical tissue-implant interfaces in diagnostic radiology.

Methods: Since our interest was the radiation dose at an interface (cortical bone-implant interface), a Monte Carlo simulation was considered to be the most suitable method for studying the problem. The Monte Carlo code employed was MCNP4B. A phantom consisting of soft tissue, cortical bone, an implant and air, with appropriate chemical compositions and densities, was described in the code. The implant simulated had a commercial name of ASTM67, grade 2 and was 1.9 mm wide. The incident photon beam was divergent of 20 cm x 20 cm at a source-to-phantom distance of 40 cm. Two energy spectra were employed (70 kVp and 100 kVp, 0.5 mm Al internal filtration) and their photon fluence distribution against energy was described in the code with an energy interval of 5 keV. The computations that led to radiation dose calculations had a spatial resolution of 0.01 cm (100 microm) to allow a detailed radiation dose distribution across the cortical bone-titanium interface. Monte Carlo runs took place both with and without an implant in the phantom and, in each case, 120 million photon histories were followed, leading to a radiation dose statistical fluctuation between 5% and 10%.

Results: The ratio of radiation dose with implant to dose without implant against depth allows a direct estimate of the effect of the implant on the radiation dose to the cortical bone surrounding the implant. At a distance >or=100 microm there was no radiation dose increase due to the titanium implant. However, in contact with the implant (i.e. the first layers of cells) there was a sharp radiation dose increase as high as 3.5 times the radiation dose compared with when the implant was absent. Also, the newly formed bone inside the implant's tiny hole received a radiation dose close to 50% of the radiation dose without the implant owing to high absorption by the implant itself.

Conclusions: Assuming that the patient received five radiographic images over a 6-month period, the maximum radiation dose at the cortical bone-titanium interface was estimated to be less than 20 mGy (0.02 Gy).