The Effect of Physical Characteristics of Spinal Implants on Spinal Cord Dose Distribution

In radiotherapy, the presence of metallic implants affects the dose distribution, depending on beam energy and type, size, material and position of the implant. In this study, the effect of spinal implants on dose distribution was evaluated by Monte Carlo simulation. Oncor accelerator and water phantom, Plexiglas phantom containing spine and Plexiglas phantom containing spine and rod implants were simulated using BEAMnrc and DOSXYZnrc user codes and were validated. The effect of the implant presence, its size material, and location on dose distribution was investigated. Comparison of dose distributions with and without implants showed an increase of 5.5% in dose up to 4 mm from metal surface, due to the back scattering of electrons. Maximum attenuation in titanium rods was 9%. When the rods were positioned closer to each other, an increase in attenuation was observed. However, dose in spine was increased, when the distance between rods and spine was reduced. Rods with smaller cross section had less effect on dose. Attenuation in vitallium and stainless steel implants were 2 times greater than titanium implants. Depending on the location of implant with respect to spine, dose perturbation due to implants can either increase or decrease the dose in spine. The effect of size and material of implant was more important when spine was located under the implant. Generally, the effect of spinal implant on dose distribution is a function of size, material and position of the implant. Therefore, it is recommended to evaluate the dose distribution for such patients individually.