Monte Carlo calculations of radiotherapy dose distributions within and around orthopaedic implants

Cancer patients often require a titanium orthopaedic implant to support or replace lost bone. In radiation treatment, the dose distribution is perturbed causing regions of high and low dose at material interfaces. Since the survival of integrating bone tissue is critical to implant success, the aim of this study was to determine the dose distribution in and around the scaffold, when constructed from titanium or Poly-ether-ether-ketone (PEEK). The dose distributions in the pores and along boundaries for three implant scaffold designs were calculated using Monte-Carlo methods in Geant4/GATE, with the material taken as titanium or PEEK. The 3D dose distributions were analysed in MATLAB and segmented using image masks, yielding the dose distributions in key regions of interest. To evaluate the effect of the predicted dose perturbations, the cell survival was calculated using the linear-quadratic model for SAOS-2 cells (bone) using experimentally determined radiation response data. High dose gradients were found along the boundaries of the titanium implants, but not for the corresponding PEEK implants. The dose to the internal cavities of the titanium implants was enhanced by 10–15% near the proximal interface whereas for PEEK, there was no significant dose perturbation. The predicted perturbation caused by the titanium implant was shown to decrease the survival for SAOS-2 cells by 7% which was not found for the PEEK implants. PEEK was shown to be a more favourable orthopaedic implant material over titanium for cancer patients considering radiation therapy.