Monte Carlo study on the impact of spinal fixation rods on dose distribution in photon beams

Background Metal spinal rods are used as fixation devices in spinal surgery. The attenuation effect of these rods has not been completely studied for patients with spinal rods and requiring spinal radiotherapy. Aim The purpose of the current study was to investigate the dosimetric perturbation effect of metallic spinal rods in different photon beams. Materials/Methods Three photon beams of 6, 9 and 15MV were modelled using MCNP4C Monte Carlo (MC) code. The geometry consisted of two spinal rods at a depth of 4cm and a water phantom was used for MC calculations. The beam profiles at depths of 5.5, 6.5 and 7.5cm were calculated. Results Dose reductions of 10.2-11.2% and 5-6.2% were observed for steel and titanium rods respectively. The insertion of metallic rods into the photon beams did not change the spinal cord received dose but the effect of both types of rods on the target region behind the rods cannot be ignored, especially for steel rods. Conclusions Our results suggest that for reliable spinal radiotherapy the dose attenuation effect of spinal rods must be taken into account in treatment planning calculations.