Dosimetric evaluation of titanium and carbon spinal fixation systems in radiotherapy: Comparison of two dose calculation algorithms.

Metallic implants in cancer patients may negatively affect radiotherapy by generating computed tomography artifacts and perturbing dose delivery through backscatter, attenuation, and secondary particle generation. Carbon-fiber implants have been proposed as an alternative to titanium due to their radiolucency. This study evaluated the dosimetric impact of titanium and carbon fixation systems in a stabilized spine phantom and compared two dose calculation algorithms, the Anisotropic Analytical Algorithm (AAA) and Acuros XB (AXB). An in-house phantom with a porcine spine was prepared in three configurations: no fixation, titanium fixation, and carbon fixation. Volumetric modulated arc therapy plans were generated for 6, 15, and 10 MV flattening filter-free (FFF) beams. The spinal cord was contoured as the main organ at risk. Dose distributions from AAA and AXB (Eclipse v15.6) were verified against Gafchromic EBT-3 film using gamma analysis (2%/2 mm) and mean dose evaluation in the spinal canal. Carbon fixation was comparable to no fixation, with mean spinal canal dose deviations as small as -1.2% at 6 MV and -0.7% at 15 MV. Titanium caused the largest error, with AXB underestimating the mean spinal canal dose by 7.2% at 6 MV. AAA showed closer agreement with film for 6 and 15 MV, while AXB performed better for 10 MV FFF in radiolucent configurations. Carbon fixation systems yielded dosimetric results comparable to those without fixation, whereas titanium introduced substantial perturbations. AAA proved more robust in conventional flattened beams, while AXB offered advantages for 10 MV FFF beam.