Purpose: To evaluate the dosimetric differences observed in spine SBRT patients when assigning a material to contoured spine hardware as compared to utilizing the Hounsfield units (HU) obtained during computed tomography simulation.
Methods: Forty-five spine SBRT patients with titanium hardware were retrospectively analyzed. The clinical plan was re-calculated with AcurosXB (version 15.6.05) set to report dose-to-medium with the spine hardware set to 3071 HU. A duplicate of the clinical plan was calculated with the material of the hardware contour set to titanium (density of 4.42 g/cm^3 and 8610 HU) via the physical materials table available. A planning target volume for evaluation (PTVeval) was created by subtracting the hardware from the clinical PTV. Dosimetric statistics for PTVeval, spinal cord, cauda equina, and esophagus were computed and analyzed. The PTVeval was used for comparison purposes as dose within the rods is not clinically significant.
Results: The compared PTVs displayed average percentage differences for maximum dose, mean dose, minimum dose, median dose, and dose to 95% of the volume (D95) of 1.95%, -0.18%, -0.44%, -0.16%, and -0.86%, respectively. Whereby a positive percent difference means that the plans with titanium assigned show higher dose. The average percent difference for the spinal cord, cauda equina, and the esophagus are 0.04%, -0.14%, and –0.22%. When the hardware encompassed more than 10% of the PTV volume, the average percentage differences for maximum dose, mean dose, minimum dose, median dose, and D95 were 2.90%, -0.34%, -0.74%, -0.30%, and –1.41%, respectively.
Conclusion: The largest observed dosimetric difference for the D95 was -3.5%, representing a decrease in coverage when assigning titanium material versus 3071 HU. Additionally, no dosimetric differences for the organ at risk metrics exceeded 2.5% when assigning titanium material versus 3071 HU.