Computed Tomography Myelosimulation Versus Magnetic Resonance Imaging Registration to Delineate the Spinal Cord During Spine Stereotactic Radiosurgery

Underestimation of the spinal cord's volume or position during spine stereotactic radiosurgery can lead to severe myelopathy, whereas overestimation can lead to tumor underdosage. Spinal cord delineation is commonly achieved by registering a magnetic resonance imaging (MRI) study with a computed tomography (CT) simulation scan or by performing myelography during CT simulation (myelosim). We compared treatment planning outcomes for these 2 techniques. Twenty-three cases of spine stereotactic radiosurgery were analyzed that had both a myelosim and corresponding MRI study for registration. The spinal cord was contoured on both imaging data sets by 2 independent blinded physicians, and Dice similarity coefficients were calculated to compare their spatial overlap. Two treatment plans (16 Gy and 18 Gy) were created using the MRI and CT contours (92 plans total). Dosimetric parameters were extracted and compared by modality to assess tumor coverage and spinal cord dose. No differences were found in the partial spinal cord volumes contoured on MRI versus myelosim (4.71 ± 1.09 vs. 4.55 ± 1.03 cm3; P = 0.34) despite imperfect spatial agreement (mean Dice similarity coefficient, 0.68 ± 0.05). When the registered MRI contours were used for treatment planning, significantly worse tumor coverage and greater spinal cord doses were found compared with myelosim planning. For the 18-Gy plans, 10 of 23 MRI cases (43%) exceeded the spinal cord or cauda dose constraints when using myelosim as the reference standard. Significant spatial, rather than volumetric, differences were found between the MRI- and myelosim-defined spinal cord structures. Tumor coverage was compromised with MRI-based planning, and the high spinal cord doses were a concern. Future work is necessary to compare thin-cut, volumetric MRI registration or MRI simulation with myelosim.