Session Item

Stereotactic body radiotherapy for spinal metastases (spine SBRT) is better than conventional palliative radiotherapy at controlling pain in the spine. In spine SBRT cases after spinal fusion surgery, the magnetic resonance (MR) image is difficult to visualize the spinal cord due to the existence of the metal artifact on it; therefore, the CT myelography is used for the visualization as an alternative to the MR imaging. In such cases, another CT without the enhanced contrast is acquired for the dose calculation in order to avoid the influence on the dose distribution due to the enhanced contrast on the CT myelography images; however, the influence has not been quantitatively clarified. In this study, we aimed to quantitatively evaluate the change in the spinal cord dose in spine SBRT treatment planning by the presence or absence of the enhanced contrast on the CT myelography image.    

Nineteen patients who had previously undergone the spine SBRT were selected randomly. All patients had scanned the CT myelography a couple of days after the treatment planning CT. The slice thickness of both image sets was 1 mm. The rigid image registration of the CT myelography that aligned at the treatment planning CT was carried out for the vertebra landmarks. The rigid image registration accuracy was evaluated by calculating the normalized mutual information (NMI) and Pearson's correlation coefficient for the vertebra landmarks. After the registration, the original plan on the treatment planning CT image was recalculated on the CT myelography image that was replaced the contrast-enhanced region (spinal fluid) by water-mass density. For the comparison, it was recalculated also on the CT myelography as the enhanced contrast left. 

For the rigid image registration accuracy, the NMI and Pearson's correlation coefficient for the landmarks were 0.38 ± 0.12 and 0.97 ± 0.03, respectively. It showed great registration accuracy. The mean D_0.035cc of the spinal cord on the CT myelography image with the contrast-enhanced region replaced by water-mass density showed a change in 0.37 ± 0.64% compared with the CT image used in the original treatment plan. On the other hand, the mean D_0.035cc in the CT myelography as the enhanced contrast left was a change in 1.41 ± 0.53%. The change between the two showed a significantly difference (p < 0.01, Wilcoxon signed-rank test).    

We quantitatively evaluated the change in the spinal cord dose in spine SBRT treatment planning by the presence or absence of the enhanced contrast on the CT myelography image. By replacing the contrast-enhanced region by water-mass density, the change in the spinal cord dose was less than 0.5%. The CT myelography would be able to use for the dose calculation for the spine SBRT by replacing the contrast-enhanced region by water-mass density.