Evaluation of deformable image registration accuracy for liver re-irradiation patients using contrast and non-contrast computed tomography images.

BACKGROUND

Re-irradiation of liver tumors is increasing in frequency, requiring clinicians to account for previous radiation dose to prevent unacceptable toxicity. Given the heterogeneity of liver morphological changes between treatments, deformable image registration (DIR) is required to accumulate dose from previous treatments onto the latest planning images for radiotherapy.

PURPOSE

The increase in re-irradiation of patients with liver cancer has led to the need to account for previous radiotherapy treatments. This feasibility study used contemporaneous intravenous contrast computed tomography images (CTs) to evaluate the accuracy of DIR dose accumulation in the re-irradiation of liver patients, via the use of structural landmarks.

METHODS

We used nine liver patients who received repeat stereotactic body radiation therapy (SBRT) liver radiotherapy, with contrast and non-contrast planning CTs, to evaluate the accuracy of dose accumulation in the liver. The initial planning CT was deformed to the second planning CT, and the deformation vector field was used to deform the initial dose map. The dose could then be accumulated by adding the deformed initial dose map to the second dose map. Three methods of performing DIR were compared, including with and without corresponding anatomical landmarks. Target registration error (TRE), dice similarity coefficient, and Hausdorff distance were used to assess the accuracy of the dose accumulation.

RESULTS

The lowest TRE was achieved with the structure guided algorithm using all of the available anatomical landmarks, with a mean + standard deviation of 1.7 mm (SD = 0.9 mm) for non-contrast (p < 0.0005) and 1.6 mm (SD = 0.9 mm) for contrast CTs (p < 0.0005). DIR based on contrast CTs reduced the TREs, the distance between the location of a given landmark on the second image, and the location of where that landmark is deformed to from the first image, with all DIR algorithms (p < 0.0005 for each contrast non-contrast pair). There were also statistically significant differences between dose accumulation errors for Contrast CTs with a mean of 0.92 Gy (SD = 3.08 Gy) and Non-Contrast CTs of 1.07 Gy (SD = 3.36 Gy) (p < 0.05), and the differences between each of the algorithms were also statistically significant, with p-values < 0.05.

CONCLUSIONS

DIR improves the dose accumulation accuracy in re-irradiation in liver SBRT. DIR accuracy can be improved using contrast CTs and corresponding anatomical landmarks. Providing additional information into the DIR in the form of corresponding anatomical landmarks dramatically improved image registration accuracy and thus reduced dose accumulation errors. Dose accumulation accuracy was dependent on the TRE, and on the dose-gradient of the mapped dose.