Study of Spinal Cord Substructure Expansion Margin in Esophageal Cancer

PURPOSE: To analyze the setup errors and residual errors of different spinal cord parts in esophageal cancer patients and to explore the necessity of spinal cord segmental expansion. METHODS AND MATERIALS: Sixty cases of esophageal cancer were included with 20 patients subdivided into the following groups: neck, chest and abdomen as per the treatment site. The patients underwent intensity modulated radiation therapy (IMRT) between 2017 and 2019. Thermoplastic mask or vacuum bag were utilized for immobilization of different groups. CTVision (Siemens CT-On-Rail system) was used to acquire pre-treatment CT, and 20 consecutive pre-treatment CT datasets were collected for data analysis for each case. Images were exported to MIM (MIM Software Inc.) for processing and data analysis. Dice coefficient, maximum Hausdorff distance and centroid coordinate values between the spinal cord contours in the pre-treatment CTs and the planning CT were calculated and extracted. The contour expansion margin value is calculated as M(PRV) = 1.3 ∑ (total) + 0.5 σ (total), where ∑ (total) and σ (total) are the systematic and random error, respectively. RESULTS: For neck, chest, abdominal segments of the spinal cord, the mean Dice coefficients (± SD) are 0.73 ± 0.06, 0.80 ± 0.06, 0.82 ± 0.06, the maximum Hausdorff distance residual error (± SD) are 4.46 ± 0.55, 3.49 ± 0.53, 3.46 ± 0.69 mm, and the mean centroid coordinate residual error (± SD) are 2.40 ± 0.53, 1.66 ± 0.47, 2.14 ± 0.95 mm, respectively. The calculated margin using residual centroid method in medial-lateral (ML), anterior-posterior (AP), and cranial-caudal (CC) direction of spinal cord in neck, chest, abdominal segments are 3.86, 5.37, 6.36 mm, 3.45, 3.83, 4.51 mm, 4.05, 4.83, 7.06 mm, respectively, and the calculated margin using residual Hausdorff method are 3.10, 5.33 and 6.15 mm, 3.30, 3.77, 4.61 mm, 3.35, 4.76, 6.87 mm, respectively. CONCLUSION: The setup errors and residual errors are different in each segment of the spinal cord. Different margins expansion should be applied to different segment of spinal cord.