Room: Exhibit Hall | Forum 8
Purpose: The purpose of this study is to investigate the benefit of CBCT in soft tissue alignment and proper selection of PTV margin to avoid irradiating the surrounding and normal tissue in vicinity of PTV. We also quantify the shifts of pancreas from planning to treatment and compare random and systematic errors if the patient is localized for treatment with respect to bony anatomy.
Methods: The images of patients who had pancreatic cancer and received radiation therapy on the pancreas were used in the study. Tumor localization of pancreas patients were retrospectively evaluated based on soft-tissue and bony anatomy alignment. We calculated the systematic (Î£) and random (Ïƒ) errors based on bony anatomy using soft tissue alignment as the accepted standard. PTV margins that would be necessary if the patient had been localized with respect to bony anatomy were also calculated by the van Herkâ€™s method (2.5 Î£ + 0.7 Ïƒ). Lastly we calculated the 3D shift of pancreas based on bony anatomy.
Results: 110 CBCT sessions for 16 patients were analyzed for this study. The systematic errors were calculated to be 2.3 mm, 2.4 mm, and 5.3 mm; the random errors 2.6 mm, 2.0 mm, and 4.3 mm; and the PTV margins 7.6 mm, 7.2 mm, and 16.3 mm in the X, Y, and Z centroids, respectively. The average 3D shift based on bony anatomy were determined to be 5.8Â±1.4 mm.
Conclusion: The translational systematic and random errors calculated are larger in the superior-inferior direction than the anterior-posterior and lateral directions. The error calculations and the average shift values in each direction indicate that the pancreas motion are largest in the superior-inferior direction. Our results show that CBCT saves 7.2 mm to 16.3 mm of PTV margin that would otherwise be necessary to be included in radiation field.