ABSTRACT
Objective To measure changes of definitive volume of rebuild CT image in three dimensional conformal radiotherapy (3DCRT) treatment planning system (TPS) under different scanning and observing conditions, which may provide a reference in selecting proper condition of scanning and observing, delineating target volume, and delivering treatment plan. Methods A self-made phantom was scanned under different CT scanning conditions, datas were imported to TPS, CT images were rebuilt in three dimensions under different observing conditions, and then the volumes of large, middle and small simulated tumors on CT images were compared with their real volumes. Results The volume defined by rebuilt CT imags in 3DCRT TPS varied according to different scanning and observing conditions. When scanning axial thickness was set at 10. 000 mm, errors of large, middle and small simulated tumor were - 13.60%, - 17. 68% and - 27. 53%, respectively. When 0. 625 mm was set, errors changed to -0. 50%, -0. 68% and -0. 81% ,respectively. When scanning pitch was set at 0. 562, 0. 938, 1. 375 and 1. 750, the errors were - 1.06%, - 1.09%, 5.52% and 4.75%, respectively. Extreme observing condition, window width and window level markedly increased errors of definitive volume of rebuilt CT images with a maximum of 80. 06%. Conclusions This study may provide clues in selecting scanning and observing condition, delineating target volume, and delivering treatment plan.