Abstract: Objective Analysis of the pendulum error of pancreatic cancer patients by spiral tomography technique under the guidance of megavolt CT (megavoltage computed tomography, MVCT) image, and calculation of planned target volume, PTV) of the margin. Methods The MVCT image scan matched with the planned CT image, the error values of the left and right (X), head and feet (Y), abdominal back (Z) axis direction and cross-section rotation (roll) direction were recorded, and the error values were analyzed and calculated. Results A total of 592 MVCT scans were conducted. The X, Y, Z and roll direction swing error values were (-0.5±2.8)mm, (-1.1±6.4)mm, (6.0±4.4)mm, and (-0.2±0.7) respectively. The average error in the three directions of X, Y and Z was less than 5 mm, with 97.13% (575/592), 88.01% (521/592) and 37.84% (224/592), and roll direction rotation error of less than 1 degree, with 93.92% (556/592). According to the formula, the extra-boundary spacing values of The PTV in X, Y and Z 3 directions were 5.16, 9.9 and 7.53 mm respectively. Conclusions In order to improve the accuracy and efficacy of tomotherapy, The margin emission values of CTV to PTV in X, Y and Z three-dimensional directions are recommended as 5, 10 and 8 mm, respectively.

Key words: pancreatic carcinoma, tomotherapy, image guided radiation therapy(IGRT), setup errors