Objective  To analyze the relationship between the calibration data of cone beam CT (CBCT) before first radiotherapy and the data representing the target fractional displacement before fractional radiotherapy for treatment of prostate cancer, so as to predict the target fractional displacement.

  Methods  Sixteen patients with prostate cancer were selected. CBCT calibration displacements before first radiotherapy and mean values of fractional radiotherapy were recorded. The correlation between the mean value and calibration displacement value was analyzed. Then a regression equation model was established initially to analyze and predict the target movement.

  Results  The absolute mean values of CBCT calibration displacement before first radiotherapy were (0.384±0.270) mm in directions of left-right, (0.980±0.570) mm in head-feet directions, (2.462±1.105) mm in anterior-posterior directions for all the patients, and were (0.719±0.464) mm, (1.387±0.757) mm, (2.884±1.607) mm, respectively before fractional radiotherapy. There was a weak correlation between absolute mean value of CBCT calibration displacement before first radiotherapy and absolute mean value for fractional radiotherapy in directions of left-right and head-feet directions(r=0.359, -0.326, P>0.05), and strong correlation between absolute mean value for fractional radiotherapy of CBCT calibration displacement and absolute mean value before first radiotherapy in anterior-posterior directions(r=0.833, P < 0.01).

  Conclusion  CBCT calibration displacement before first radiotherapy in anterior-posterior directions could be used to predict the target displacement of fractional radiotherapy(posterior direction). It is necessary to optimize the experiment process.