Comparison of displacement of the geometry constituted by silver clips in cavity in the different state of respiration assisted by active breathing control system in external-beam partial breast irradiation

Objective: To investigate the effect of the displacement of the selected silver clip in the different respiratory state achieved by active breathing control (ABC) system on the displacement of the geometry constituted by all of the silver clips at the border of the cavity in external-beam partial breast irradiation (EB-PBI). Methods: Two sets of CT images in state of moderate deep inspiratory breathing hold (mDIBH), deep expiratory breathing hold (DEBH), and free breath (FB) were acquired in the same CT simulation assisted by ABC system for each of the 27 patients after breast conservative surgery. All silver clips in the cavity were delineated based on each set of CT images. Thereafter, the irregular geometry based on the silver clips as the vertices was automatically formed. Four selected clips located at the top, bottom, lateral border and medial border of the cavity were correspondingly manually registered based on automatic registration of the CT images acquired in the same or different state of respiration including mDIBH, FB, and DEBH. The displacement of center of the geometry in the direction of right-left (RL), anterior-posterior (AP), and superior-inferior (SI) separately based on automatic registration and manual registration was evaluated. The difference of the displacement was analyzed by Kruskal-Wallis H-test and Kolmogorov-Smirnov Z-test. Results: When registered between mDIBH and mDIBH, FB and FB, DEBH and DEBH, the differences of the displacement of the center of geometry were not statistically significant (H =0.00 - 1.76, P=0.184-0.954). When registered between mDIBH and DEBH,the differences were statistically significant (Z =11.31 - 23.00, P =0.000 - 0.001). There were statistically significant differences in the displacement of geometry center based on the selected silver clip between different registration forms in AP and SI directions (Z=4.76-25.54, P=0.000-0.029). Conclusions: The difference of intrafraction displacement of the geometry constituted by the clips between the same respiratory states in the three dimensional direction is not statistically significant, but the difference is statistically significant between the different respiratory states in AP and SI directions. (authors)