[Study on the target movement in external-beam partial breast irradiation with active breathing control after breast-conserving surgery].

Objective: To explore the displacement of surgical clip and cavity during different respiratory status, intrafraction and interfraction for the patients treated by external-beam partial breast irradiation (EB-PBI) assisted by active breathing control (ABC) device after breast-conserving surgery.

Methods: After breast-conserving surgery, twenty-five patients with stage I/II breast cancer who was identified to be feasible for EB-PBI underwent CT-simulation positioning assisted with ABC device. Five series of CT images were done during three respiratory status, which included one series of images during free breathing status (FB), two series of images during moderate deep inspiration breath hold (mDIBH), two series of images during deep expiration breath hold (DEBH) at 75% maximum inspiration capacity, and all of the five series of images were transferred to ADAC Pinnacle3 treatment planning system. This procedure was repeated after 10 - 15 days. The targets were delineated and the first appeared clips were marked by the same radiotherapist on the CT images in mDIBH and DEBH status from the first CT simulation positioning to analyze the displacement of the targets produced by respiratory status. The targets were also delineated and all of the clips were marked by the same radiotherapist on another two series of CT images during mDIBH from the first and repeated CT-simulation positioning for analyzing the intrafraction and interfraction movement of cavity and clips, respectively.

Results: There was a significant difference between mDIBH and DEBH in the movement of the point of interest (POI) of the first appeared clip in the directions of X (0.22 cm), Y (1.27 cm) and Z axis (0.50 cm) (chi2 =9.558, P = 0.008); and this was also found to be present in the movement of POI of the cavity in the directions of X (0.10 cm), Y (1.08 cm) and Z axis (0.50 cm) (chi2 = 20.44, P <0.01). Regardless of the clip and the cavity, the movement extent along the direction of Y axis was the biggest. However, no significant difference was found in the displacements of POI of the geometric form constructed by all clips in the directions of X (0.09 cm, 0.68 cm), Y (0.14 cm, 0.37 cm) and Z axis (0.25 cm, 0.50 cm) in the same respiratory status of mDIBH at different moment in the intrafraction and interfraction, respectively; and this was also found in the displacements of POI of the cavity in the direction of X (0.15 cm, 0.66 cm), Y (0.17 cm, 0.45 cm) and Z axis (0.25 cm, 0.75 cm)in the same respiratory status of mDIBH at different moment in the intrafraction and interfraction, respectively.

Conclusion: The margin of planning target volume for external beam partial breast irradiation in different respiratory status is found to be different, the margin displacement along the different axis is also different in free breathing status. However, it can be controlled almost at the same extent in moderate deep inspiration breath hold if assisted by active breathing control device.