A study on the displacements of the clips in surgical cavity for external-beam partial breast irradiation after breast-conserving surgery based on 4DCT.

To explore the clip and the geometrical center displacements based on the four-dimensional computed tomography (4DCT) for external-beam partial breast irradiation (EB-PBI), fourteen breast cancer patients treated with breast-conserving surgery were recruited for EB-PBI and undertook 4DCT simulation during free breathing. The displacements of the selected clips and the geometrical center at left-right (LR), anterior-posterior (AP) and superior-inferior (SI) directions were measured. The comparison and the correlation of the displacement between the selected clips and the geometric center were analyzed. The displacements in AP and SI were greater than that in LR for the same selected clip (P < 0.05). Almost all the displacements of the geometrical center were greater than that of the selected clips in the same direction, (P < 0.05), except the displacements of lower clip in SI direction. The displacement of the geometric center showed a statistical correlation with the upper clip and lower clip in SI direction (both P < 0.05). Therefore, the internal margin in AP and SI directions should be greater than LR direction for the purpose of adequate target coverage and sparing more normal tissue. This study also indicates that the displacement of a single clip was not qualified to substitute for the displacement of the target based on all clips of the surgical cavity.