Quantitative Assessment of Respiratory-Induced Clinical Target Volume Motion During Postoperative Whole-Breast Radiotherapy Using Four-Dimensional Computed Tomography

Background/Objectives: We quantitatively evaluated respiratory-induced clinical target volume (CTV) motion during postoperative whole-breast radiotherapy in patients undergoing breast-conserving surgery (BCS) using four-dimensional computed tomography (4DCT) and determined the internal margins for assessing respiratory motion. Materials and Methods: We enrolled 100 patients who underwent postoperative whole-breast breast RT after BCS. Simulation CT was performed using 4DCT, and the CTV was delineated for the 10 respiratory phases (0–90%). CTV center displacement was measured in the lateral, anteroposterior, and superoinferior directions and three-dimensional (3D) vector magnitudes were calculated. Internal margins for respiratory motion were calculated using the Van Herk formula (2.5Σ + 0.7σ). Results: Mean CTV center displacement (mm) was 0.94 ± 0.52 (lateral), 1.29 ± 0.59 (anteroposterior), and 1.00 ± 0.51 (superoinferior). The mean 3D vector magnitude was 1.99 ± 0.68 (range, 0.70–3.51) mm. Anteroposterior motion was significantly greater than the lateral and superoinferior motion (p < 0.001). No patient exhibited motion exceeding 3 mm in any direction. Right-breast 3D motion was significantly larger than the left-breast motion (2.17 ± 0.66 vs. 1.82 ± 0.65 mm, p = 0.008). Internal margins (mm) for respiratory motion were 1.02 (lateral), 1.24 (anteroposterior), and 1.03 (superoinferior). Conclusions: Respiratory-induced CTV motion during whole-breast RT after BCS was small, with anteroposterior dominance. Calculated internal margins were approximately 1 mm in all directions, suggesting that a conventional 5 mm CTV-to-planning target volume (PTV) margin is adequate to compensate for respiratory motion and indicating that routine 4DCT or respiratory motion management may be unnecessary for target coverage.