Three-Dimensional Intrafractional Motion of Breast During Tangential Breast Irradiation Monitored With High-Sampling Frequency Using a Real-Time Tumor-Tracking Radiotherapy System

Purpose To evaluate the three-dimensional intrafraction motion of the breast during tangential breast irradiation using a real-time tracking radiotherapy (RT) system with a high-sampling frequency. Methods and Materials A total of 17 patients with breast cancer who had received breast conservation RT were included in this study. A 2.0-mm gold marker was placed on the skin near the nipple of the breast for RT. A fluoroscopic real-time tumor-tracking RT system was used to monitor the marker. The range of motion of each patient was calculated in three directions. Results The mean ± standard deviation of the range of respiratory motion was 1.0 ± 0.6 mm (median, 0.9; 95% confidence interval [CI] of the marker position, 0.4–2.6), 1.3 ± 0.5 mm (median, 1.1; 95% CI, 0.5–2.5), and 2.6 ± 1.4 (median, 2.3; 95% CI, 1.0–6.9) for the right–left, craniocaudal, and anteroposterior direction, respectively. No correlation was found between the range of motion and the body mass index or respiratory function. The mean ± standard deviation of the absolute value of the baseline shift in the right–left, craniocaudal, and anteroposterior direction was 0.2 ± 0.2 mm (range, 0.0–0.8 mm), 0.3 ± 0.2 mm (range, 0.0–0.7 mm), and 0.8 ± 0.7 mm (range, 0.1–1.8 mm), respectively. Conclusion Both the range of motion and the baseline shift were within a few millimeters in each direction. As long as the conventional wedge-pair technique and the proper immobilization are used, the intrafraction three-dimensional change in the breast surface did not much influence the dose distribution.