Effect of breathing motion in radiotherapy of breast cancer: 4D dose calculation and motion tracking via EPID.

PURPOSE: To evaluate the influence of breathing motion in postoperative whole-breast radiotherapy.

PATIENTS AND METHODS: For ten patients with left-sided breast cancer, radiotherapy treatment plans were generated based on conventional three-dimensional computed tomography (3D CT) studies: two techniques (segmented and wedge-based tangential fields) were compared. The influence of breathing motion on the dose to the target and organs at risk (OARs) was evaluated with four-dimensional (4D) dose calculation based on respiration-correlated CTs. Reproducibility of breathing motion was evaluated with electronic portal images (EPID) acquired in cine mode during treatment.

RESULTS: Differences in dose distributions were small between segmented and wedge techniques based on 3D studies. Because of small motion amplitude of the chest in the 4D CT studies (1.8 mm +/- 0.9 mm), target coverage was reduced by < 5% due to breathing motion. Differences between 3D and 4D dose calculation were similar for segmented and wedge techniques. Blurring of the dose distribution in 4D dose calculation resulted in lower doses to the OARs. Analysis of EPID movies proved good reproducibility of breathing motion observed in the 4D CT study.

CONCLUSION: Breathing motion was of minor relevance in postoperative radiotherapy treatment of breast cancer for both segmented and wedge tangential field techniques.