Analysis of Correlation Between Breast Surface Position and Internal Organ Deformation Using Deep-Inspiration Breath Holding for Left-Sided Breast Irradiation

Abstract

An increased risk of cardiac toxicity and radiation pneumonitis is related to the mean dose of radiation to the heart and lungs during left-sided breast irradiation. The deep-inspiration breath holding (DIBH) technique was investigated with focus on reducing the radiation dose to the heart and its substructures, such as the left anterior descending coronary artery (LAD). Three-dimensional surface-imaging systems have been used to facilitate image-guided breast radiotherapy. However, internal organ position during DIBH depends on the compliance of the patient. Previous research has shown that relative craniocaudal distance changes during inspiration and expiration normalized to the mean distance in quiet breathing was maximum 44.3% ± 4% and minimum 4% ± 5%. We investigated positional correlations between limited breast surface position and internal organs and evaluated the effect of doses in left-sided breast irradiation with the DIBH technique. In this study, breast surface position was measured using an infrared range finder and the heart position was retrospectively measured using CT data. Five patients who had undergone left-sided lumpectomy practiced DIBH during CT. Simultaneously, breast surface distance (BSD) was monitored by a range finder continually during CT acquisition. The range finder was attached to the CT couch and measured the distance from one point of the breast surface to the scanner. CT with voluntary DIBH and free breathing in the treatment position was performed three times in each patient. To measure heart distances (HD), the maximum distance from the LAD to the chest wall (lateral edge of the lung) was calculated in each CT image and associated with the location to the corresponding BSD. Each free-breathing CT image was deformed to each DIBH CT image, and the deformation vector field (DVF), which was associated with the HD and the BSD, was calculated. Moreover, we evaluated dosimetric effects of variations of internal organ deformation by the voluntary DIBH. Compared with the mean BSD during free breathing, mean BSD during DIBH was reduced by 12 mm and the reproducibility was shown to be stable at approximately ± 0.2～3.9 mm. The mean HD was increased by 5.4 ± 2.4 mm. The correlation coefficient between BSD and HD during DIBH was 0.23, which indicated a weak positive correlation. According to the results of DVF, the heart and diaphragm positions moved forward toward the front caudal side; however, the variation in HD and internal organ motion depended on the characteristics of each patient. Moreover, in cases of abdominal breathing, breast surface motion was significantly small and no correlation between BSD and HD was observed. However, dosimetric errors of target and normal tissue were small, suggesting that DIBH irradiation while monitoring BSD is feasible. Correlations among BSD, HD, and internal organ motion were weak and depended on patient characteristics and respiratory method.