The aim of this study was to develop a method for accurately measuring the intra-fraction motion in cine electronic portal imaging device (EPID) images of tangential breast irradiation.

The cine EPID images were acquired for 18 breast cancer patients during medial tangential breast radiotherapy. The skin surface and the chest wall were segmented separately in each EPID image using an automated MATLAB algorithm to obtain the magnitude of intra-fraction motion.

The patients had an average age of 55 years (range 43–69 years), with the highest observed displacement at 3·50 mm. The mean intra-fraction motion due to respiration for the skin surface and chest wall was 1·04 ± 0·24 mm and 1·10 ± 0·27 mm, respectively. The amount of intra-fraction motion showed no significant association with either the patient’s age or the side of the breast receiving the treatment.

In this study, by utilising cine EPID technology and the automated MATLAB algorithm, the intra-fraction motion during breast radiotherapy treatment was accurately measured and analysed. Results indicate minimal skin surface and chest wall motion (approximately 1 mm), confirming negligible intra-fraction motions during conformal radiotherapy in breast cancer patients.